From Farm to Pharmacy: Controversial Antiparasitics in Cancer Care

Interest in alternative medicine in cancer care is at an all-time high in the United States, despite the plethora of grounded clinical evidence supporting conventional cancer therapies. Patients often turn to these controversial therapies in response to misinformation, lack of trust in medicine, and loss of agency.

Controversial medicine refers to repurposed, non–evidence‑based therapies promoted for cancer, especially ivermectin and the benzimidazole anthelmintic agents fenbendazole and mebendazole. These drugs are antiparasitics used in humans and animals, but have become controversial due to their online promotion as anticancer treatments without solid clinical evidence. The health misinformation surrounding these touted therapies may include false, inaccurate, or misleading claims, based on the best available evidence at the time. Additionally, the benefits of unproved therapies are often overstated.

The controversy lies around mismatched data between laboratory and real‑world evidence and the misuse of these medications. The reality is that, especially for patients with cancer receiving multiple therapies, there is little understanding of how drugs such as ivermectin interact with these other treatments, and their potential benefit remains unfounded.

As social media continues to be a central source of cancer information—both accurate and inaccurate—it is crucial for pharmacists and health care providers to understand and effectively communicate the evidence, or lack thereof, behind controversial medicine.

Ivermectin

Ivermectin gained significant attention during the COVID-19 pandemic, and public interest in it continues. Now the use of ivermectin has expanded into an alternative to cancer treatment. In some cases, individuals are turning to ivermectin as a supplement despite a lack of studies demonstrating its effectiveness. Ivermectin works by paralyzing muscles in parasites, causing them to die; it does not affect mammals because it cannot enter the mammalian central nervous system, where the relevant channels are located.

Ivermectin is approved as an antiparasitic drug for the treatment of infections such as Strongyloides stercoralis (a roundworm) and onchocerciasis (river blindness) and, topically, for some forms of scabies and lice. These oral or topical formulations of ivermectin are often used short-term at a dose of approximately 150 to 200 µg/kg. For example, a 70‑kg adult would receive approximately 14 mg total as a single dose, which may be repeated.

Mechanism of Action

Ivermectin primarily targets invertebrate nerve and muscle cells by binding to glutamate-gated chloride channels. This interaction increases chloride ion influx, causing hyperpolarization of the cell membrane. The resulting inhibition of nerve transmission leads to paralysis and ultimately the death of the parasite.

These specific glutamate-gated chloride channels are found in invertebrates, not humans. At standard antiparasitic doses, ivermectin does not significantly cross the human blood-brain barrier because it is a P-glycoprotein (PGP) substrate, meaning PGP actively pumps ivermectin out of the central nervous system, keeping brain levels low.

Clinical Evidence

Studies on ivermectin use beyond parasites are primarily laboratory and animal based, making it difficult to gauge its safety in humans. Lab data suggest ivermectin may affect multiple oncogenic signaling pathways involved in tumor growth and metastasis. These findings also suggest ivermectin’s potential ability to modulate the immune system, increase T‑cell infiltration, and “turn cold tumors hot.”

However, these effects are mostly seen at much higher concentrations than those used for parasitic infections. In lab studies, ivermectin is used at a dose of 3 to 40 mg/kg, vs the 0.15 to 0.2 mg/kg used for human antiparasitic treatment. So the concentrations with anticancer effects in the lab are far above what is known to be safe in humans.

Some case reports and case series are investigating ivermectin treatment in patients with cancer. However, the outcomes are questionable. Regimens often included ivermectin alongside multiple other agents, such as dichloroacetate, omeprazole, and tamoxifen. Reported outcomes included symptom improvement, stabilization of pleural effusion, or decreased tumor markers.

However, these reports have major problems. Patients were frequently receiving concomitant standard cancer therapy, making it impossible to attribute any observed benefit to ivermectin specifically. Outcomes were subjective or nonstandard, with descriptions such as “all symptoms were relieved” and poorly defined follow-up durations. Safety reporting was absent or minimal, and the patient numbers were very small, often just 1 to 3 individuals per report.

The highest-level cancer evidence cited was from a phase 1/2 study presented at the 2025 American Society of Clinical Oncology Annual meeting. The trial enrolled patients with unresectable or metastatic triple-negative breast cancer who were heavily pretreated, with a median of 5 prior lines of therapy. Treatment consisted of a PD-1 inhibitor, balstilimab (Agenus Inc), combined with high-dose ivermectin (30, 45, or 60 mg) given daily in a cyclic schedule until progression or intolerance.

Among 9 patients, results showed 1 partial response, 1 with stable disease, and progression in the remaining 7. The median progression-free survival was approximately 2.5 months, and the 4-month clinical benefit rate was 37.5%. On the safety side, adverse events were mostly grade 1, including gastrointestinal issues, rash, and muscle weakness, with 1 case of grade 3 anemia.

“This is the highest level of evidence that we have for ivermectin as an adjuvant cancer treatment,” said Christy Harris, PharmD, BCOP, FHOPA, associate professor at Massachusetts College of Pharmacy and Health Sciences in Boston, and a clinical pharmacist at Dana-Farber Cancer Institute. “And I personally don’t find [these] data encouraging, but the authors did conclude that the clinical benefit rate was encouraging and warrants continued investigation.”

Safety of Ivermectin

At standard antiparasitic doses, ivermectin’s safety profile is relatively manageable, but that changes substantially at higher doses, with prolonged use, or when veterinary formulations are involved. The core concern is central nervous system (CNS) toxicity. At very high exposures, the P-glycoprotein (PGP) pump at the blood-brain barrier can be overwhelmed, allowing ivermectin to accumulate in the CNS at toxic levels. Symptoms range from drowsiness and confusion to ataxia, muscle weakness, and seizures, with severe cases progressing to coma or death.

