Ivermectin and Fenbendazole in Veterinary Oncology: A Review of Preclinical Evidence and Anecdotal Reports

I. Introduction: Exploring Antiparasitics in Veterinary Cancer Care

A. The Emerging Interest in Repurposed Drugs

The field of oncology is continually seeking novel therapeutic strategies to combat cancer in both human and veterinary patients. One area that has garnered increasing attention is drug repurposing—the investigation of existing drugs, approved for other medical conditions, for new applications such as cancer treatment.¹ This approach offers several advantages, primarily because these drugs often have well-established pharmacokinetic profiles, known safety parameters from their original indications, and potentially lower development costs and shorter timelines to clinical application compared to entirely new chemical entities.¹

Among the candidates for repurposing in oncology are the broad-spectrum antiparasitic agents ivermectin and fenbendazole. Ivermectin, a macrocyclic lactone, and fenbendazole, a benzimidazole, have long histories of safe and effective use against a wide range of internal and external parasites in various animal species, and ivermectin is also used in human medicine.³ Emerging preclinical research has suggested that these compounds may possess anticancer properties, leading to considerable interest within the scientific community and among pet owners seeking alternative or complementary cancer therapies.¹ This report aims to synthesize the available scientific evidence and anecdotal information regarding the use of ivermectin and fenbendazole for cancer in dogs and cats, with the goal of providing a balanced overview that may stimulate further, rigorous research into these potential alternative modalities.

The research landscape for ivermectin and fenbendazole in veterinary oncology is characterized by a notable dichotomy. On one hand, a growing body of preclinical evidence from in vitro (cell culture) studies and in vivo animal models (predominantly mice) suggests plausible anticancer mechanisms and efficacy for both drugs.⁴ This generates legitimate scientific interest and hope for new therapeutic avenues. On the other hand, this is paralleled by fervent anecdotal support, particularly fueled by personal testimonials like the “Joe Tippens protocol” for fenbendazole, which circulates widely on social media and online forums.¹ However, there is a stark lack of robust, controlled veterinary clinical trials specifically evaluating these drugs for cancer in dogs and cats.¹ This disparity creates a challenging environment where the genuine hope for novel, accessible treatments can easily blur into unsubstantiated hype, potentially leading pet owners and some practitioners to consider these therapies based on incomplete or misinterpreted information. Such decisions may carry risks to patient safety or lead to the delay or abandonment of established, evidence-based treatments. Therefore, this report will meticulously navigate this complex landscape, presenting scientific findings with objectivity, contextualizing anecdotal claims, and consistently underscoring the paramount importance of evidence-based veterinary medicine and the urgent need for well-designed clinical trials.

B. Scope and Disclaimer

This report focuses on synthesizing the available scientific evidence (preclinical and limited clinical) alongside anecdotal reports concerning the use of ivermectin and fenbendazole in the context of canine and feline cancer. It will cover proposed mechanisms of action, efficacy data from in vitro and in vivo studies (primarily in non-veterinary species or xenograft models), reported dosages for both antiparasitic and investigational anticancer uses, cancer types with some level of investigated or reported efficacy, and critical safety considerations.

It is crucial to state that this report is for informational and research-stimulation purposes only and does not constitute veterinary medical advice. The use of ivermectin and fenbendazole for cancer treatment in pets is largely experimental and considered off-label. Any consideration of such therapies must be undertaken only after thorough consultation with a qualified veterinarian, preferably a board-certified veterinary oncologist, who can provide guidance based on the individual patient’s condition, current scientific understanding, and a comprehensive risk-benefit assessment.

II. Ivermectin: A Review of Anticancer Potential in Dogs and Cats

Ivermectin, a member of the avermectin class of drugs, is a potent, broad-spectrum antiparasitic agent.³ Its traditional use in veterinary medicine includes the prevention of heartworm disease and treatment of various internal and external parasites in dogs and cats.⁵¹ Beyond its antiparasitic effects, emerging research has highlighted its potential anti-inflammatory, antiviral, and, notably, anticancer properties.³

A. Mechanisms of Anticancer Action (Preclinical Evidence)

Preclinical studies, primarily using human cancer cell lines and mouse models, have elucidated several potential mechanisms through which ivermectin may exert anticancer effects. These are multifaceted and suggest ivermectin can interfere with multiple pathways crucial for cancer cell survival and proliferation.

1. P-glycoprotein (MDR) Inhibition: Cancer cells often develop resistance to chemotherapy drugs by overexpressing efflux pumps like P-glycoprotein (P-gp), which is encoded by the Multidrug Resistance (MDR1 or ABCB1) gene. Ivermectin has been shown to inhibit P-gp, potentially reversing multidrug resistance and sensitizing cancer cells to conventional chemotherapeutics.⁵ This is a significant finding, as chemoresistance remains a major obstacle in successful cancer treatment.

2. Ionophore Activity and Chloride Channel Modulation: Ivermectin can act as an ionophore, altering ion transport across cell membranes. It has been reported to up-regulate chloride channels, leading to increased intracellular chloride concentrations. This influx can induce plasma membrane hyperpolarization and osmotic stress, ultimately triggering apoptotic cell death in cancer cells.⁷ Malignant cells, which often have altered chloride channel expression, may be particularly susceptible to this mechanism.

3. Mitochondrial Dysfunction and Oxidative Stress: Mitochondria play a central role in cellular energy production and apoptosis. Ivermectin has been shown to target mitochondrial function by inhibiting respiratory complex I. This leads to decreased ATP production, increased generation of reactive oxygen species (ROS), and oxidative stress, which can damage DNA and other cellular components, pushing cancer cells towards apoptosis.⁵

4. Induction of Immunogenic Cell Death (ICD): ICD is a form of cancer cell death that elicits an adaptive immune response against tumor antigens. Ivermectin has been reported to promote ICD by stimulating the release of damage-associated molecular patterns (DAMPs), such as ATP and high-mobility group box 1 (HMGB1), into the tumor microenvironment. These signals can recruit and activate immune cells, fostering an anti-neoplastic immune response.⁷

5. Autophagy Induction: Autophagy is a cellular self-degradation process that can either promote cell survival or lead to cell death, depending on the context. Ivermectin can induce autophagy in cancer cells, partly by promoting the ubiquitination and degradation of the oncogenic kinase PAK1 (p21-activated kinase 1). PAK1 downregulation, in turn, inhibits the Akt/mTOR pathway, a known repressor of autophagy, thereby leading to autophagic cell death.⁵

6. WNT/β-catenin Pathway Inhibition: The WNT/β-catenin signaling pathway is frequently dysregulated in various cancers and plays a critical role in tumor initiation, progression, and the maintenance of cancer stem cells. Ivermectin has been demonstrated to inhibit this pathway by, for instance, reducing the nuclear translocation of β-catenin, thereby downregulating target genes involved in cell proliferation.⁵

7. Epigenetic Modulation: Ivermectin may influence the epigenetic landscape of cancer cells. It has been suggested to mimic the SIN3-interaction domain, thereby interfering with the activity of epigenetic deregulators SIN3A and SIN3B. This interaction can lead to a reduction in the expression of stem cell markers like NANOG and SOX2, potentially diminishing cancer cell pluripotency and self-renewal capabilities.⁷

8. RNA Helicase Inhibition: Certain RNA helicases are implicated in ribosome biogenesis and the regulation of oncogenic microRNAs, such as miR-21. Ivermectin has been identified as an inhibitor of RNA helicases like DDX23, potentially disrupting these processes and reducing the levels of tumor-promoting miRNAs.⁵

9. Targeting Cancer Stem Cells (CSCs): CSCs are a subpopulation of tumor cells believed to be responsible for tumor initiation, metastasis, and recurrence. Ivermectin has shown a preferential inhibitory effect on CSCs in several cancer models, potentially by targeting pathways like Hedgehog signaling, which are vital for CSC maintenance.⁵

B. In Vitro and In Vivo Efficacy Studies (Primarily non-veterinary species)

The anticancer potential of ivermectin has been investigated in numerous in vitro and in vivo studies, predominantly using human cancer cell lines and immunodeficient mouse xenograft models. These studies have generally shown that ivermectin can inhibit cancer cell proliferation, induce apoptosis, and reduce tumor growth across a range of cancer types.

Table 4 from the review by Juarez et al. (2018) summarizes several in vivo studies where ivermectin treatment resulted in significant tumor volume reduction (often >50%) in mouse models of murine leukemia (MDAY-D2), human glioblastoma (U87, T98G), breast cancer (MDA-MB-231-GFP), human glioma (U87MG), and human colon cancer (LDL1).⁷ Doses in these studies ranged from 2.4 mg/kg to 40 mg/kg, administered via intraperitoneal (i.p.), oral, or intratumoral (i.t.) routes.⁷ Notably, in a human glioma model, a 10 mg/kg i.t. dose led to undetectable tumors after 42 days.⁷

Further supporting its potential, ivermectin has been shown to augment the efficacy of conventional chemotherapy. For instance, it enhanced the inhibitory effect of cisplatin on ovarian cancer cells in vitro and in a xenograft mouse model by suppressing Akt/mTOR signaling.⁶ Additionally, ivermectin exhibited potent anticancer activity against gemcitabine-resistant cholangiocarcinoma cells in vitro.⁶

A notable preclinical study from City of Hope demonstrated that combining ivermectin with anti-PD1 monoclonal antibodies successfully treated triple-negative breast cancer in animal models.¹³ Ivermectin appeared to convert immunologically “cold” tumors (low T-cell infiltration) into “hot” tumors, thereby enabling the anti-PD1 therapy to be effective. This combination led to complete tumor eradication in 40-60% of the treated animals.¹³

C. Canine-Specific Research: Focus on Mammary Tumors

While most anticancer research on ivermectin has utilized human cell lines or non-veterinary models, a significant study by Diao et al. (2019) specifically investigated its effects on canine mammary tumors (CMT).⁶ CMTs are the most common spontaneous tumors in intact female dogs and often carry a poor prognosis, making the search for novel therapies critical.⁶

The study by Diao et al. reported the following key findings:

• In Vitro Efficacy: Ivermectin inhibited the growth of canine mammary tumor cell lines (CMT7364 and CIPp) in a dose- and time-dependent manner. The concentrations used in vitro included 8 µM and 12 µM.⁶
• Mechanism of Action in CMT Cells: The antitumor effects were primarily associated with cell cycle arrest at the G1 phase. This was mediated by the downregulation of cyclin-dependent kinase 4 (CDK4) and cyclin D1 expression. Interestingly, significant induction of apoptosis was not observed in this model.⁶ Furthermore, ivermectin treatment led to a significant reduction in the nuclear translocation of β-catenin, resulting in the inactivation of the WNT signaling pathway, a known driver of various cancers.⁶
• In Vivo Efficacy (Xenograft Model): Consistent with the in vitro results, ivermectin administration led to a significant suppression of tumor growth in a canine mammary tumor xenograft model in mice.⁶ The specific dosage used in the in vivo part of this xenograft study was not detailed in the readily available abstracts but the results showed a significant reduction in CIPp tumor volume in mice treated with ivermectin.¹⁵

The authors concluded that ivermectin is a promising anticancer agent for canine mammary tumors, acting by regulating cell cycle progression and WNT signaling.⁶ This study provides the most direct evidence to date for ivermectin’s potential in a specific canine cancer.

D. Reported Dosages

Dosage information for ivermectin varies widely depending on the species, intended use (antiparasitic vs. anticancer), and the source of information (FDA-approved labels, scientific literature, or anecdotal reports).

Table 1: Reported Dosages of Ivermectin for Antiparasitic and Investigational Anticancer Use in Dogs and Cats

Note: mcg = microgram, mg = milligram, kg = kilogram, lb = pound, SQ = subcutaneous, i.p. = intraperitoneal, i.t. = intratumoral. Dosages for cancer are largely investigational or anecdotal and carry significant risk, especially in MDR1-sensitive breeds.

It is evident from the table that doses explored for anticancer effects in animal models (mice) are substantially higher (mg/kg range) than those used for routine parasite prevention in dogs and cats (mcg/kg range). This dosage disparity is a critical factor when considering safety, particularly in MDR1-sensitive dog breeds.