Real-world data from an Oregon Poison Center series covering 2021 to 2022 illustrated how frequently this plays out in practice. Many cases involved people using veterinary formulations or taking high or repeated doses for COVID-19 infection or other off-label reasons, and a significant number required hospitalization with frequent neurotoxic effects. Even chronic regimens at human doses—such as 13 mg daily for 4 weeks—produced mild toxicity, but patients using veterinary products faced a much higher risk of severe CNS effects simply because the doses were far larger and less controlled.

A case discussed in the talk made these risks concrete: A patient with osteosarcoma took 12 mg of ivermectin 3 times over 5 days based on social media guidance, while also on regorafenib (Stivarga; Bayer HealthCare Pharmaceuticals Inc). They presented with anorexia, nausea, fatigue, near-syncope, drowsiness, and acute kidney injury, and were ultimately determined to have ivermectin-related neurotoxicity. Harris also noted potential drug interactions via PGP, CYP3A4, and protein-binding pathways when ivermectin is combined with cancer therapies, though the data on those interactions are limited.

Fenbendazole and Mebendazole

Fenbendazole and mebendazole are both benzimidazole antiparasitics, meaning they share the same core drug class and mechanism. The key difference is their approval status: Mebendazole is FDA approved for human use in certain intestinal worm infections, whereas fenbendazole is approved only for animals such as cattle, horses, dogs, cats, and exotic species. Because of this, if a patient is taking fenbendazole, the immediate practical question is where they obtained it, as there is no sanctioned human formulation.

In terms of safety profile, mebendazole has historically been considered relatively low-risk in humans largely because it is poorly absorbed at standard doses and most of the drug stays in the gut rather than reaching systemic circulation. Fenbendazole doesn’t have that same human safety track record simply because it was never developed or studied for human use in the first place.

Mechanism of Action

Fenbendazole and mebendazole share essentially the same mechanism of action. Both bind to β-tubulin in parasitic worms and inhibit microtubule polymerization, disrupting the structural scaffolding that cells depend on for division, transport, and integrity. The downstream consequences are also the same––glucose uptake is blocked, glycogen stores are depleted, and essential cellular processes break down, ultimately leading to immobilization and death of the parasite.

The primary distinction is the target tissue. Fenbendazole acts broadly on helminths throughout the body, while mebendazole works more locally, specifically on nematode intestinal cells, which is also why systemic absorption matters less with mebendazole at standard doses.

Clinical Evidence

For mebendazole, there are some human studies, but they are mostly early-phase with small patient numbers and inconsistent designs. Dosing is often based on convenience (100 mg 3 times daily, which reflects the available tablet strength), though some studies push higher to reach targeted blood levels.

Trials in glioma, colorectal, and gastric cancers have used doses up to 500 mg twice daily or as high as 4 g/day to hit a target serum concentration of approximately 300 ng/mL. Results across these trials have been mixed, with very small patient numbers and some showing hyperprogression rather than benefit.

For fenbendazole, the evidence base is even thinner. As of late 2025, when the talk was prepared, there were no human clinical trials at all, only case series and individual reports. Doses in these reports are often framed around animal formulations, typically in the range of 222 to 444 mg/day or 1 g three times a week. Toxicity is frequently described as absent or simply not reported, and although some reports mention promising signals, such as ctDNA changes or apparent disease stability, the numbers are small, regimens are inconsistent, and methods are not rigorous.

Safety

Mebendazole and fenbendazole share a similar toxicity profile. Both carry gastrointestinal effects, including abdominal pain, nausea, vomiting, and diarrhea, as well as hepatic concerns, including elevated aspartate aminotransferase and alanine aminotransferase levels, and a risk of drug-induced liver injury that grows with higher or prolonged dosing. Rare seizures have been reported with mebendazole, and bone marrow suppression is a class-level concern for benzimidazoles broadly, meaning fenbendazole carries that risk by association, even without robust human data to quantify it.

The key difference is that mebendazole at least has an established human safety profile from its approved uses, giving clinicians some framework for what to expect and monitor. Fenbendazole has no approved human dose, no standardized formulation, and no monitoring framework, with product quality varying widely across veterinary and supplement sources.

Both drugs carry greater risk in patients with preexisting liver disease, renal impairment, or advanced age, but fenbendazole’s risks in these populations are essentially uncharacterized, which is itself the problem when patients are self-administering it alongside active cancer treatment.

What Pharmacists Can Do

Pharmacists are uniquely positioned to address the growing use of controversial therapies in cancer care. When a patient brings up ivermectin, fenbendazole, or mebendazole, the conversation should start with curiosity rather than dismissal. From there, pharmacists can clearly communicate what the evidence actually shows and that the risks are real and potentially serious in patients already taking complex oncology regimens.

Pharmacists should also screen for these agents during medication reconciliation, as patients may not voluntarily disclose use of a veterinary antiparasitic or off-label supplement. Identifying use early on allows the care team to monitor appropriately, counsel proactively, and evaluate interaction potential with existing cancer therapies. A pharmacist who engages with empathy and provides honest, clear information without condescension can help keep patients connected to evidence-based care.

REFERENCE

Harris C, Feaster R. Controversial therapies in oncology: evidence, ethics and effective communications surrounding ivermectin, fenbendazole, and mebendazole. Presented at: 2026 Hematology/Oncology Pharmacy Association Annual Meeting; March 25-27, 2026; New Orleans, LA.