E. Cancer Types with Investigated or Reported Efficacy

The range of cancer types against which ivermectin has shown some level of activity in preclinical or limited veterinary settings includes:

• Dogs:
  • Canine Mammary Tumors: As discussed, Diao et al. (2019) demonstrated efficacy in vitro and in a mouse xenograft model using canine mammary tumor cells.⁶
  • Canine Transmissible Venereal Tumor (CTVT): A case report described successful management of CTVT using ivermectin ($0.3\text{ mg/kg}$ SQ) as an adjunct to vincristine chemotherapy.⁵⁸ The proposed mechanism was ivermectin’s P-glycoprotein inhibition, potentially enhancing vincristine’s efficacy and reducing its side effects.
  • Canine Osteosarcoma: A study showed ivermectin acted synergistically with doxorubicin in osteosarcoma cells in vitro and significantly inhibited osteosarcoma growth in vivo in a mouse xenograft model, with the effective dose being clinically feasible and non-toxic in mice.²⁵
  • Canine Lymphoma / Cutaneous T-cell Lymphoma (CETL): Anecdotal suggestions for lymphoma exist (e.g., 200 mcg/kg daily) ⁴⁹, but veterinary oncologists strongly caution against this due to lack of efficacy data in dogs and high toxicity risk.⁴⁹ One case report on CETL mentioned an ivermectin injection as part of initial, non-specialist treatment before referral to oncology, but its specific contribution to the outcome is unclear.⁵⁹
• Cats:
  • Feline T-cell Lymphoma: One recent study (2025 abstract) using Dalton’s lymphoma cells (a mouse T-cell lymphoma line, but the study implies relevance to T-cell lymphoma broadly) showed ivermectin induced G0-G1 cell cycle arrest and apoptosis in vitro, and in vivo treatment in tumor-bearing mice led to tumor size reduction.²³ This is not direct feline data but suggests a potential area for investigation.
  • General: Information on specific feline cancers successfully treated with ivermectin is scarce in the provided materials. While ivermectin is used for parasites in cats ⁵¹, its application in feline oncology is not well-documented in these sources beyond the T-cell lymphoma model. One article mentions metronomic chemotherapy having positive results in soft tissue sarcomas in cats, but this is a general therapy class, not specific to ivermectin.²⁸
• General (Human/Mouse Models – Potential for Translation):
  • Leukemia (murine and human cell lines) ⁷
  • Glioblastoma/Glioma (human cell lines) ⁷
  • Breast Cancer (human cell lines, including triple-negative) ⁷
  • Colon Cancer (human cell lines) ⁶
  • Ovarian Cancer (human cell lines) ⁶
  • Cholangiocarcinoma (gemcitabine-resistant, in vitro) ⁶

The broad in vitro activity of ivermectin against various cancer cell lines is promising. However, the journey from a petri dish to a patient is long and fraught with challenges. For canine lymphoma, specifically, the perspective of veterinary oncologists is critical. Dr. Brooke Britton, for instance, strongly refutes the use of ivermectin for canine lymphoma, highlighting the absence of in vivo data in dogs and the substantial risk of toxicity at doses that might theoretically be effective.⁴⁹ In vitro studies might show ivermectin can kill cancer cells, but achieving those same drug concentrations at the tumor site within a living animal, without causing unacceptable systemic harm, is a formidable hurdle.⁴⁹ Dr. Britton specifically notes that the ivermectin concentrations needed in vivo to replicate in vitro effects against carcinoma cells (a different tumor type) would be highly toxic to a dog, and there is no data to support efficacy against lymphoma cells in dogs.⁴⁹ This underscores a critical translational gap: positive laboratory findings do not guarantee clinical utility, and caution is essential when considering off-label use based solely on such preclinical data, especially for conditions like lymphoma where standard-of-care treatments exist.

F. Safety, Toxicity, and the Critical MDR1 Gene Mutation in Dogs

The safety profile of ivermectin is heavily influenced by dose, species, and individual genetic predispositions, most notably the MDR1 (ABCB1) gene mutation in dogs.

• MDR1 Gene (ABCB1-1Δ) Mutation:
  • Background: The MDR1 gene encodes P-glycoprotein (P-gp), an efflux pump crucial for limiting drug penetration into the brain and other sensitive tissues.³ A deletion mutation in this gene (often referred to as ABCB1-1Δ) results in a non-functional P-gp.⁶⁵
  • Consequence: Dogs homozygous for this mutation are unable to effectively pump ivermectin (and other P-gp substrate drugs) out of the central nervous system, leading to drug accumulation and severe neurotoxicity even at doses considered safe for wild-type dogs.³ Heterozygous dogs may also show increased sensitivity, though typically less severe than homozygous individuals.⁶⁷
  • Affected Breeds: This mutation is prevalent in Collies (up to 75% carry the mutation), Australian Shepherds, American Shepherds, German Shepherds, Shetland Sheepdogs, Old English Sheepdogs, Long-haired Whippets, Silken Windhounds, McNabs, and other herding breeds, often associated with “white feet”.⁵⁴
  • Implications for Cancer Dosing: While standard heartworm prevention doses of ivermectin (e.g., $6-12\text{ mcg/kg}$ monthly) are generally considered safe even for MDR1-mutant dogs ⁵¹, the significantly higher doses that would likely be required for anticancer effects (potentially in the $0.2-0.6\text{ mg/kg}$ or higher range, based on off-label parasitic uses or mouse cancer studies) could be dangerous or lethal to these sensitive dogs.⁴⁹ Genetic testing for the MDR1 mutation is commercially available and strongly recommended before considering any high-dose or off-label ivermectin use in susceptible breeds.⁵²

The MDR1 gene mutation poses a substantial barrier to the widespread or high-dose use of ivermectin in canine oncology. The anticancer doses suggested by preclinical mouse models (e.g., $2.4-40\text{ mg/kg}$) ⁷ are orders of magnitude higher than safe antiparasitic doses for MDR1-affected dogs. This genetic sensitivity means a significant portion of the canine population might be ineligible for such ivermectin-based cancer therapies, or would necessitate such low doses to avoid catastrophic neurotoxicity that any potential anticancer efficacy would likely be compromised. This reality underscores that any future investigation into ivermectin for canine cancer must rigorously stratify study populations by MDR1 genotype or focus on developing innovative drug delivery systems that can target tumors while bypassing or mitigating systemic neurotoxicity in sensitive breeds. This highlights the indispensable role of pharmacogenetics in the responsible exploration of repurposed drugs in veterinary medicine.

• General Ivermectin Toxicity (Dogs & Cats):
  • Dogs (Non-MDR1 or at high doses): Even in dogs without the MDR1 mutation, high doses of ivermectin can lead to neurotoxicity. Signs include ataxia (incoordination), stumbling, tremors, dilated pupils, blindness, seizures, drooling, head pressing, lethargy, coma, and potentially death.³ Gastrointestinal upset (vomiting, diarrhea) can also occur.⁵² A rapid die-off of heartworm microfilariae can cause a shock-like reaction (weakness, vomiting, diarrhea, pale gums).⁵²
  • Cats: Cats are generally considered more tolerant to ivermectin than dogs, particularly MDR1-sensitive dogs, and standard antiparasitic doses are usually well-tolerated.⁵³ However, neurotoxicity can occur at doses above $2.5\text{ mg/kg}$, with more severe signs (tremors, blindness, seizures, respiratory failure, coma) at doses exceeding $5\text{ mg/kg}$.⁶⁰ Accidental overdose, often from using formulations intended for larger animals like horses or cattle, is a significant risk.⁵³ The MDR1 mutation has also been identified in some cats, with a homozygous deletion (ABCB11930_1931del TC) associated with ivermectin sensitivity.⁷¹
• Drug Interactions: Certain drugs can inhibit P-glycoprotein or other metabolic pathways, increasing ivermectin’s concentration in the brain and the risk of toxicity. These include ketoconazole, itraconazole, cyclosporine, and erythromycin.⁵²

III. Fenbendazole: A Review of Anticancer Potential in Dogs and Cats

Fenbendazole is a broad-spectrum benzimidazole anthelmintic widely used in veterinary medicine to treat a variety of gastrointestinal and other parasites in dogs, cats, livestock, and other animals.¹ Like ivermectin, it has recently gained attention for its potential anticancer properties, largely driven by preclinical studies and compelling anecdotal reports, most notably the “Joe Tippens protocol”.¹

A. Mechanisms of Anticancer Action (Preclinical Evidence)

Fenbendazole’s anticancer activity is thought to stem from several mechanisms, with microtubule disruption being the most well-characterized.

1. Microtubule Disruption: This is considered the primary anticancer mechanism of fenbendazole and other benzimidazoles.¹ Microtubules are essential components of the cellular cytoskeleton, involved in cell division (mitosis), cell structure maintenance, intracellular transport, and motility. Fenbendazole binds to β-tubulin, a protein subunit of microtubules, and inhibits its polymerization. This disruption preferentially affects rapidly dividing cells, such as cancer cells and parasites, more than normal mammalian cells, due to differences in tubulin structure and binding affinity.⁴ This mechanism is shared by established anticancer drugs like vinca alkaloids (e.g., vincristine) and taxanes (e.g., paclitaxel).⁴ By disrupting microtubule dynamics, fenbendazole can induce cell cycle arrest, typically in the G2/M phase, and subsequently trigger apoptosis.⁹

2. P53 Activation and Apoptosis Induction: Fenbendazole has been reported to increase the activation of the tumor suppressor protein p53.⁹ Activated p53 can induce apoptosis (programmed cell death). Studies in colorectal cancer (CRC) cells have shown that fenbendazole triggers apoptosis through mitochondrial injury and the caspase 3-PARP pathway.⁹ In wild-type CRC cells, it can activate p53-mediated apoptosis, while in 5-FU-resistant CRC cells, it may trigger apoptosis through p53-independent pathways, potentially enhancing ferroptosis-augmented apoptosis.⁹

3. Glucose Metabolism Inhibition (“Starvation” of Cancer Cells): Cancer cells often exhibit altered glucose metabolism, relying heavily on glycolysis even in the presence of oxygen (the Warburg effect). Fenbendazole can interfere with this by:

   • Inhibiting the GLUT1 transporter: p53 activation by fenbendazole can lead to the inhibition of GLUT1 transporter expression, thereby reducing glucose uptake into cancer cells.⁹
   • Impeding Hexokinase II (HKII): HKII is a key enzyme in the glycolytic pathway. Fenbendazole, through p53 activation, is believed to impair HKII activity.⁹ By reducing glucose uptake and utilization, fenbendazole can effectively “starve” cancer cells of their primary energy source and building blocks for proliferation, leading to reduced lactate levels.¹

4. Induction of Oxidative Stress: Fenbendazole administration has been linked to increased oxidative stress in cancer cells. This, coupled with the activation of stress-related pathways like the MEK3/6-p38MAPK pathway, can further contribute to the inhibition of cancer cell proliferation and enhancement of apoptosis.⁹

5. Impairment of Proteasomal Function: Some evidence suggests fenbendazole may also impair proteasomal function, which is critical for protein degradation and cellular homeostasis. Disruption of this system can be detrimental to cancer cells.⁹

B. In Vitro and In Vivo Efficacy Studies

Fenbendazole’s anticancer activity has been evaluated in a variety of in vitro cell line studies and in vivo animal models, with mixed but often promising results, particularly in vitro.

• In Vitro Studies:Fenbendazole has demonstrated cytotoxic effects and growth inhibition against a broad range of human cancer cell lines. These include non-small cell lung cancer (NSCLC) lines (A549, H460, H1299), cervical cancer lines (HeLa, C-33A, CaSki), colorectal cancer lines (SNU-C5, including 5-FU resistant variants), skin cancer (melanoma A375 cells), leukemia (HL60 cells), hepatocellular carcinoma cells, and breast cancer (MCF-7 cells).9 It has also shown cytotoxicity in canine glioma cell lines.16 The concentrations required for these effects vary but are often in the low micromolar range. The drug has also shown efficacy against cancer cells resistant to conventional chemotherapies like 5-FU, paclitaxel, and docetaxel.9
• In Vivo Animal Model Studies (Primarily Mice):The translation of in vitro findings to in vivo efficacy has been more complex.
  • EMT6 Mouse Mammary Tumor: In one study using the EMT6 mouse mammary tumor model, intensive fenbendazole treatment was toxic to cells in vitro, especially under hypoxic conditions. However, when administered in vivo using maximally intensive regimens, fenbendazole did not alter tumor growth nor did it enhance the anticancer effects of radiation therapy.⁴ This study represents a critical example of where promising in vitro results did not translate to in vivo benefit under the tested conditions.
  • Human Lymphoma Xenograft in SCID Mice: A pivotal study by Gao et al. (often cited in discussions about fenbendazole) reported that a fenbendazole-containing diet (150 ppm), when combined with supplemental high doses of vitamins, significantly inhibited the growth of a human lymphoma xenograft in SCID mice.⁴ Importantly, in this study, fenbendazole administered alone (without the vitamin supplementation) did not show a clear benefit and, in one observation, even trended towards increasing tumor growth, although this was attributed to an outlier.²⁹ The vitamins themselves also showed some anticancer effects, but the combination appeared most effective.³² This study strongly suggests that co-factors or combination therapy might be essential for fenbendazole’s in vivo anticancer activity, at least in this model.
  • Human NSCLC Xenograft: In contrast to the EMT6 model, oral administration of fenbendazole ($1\text{ mg/mouse}$ every 2 days for 12 days) to mice with human NSCLC xenografts (A549, H460 cells) resulted in marked shrinkage of tumor size and weight.⁹
  • Mouse T-cell Lymphoma Model (EL-4 cells): More recently, a study investigating fenbendazole in an EL-4 mouse lymphoma model found that while fenbendazole induced G2/M arrest and cell death in vitro, it had no anticancer effects in vivo. The tumor growth was comparable to controls, and treated mice experienced rapid weight loss. This study also noted contradictory changes in immune cell populations within the tumor microenvironment in the fenbendazole-treated group compared to in vitro expectations.²⁰

The conflicting in vivo results, particularly the observation from the Gao et al. study where fenbendazole’s efficacy in lymphoma xenografts appeared dependent on co-administered vitamins ⁴, and the lack of efficacy or even potential for increased tumor growth with fenbendazole alone in some contexts ²⁹, highlight a significant complexity. Fenbendazole’s poor water solubility and consequent low bioavailability are known challenges.⁹ It is plausible that the vitamins in the Gao et al. study may have enhanced fenbendazole’s absorption, possessed synergistic anticancer properties, or counteracted some metabolic stress induced by fenbendazole that might otherwise hinder its efficacy or promote tumor growth. The popular Joe Tippens protocol also involves multiple supplements (curcumin, Vitamin E, CBD oil) ¹, making it difficult to attribute observed effects solely to fenbendazole. This “paradox of efficacy” suggests that future research should not only focus on fenbendazole as a monotherapy but also rigorously investigate these potential synergistic combinations to delineate the true therapeutic potential and develop optimized, safe protocols.

C. Reported Dosages

Similar to ivermectin, fenbendazole dosages vary significantly based on intended use (antiparasitic vs. investigational anticancer) and the source of the information.

Table 2: Reported Dosages of Fenbendazole for Antiparasitic and Investigational Anticancer Use in Dogs and Cats

Note: Dosages for cancer are largely investigational or anecdotal and derived from human protocols or preclinical studies. These carry risks and should only be considered under strict veterinary supervision.

The “Joe Tippens protocol,” originally for human use, often involves 222 mg of fenbendazole (commonly found in one packet of Panacur C 1-gram size, which contains 22.2% or 222 mg fenbendazole per gram of powder) taken daily, often in a cycle of 3 days on and 4 days off, along with supplements like Vitamin E, curcumin, and CBD oil.⁹ Adaptations for dogs often involve scaling this based on weight, with some holistic veterinarians suggesting around $50\text{ mg/kg}$ for the 3-days-on/4-days-off cycle, or a much lower daily dose of $1-2\text{ mg/lb}$.⁴⁰ Fenben.pro also mentions a $25-50\text{ mg/kg}$ daily dose, often cyclical.⁴⁸ It is critical to note that these anticancer dosages are off-label and not validated by extensive veterinary clinical trials.

D. Cancer Types with Investigated or Reported Efficacy

Fenbendazole has been investigated or anecdotally reported to have effects against a variety of cancer types, though rigorous veterinary clinical data is largely absent.

• Dogs:
  • Transitional Cell Carcinoma (TCC) / Bladder Cancer: Anecdotal reports from TCC Facebook groups suggest some owners use fenbendazole, often as part of the Joe Tippens protocol.³⁵
  • Lymphoma: The Gao et al. mouse xenograft study showed inhibition of human lymphoma growth when fenbendazole was combined with vitamins.⁴ However, a more recent mouse T-cell lymphoma model (EL-4 cells) showed no in vivo anticancer effect of fenbendazole alone, despite in vitro activity.²⁰ Lymphoma is among the most commonly diagnosed cancers in dogs.²⁴
  • Mast Cell Tumors: Mentioned in discussions about repurposed drugs, but specific efficacy data for fenbendazole in canine mast cell tumors is limited in the provided snippets.⁸⁷
  • Canine Melanoma: In vitro studies have included canine melanoma cell lines.³⁷
  • Canine Glioma: Cytotoxic effects observed in canine glioma cell lines in vitro.¹⁶
  • Metastatic Breast Cancer: One anecdotal report describes a 10-year-old German Shepherd with metastatic breast cancer treated with a combination of fenbendazole (5 days/week), ivermectin (once/week), and artemisia, reportedly resulting in a 60% decrease in node size and increased energy.⁴²
  • Osteosarcoma, Hemangiosarcoma: While mentioned in broader discussions about fenbendazole’s potential ¹¹, specific positive outcomes for fenbendazole against these aggressive canine cancers are not detailed in the provided research.
• Cats: Specific data on fenbendazole for feline cancer types is very scarce. Its safety for deworming is established ²⁰, but its anticancer use in cats is not well-documented in these sources.
• General (Human/Mouse Models – Potential for Translation):
  • Non-small cell lung cancer (NSCLC) ⁹
  • Colorectal cancer (including 5-FU resistant) ⁹
  • Cervical cancer ⁹
  • Leukemia ⁹
  • Hepatocellular carcinoma ⁹
  • Breast cancer ⁹
  • Glioblastoma ¹¹

E. Safety, Toxicity, and Bioavailability Challenges

While fenbendazole is generally considered safe for short-term antiparasitic use, its safety profile for long-term, potentially higher-dose cancer treatment protocols is not well established and raises concerns.

• General Safety (Antiparasitic Use): For routine deworming in dogs and cats at labeled doses ($50\text{ mg/kg}$ for 3 days for dogs; similar off-label for cats), fenbendazole is well-tolerated.²⁷ Side effects such as vomiting, diarrhea, or salivation are rare (affecting about 1% of dogs) and usually mild.³⁷ Allergic reactions to dying parasites can occasionally occur, especially with high parasite loads.⁷⁶ A study in healthy cats showed no adverse reactions even at doses up to $250\text{ mg/kg}$ for 9 days.²⁰

• Long-Term/High-Dose Concerns (Potential Anticancer Use):The safety profile for short-term deworming cannot be directly extrapolated to the prolonged, often off-label, use patterns considered for cancer therapy.
  • Liver Toxicity: A significant concern is potential hepatotoxicity. An 80-year-old human patient with NSCLC self-administering fenbendazole (1 g/day, 3 days on, 4 days off for a month) developed severe liver injury (elevated AST, ALT, bilirubin), which resolved upon discontinuation of the drug.⁸ This patient was also on pembrolizumab, an immunotherapy agent also known to cause liver issues, making direct causation by fenbendazole alone complex but suggestive. Lifetime studies in rats indicated fenbendazole could cause morphologic changes like hepatocellular hypertrophy and hyperplasia, though no carcinogenesis.⁹ The risk of liver damage in dogs with long-term use is a noted concern.³⁷
  • Bone Marrow Suppression (Hypoplasia/Pancytopenia): The U.S. Food and Drug Administration (FDA) has issued a letter to veterinarians highlighting reports of bone marrow hypoplasia and pancytopenia in dogs treated with fenbendazole for longer than the labeled 3-day duration (extra-label use ranging from 5 to 14 days for various parasitic conditions).⁹⁰ As of October 2023, the FDA had received 12 such reports in dogs, with some cases confirmed by bone marrow biopsy.⁹⁰ Myelosuppression has also been reported in other species and in the literature.⁷⁹ Febantel, a drug metabolized to fenbendazole, has also been linked to suspected bone marrow toxicity in a dog, presenting with pancytopenia.⁷⁹ These reports underscore that prolonged administration, as would likely be required for cancer treatment, carries a distinct risk of severe hematological side effects.

• Bioavailability Challenges: Fenbendazole has poor water solubility, which significantly hinders its oral absorption and ability to reach therapeutic concentrations in systemic circulation and, consequently, in tumors.⁹ Reports suggest only 10-50% absorption from the intestine.³⁷ This poor bioavailability is a major hurdle for its potential efficacy as a systemic anticancer agent and may necessitate research into improved formulations or co-administration strategies to enhance absorption.

• Drug Interactions: For standard short-term deworming, no significant drug interactions with fenbendazole are commonly reported.³⁷ However, the potential for interactions during long-term administration, especially in conjunction with other chemotherapeutics or supplements used in cancer protocols, is largely unknown and warrants caution.³⁷

The distinct difference in reported toxicity between short-term antiparasitic use and the emerging concerns with prolonged, off-label use for cancer is critical. The established safety for deworming cannot be assumed for chronic cancer therapy regimens. Future veterinary research must prioritize investigating these chronic toxicities, and pet owners considering such off-label use must be fully informed of these potential risks by their veterinarian.

IV. Combined Use and Adjunctive Therapies

The interest in ivermectin and fenbendazole extends beyond their potential as standalone agents to their use in combination with each other, with various supplements, or as adjuncts to conventional allopathic cancer treatments like chemotherapy and radiation.

A. Potential for Ivermectin and Fenbendazole Combination

Anecdotal reports and online discussions suggest that some pet owners and holistic practitioners are exploring the combined use of ivermectin and fenbendazole for treating cancer in pets, often alongside other supplements.⁴² One anecdotal account describes a 10-year-old German Shepherd with metastatic breast cancer receiving fenbendazole 5 days a week, ivermectin once a week, and an artemisia combination, with reported good results including a 60% reduction in lymph node size and increased energy levels.⁴²

From a pharmacological perspective, ivermectin and fenbendazole have different primary mechanisms of anticancer action. Ivermectin affects multiple signaling pathways, ion channels, and immune responses, while fenbendazole’s main action is potent microtubule disruption.¹ Theoretically, combining drugs with different mechanisms could lead to synergistic anticancer effects or help overcome resistance. However, there is a lack of formal scientific studies in veterinary species specifically evaluating this combination for cancer efficacy or safety. One blog post mentions a peer-reviewed study (context appears human-focused but mentions pets) highlighting a protocol combining ivermectin, fenbendazole, and natural compounds (Vitamin E, curcumin, CBD) showing potential in supporting cancer care through immune modulation, cellular repair, and limiting cancer cell proliferation.⁴⁵

B. The “Joe Tippens Protocol” Context: Fenbendazole with Curcumin, Vitamin E, CBD Oil

The “Joe Tippens protocol” has significantly influenced the off-label use of fenbendazole for cancer, including in pets. This protocol, based on Mr. Tippens’ anecdotal remission from human lung cancer, combines fenbendazole with several supplements: curcumin, Vitamin E, and CBD oil.¹ Pet owners and some holistic practitioners have adapted this protocol for dogs and cats with cancer.³⁵

• Fenbendazole Dosage (in Tippens-like protocols for pets): As previously discussed, adaptations often involve around $50\text{ mg/kg}$ daily for 3 days on, 4 days off, or lower daily doses.⁴⁰
• Curcumin:
  • Rationale: Curcumin, the active compound in turmeric, has demonstrated anti-inflammatory and in vitro anticancer effects, including against bladder cancer cells in rats and potentially potentiating chemotherapy.³⁵
  • Dosage for Dogs: Generally suggested at $15-20\text{ mg}$ per pound of body weight per day. For “Golden Paste,” starting doses are roughly $1/4$ tsp for small dogs, $1/2$ tsp for medium, $3/4$ tsp for large, and $1$ tsp for giant dogs, potentially increasing up to $1$ Tbsp for larger dogs, given in smaller amounts multiple times a day due to rapid clearance.³⁵
  • Bioavailability: Curcumin absorption is poor but can be enhanced by administration with fatty foods (e.g., coconut oil, olive oil) or black pepper (piperine), as in Golden Paste recipes.³⁵
• Vitamin E:
  • Rationale: An antioxidant that may protect cells from damage.²⁹
  • Dosage: The original Joe Tippens human protocol reportedly used $800\text{ mg}$ of Vitamin E daily.⁴⁴ The Gao et al. mouse study used a diet supplemented with $126\text{ mg/kg}$ of Vitamin E (compared to $101\text{ mg/kg}$ in the regular diet).²⁹ Specific, widely accepted pet dosages for cancer within this protocol are not clearly defined in the provided materials.
• CBD Oil:
  • Rationale: Potential benefits include aiding chemotherapy’s tumor-shrinking action, reducing pain and inflammation, boosting the immune system, improving sleep, and increasing appetite while reducing nausea.³⁵
  • Dosage for Pets: Generally $0.1\text{ to }2\text{ mg/kg}$ twice daily, starting at the low end and gradually increasing.³⁵
  • Quality: Important to source high-quality, third-party tested, full or broad-spectrum CBD oil.³⁵

The use of multiple agents in protocols like Joe Tippens’ creates a “protocol soup” scenario. While combination therapy is a cornerstone of conventional oncology, the specific combinations in these anecdotal protocols lack controlled study. It becomes scientifically challenging to isolate the effects of ivermectin or fenbendazole from those of the concurrently administered supplements or even conventional treatments (as Joe Tippens was also enrolled in a Keytruda clinical trial ³²). Observed outcomes in anecdotal cases could be due to one of the supplements, a true synergistic interaction, the conventional treatment, spontaneous remission, or even misdiagnosis. This complexity underscores the need for systematic research to dissect these combinations or to test the primary antiparasitic agents in well-defined veterinary contexts.

C. Use as Adjuncts to Conventional Allopathic Treatments

A potentially more scientifically grounded and safer approach for these repurposed drugs in veterinary oncology may be their use as adjuncts to established conventional therapies, aiming to enhance efficacy or reduce toxicity.

• Ivermectin as an Adjunct:
  • Chemosensitization: Ivermectin has been shown to augment the efficacy of cisplatin in ovarian cancer models ⁶ and doxorubicin in osteosarcoma models.²⁵ Its P-glycoprotein inhibitory action could theoretically overcome chemoresistance to various drugs.⁵
  • Immunotherapy Enhancement: The City of Hope study on ivermectin combined with anti-PD1 antibodies in preclinical triple-negative breast cancer models suggests it can convert “cold” tumors into “hot,” immune-responsive tumors.¹³ This is a promising avenue given the rise of immunotherapy in cancer treatment.
  • Canine Transmissible Venereal Tumor (CTVT): In a case report, ivermectin ($0.3\text{ mg/kg}$ SQ weekly) combined with vincristine sulfate led to complete healing of CTVT in two dogs, with the authors suggesting ivermectin’s P-gp inhibition might have reduced vincristine-associated immunosuppression.⁵⁸
• Fenbendazole as an Adjunct:
  • Interaction with Chemotherapy/Radiation: The study on EMT6 mouse mammary tumors found that fenbendazole did not alter the dose-response curves for radiation or docetaxel, producing only additive cytotoxicities rather than synergistic enhancement.⁴
  • Anecdotal Human Use: There are anecdotal reports of humans using fenbendazole in combination with standard cancer therapies.¹¹
  • Shared Mechanisms: Fenbendazole’s primary mechanism of microtubule disruption is similar to that of vinca alkaloids (like vincristine) and taxanes (like paclitaxel), which are common chemotherapeutic agents.¹ This similarity suggests potential for synergistic effects if combined carefully, but also a risk of overlapping toxicities if not properly managed and studied.

The exploration of these antiparasitics as chemo- or immuno-sensitizers, or as agents that could allow for reduced dosages of more toxic conventional drugs, appears to be a more scientifically robust and potentially safer avenue for future veterinary research. This approach leverages their known (albeit antiparasitic) safety profiles at lower doses while aiming for enhanced therapeutic outcomes in combination with proven treatments.

V. Navigating Anecdotal Evidence and Scientific Rigor

The discussion surrounding ivermectin and fenbendazole for cancer in pets is heavily influenced by anecdotal reports, which, while compelling, must be approached with scientific caution.

A. The Power and Pitfalls of Anecdotes

Personal success stories, such as Joe Tippens’ widely publicized experience with fenbendazole for lung cancer ¹, and similar testimonials from pet owners shared on social media and online forums ³², are powerful narratives. They can inspire hope and are valuable for generating hypotheses for scientific investigation. However, anecdotes have significant limitations:

• Lack of Controls: They do not involve a comparison group, making it impossible to determine if the observed outcome was due to the treatment, other factors, or would have occurred anyway.
• Confounding Variables: Patients (human or animal) are often concurrently using multiple treatments (conventional and alternative), various supplements, and undergoing dietary changes. Attributing success to a single agent in such scenarios is difficult.⁸
• Misdiagnosis or Spontaneous Remission: While rare, misdiagnosis or spontaneous remission of cancer can occur, and these instances might be mistakenly attributed to an alternative therapy.
• Reporting Bias: Positive outcomes are more likely to be shared and publicized than negative outcomes or instances where the treatment had no effect or caused harm. This creates a skewed perception of efficacy.
• Lack of Standardization: Dosages, product quality, and adjunctive therapies used in anecdotal cases vary widely, making it impossible to replicate or systematically evaluate the approach.

The “social media effect” plays a significant role in the dissemination of information—and misinformation—about these treatments. Platforms like Facebook and TikTok, along with blogs, rapidly spread testimonials and protocols, heavily influencing pet owner decisions, often without adequate veterinary consultation or understanding of the underlying science and risks.¹ This can lead to owners self-administering these drugs to their pets, potentially using inappropriate dosages or formulations sourced online, which may be ineffective or even dangerous.¹ Veterinarians and animal naturopaths must be cognizant of this powerful influence and be prepared to engage in informed discussions with clients, providing balanced perspectives and cautioning against unverified online protocols. This also highlights an urgent need for accessible, scientifically accurate information for the public regarding such treatments.

B. The Importance of the Scientific Method in Veterinary Medicine

In contrast to anecdotal reports, the scientific method provides a structured approach to evaluating potential therapies. This involves:

• Preclinical Studies: In vitro experiments on cancer cell lines to understand mechanisms and identify potential efficacy, followed by in vivo studies in animal models (typically rodents) to assess effects in a living system, pharmacokinetics, and preliminary safety.
• Veterinary Clinical Trials: If preclinical data is promising, well-designed clinical trials in the target species (dogs or cats with naturally occurring cancer) are essential. These should ideally be randomized, controlled (e.g., against a placebo or standard-of-care treatment), and blinded to minimize bias. Such trials aim to definitively establish both safety and efficacy for a specific condition and dosage regimen.

Currently, for both ivermectin and fenbendazole, there is a significant gap between preclinical research (often in non-target species or artificial models) and robust veterinary clinical trial data for cancer treatment.

VI. The Path Forward: Enticing and Guiding Future Research in Veterinary Oncology

To move beyond anecdotal claims and harness any true potential of ivermectin and fenbendazole in veterinary cancer care, focused and rigorous research is paramount.

A. Identifying Key Research Gaps

Several critical knowledge gaps need to be addressed:

1. Lack of Veterinary Clinical Trials: The most significant gap is the absence of well-designed, prospective clinical trials evaluating ivermectin and fenbendazole for specific cancer types in dogs and cats.
2. Pharmacokinetics and Pharmacodynamics (PK/PD) in Target Species: Comprehensive PK/PD studies are needed in dogs and cats to establish safe and potentially effective anticancer dosing regimens. This includes understanding absorption, distribution, metabolism, excretion, and target engagement at various doses.
3. Mechanisms in Canine/Feline Cancers: While mechanisms are inferred from human cell line studies, direct investigation of these drugs’ effects on canine and feline cancer cells and their tumor microenvironments is crucial.
4. Bioavailability and Formulation: Particularly for fenbendazole, its poor water solubility and low bioavailability are major hurdles.⁹ Research into improved formulations or co-administration strategies to enhance absorption is needed.
5. Long-Term Safety: The safety of chronic administration at doses potentially required for anticancer effects needs thorough evaluation, focusing on potential liver, bone marrow, and neurological toxicities.
6. Combination Therapies: Controlled studies are needed to assess the safety and efficacy of these drugs when combined with each other, with supplements (like those in the Joe Tippens protocol), or with conventional chemotherapies and radiotherapies.

B. Suggesting Potential Avenues for Future Studies

Future research efforts could productively focus on:

1. Dose-Escalation Studies: Carefully designed dose-escalation studies in dogs and cats with naturally occurring cancers to determine maximum tolerated doses and identify dose-limiting toxicities for anticancer regimens.
2. Targeted Clinical Trials: Based on promising preclinical data (e.g., ivermectin for canine mammary tumors as per Diao et al. ⁶), conduct randomized, controlled clinical trials.
3. MDR1 Genotype Considerations: For ivermectin, studies in dogs must stratify by MDR1 genotype or investigate novel delivery systems (e.g., nanoparticle formulations) or P-gp inhibitor co-administration to safely achieve therapeutic concentrations in MDR1-mutant dogs without neurotoxicity.
4. Investigating Co-factors for Fenbendazole: Controlled studies are needed to determine if vitamins or other supplements (e.g., curcumin, Vitamin E) genuinely enhance fenbendazole’s bioavailability or efficacy, or possess independent/synergistic anticancer effects.
5. Adjunctive Therapy Research: Focus on using these drugs to sensitize tumors to existing chemotherapy or radiation, or to modulate the immune response in conjunction with immunotherapy, potentially allowing for lower doses of more toxic conventional agents.

Much of the current enthusiasm for these drugs in pets is driven by human anecdotal data (like the Joe Tippens story) or basic science findings being extrapolated to companion animals. A “reverse translation” approach is also warranted. This involves taking promising veterinary-specific anecdotal observations or preclinical findings (such as the Diao et al. study for ivermectin in canine mammary tumors ⁶) and rigorously testing them in controlled veterinary clinical settings. Systematically collected veterinary case reports, if detailed and comprehensive, could help identify specific cancer types, breeds, or co-administered agents that appear associated with positive responses, thereby guiding hypothesis generation for formal studies. This approach prioritizes the unique physiological and genetic contexts of dogs and cats.

C. The Importance of Standardized Protocols and Reporting

To advance the field, even in the absence of large-scale trials, improved data collection and reporting are essential. Veterinarians and animal naturopaths observing outcomes (positive or negative) with these agents should be encouraged to:

• Meticulously document cases: This includes confirmed diagnosis with histopathology, full staging information, precise dosage and formulation of the agent(s) used, details of all concurrent therapies (conventional and alternative), objective monitoring parameters (e.g., tumor measurements via imaging, bloodwork, quality of life scores), and clear outcome data.
• Consider publishing well-documented case reports or case series in peer-reviewed veterinary journals. While not as robust as clinical trials, such reports can contribute to the body of knowledge and highlight areas for further investigation.
• Advocate for and support collaborative efforts to pool data from individual cases, while always acknowledging the inherent limitations of anecdotal data.

VII. Concluding Remarks for the Animal Naturopath

The exploration of ivermectin and fenbendazole as potential anticancer agents in veterinary medicine is an area of active interest, fueled by a combination of preclinical scientific inquiry and compelling, though largely unverified, anecdotal reports.

A. Summary of Key Takeaways

• Ivermectin: Shows broad in vitro anticancer activity through multiple mechanisms, including P-gp inhibition, chloride channel modulation, mitochondrial dysfunction, and WNT pathway inhibition. A study on canine mammary tumor cells and xenografts is a notable piece of veterinary-specific research. However, its use is significantly complicated by the MDR1 gene mutation in many dog breeds, which can lead to severe neurotoxicity at doses that might be required for anticancer effects. Data on specific feline cancers is sparse.
• Fenbendazole: Primarily acts by disrupting microtubule function, similar to some conventional chemotherapeutics. It also appears to affect p53 activation and glucose metabolism in cancer cells. In vitro studies show broad activity. In vivo studies in mice have yielded mixed results, with some showing efficacy (especially when combined with vitamins in a lymphoma model, or alone in an NSCLC model), while others showed no benefit or even potential for increased tumor growth if not combined appropriately. Its poor bioavailability is a challenge. Long-term use, as would be needed for cancer, raises concerns about liver and bone marrow toxicity in dogs, distinct from its excellent safety profile in short-term deworming.
• Limited Scientific Evidence in Pets: For both drugs, robust, controlled veterinary clinical trial data demonstrating safety and efficacy for cancer treatment in dogs and cats is currently lacking. Much of the enthusiasm is based on preclinical data (often in non-target species) or human anecdotal reports.

B. Reinforcement of Caution and Collaboration

Given the current state of evidence, extreme caution is warranted. The off-label use of ivermectin and fenbendazole for cancer in pets carries potential risks, including severe toxicity (especially ivermectin in MDR1-mutant dogs, and fenbendazole with long-term use) and the possibility of owners forgoing or delaying proven conventional therapies.

Collaboration between naturopathic practitioners, pet owners, and conventional veterinary oncologists is crucial. An integrated approach, where all treatment modalities are discussed openly and patient safety is paramount, is essential. Veterinarians should be informed of any alternative therapies being considered or used.

C. Call to Action for Research

The user’s goal of stimulating research in these areas is commendable. This review highlights that while there is preclinical rationale and anecdotal suggestion of potential for these antiparasitic drugs in veterinary oncology, significant research is needed to:

• Establish safe and effective dosing regimens for anticancer purposes in dogs and cats, considering factors like MDR1 status for ivermectin and bioavailability for fenbendazole.
• Conduct well-designed veterinary clinical trials to evaluate efficacy against specific cancer types.
• Investigate the mechanisms of action directly in canine and feline cancer cells.
• Explore the true impact of combination therapies, including with supplements or conventional treatments, in a controlled manner.

The journey from antiparasitic to anticancer agent is complex. While the hope for new, affordable, and effective treatments is strong, this must be tempered with a commitment to rigorous scientific investigation to ensure that any potential benefits are realized safely and ethically for pets with cancer. Meticulous documentation of cases by practitioners and advocacy for veterinary-specific clinical trials are vital steps in this process.

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