Fenbendazole and Cancer: A Closer Look at Its Use and Risks

Co-authored with Mark Taylor of Patient Led Oncology

CancerChoices reviews and synthesizes the evidence behind complementary therapies including repurposed drugs. Given the significant interest in our review of mebendazole and fenbendazole, and our accompanying article on repurposed drugs, we’ve partnered with Mark Taylor of Patient Led Oncology to provide a deeper analysis of fenbendazole.  

Fenbendazole, originally developed as a deworming medication for animals, has gained traction as a popular repurposed drug for cancer treatment. Its rise in popularity is driven by testimonials shared in Facebook groups with over 100,000 members, as well as its low cost and ease of access. While laboratory studies demonstrate fenbendazole’s anticancer effects in petri dishes and cancer sensitivity assays, its real-world impact on patient outcomes remains unclear and can potentially be harmful in certain contexts.

The popularity of fenbendazole and its connection to mebendazole

Fenbendazole’s rise in popularity can be attributed to two key factors: its mention in Jane McLelland’s book How to Starve Cancer, which highlighted the anticancer potential of related drug mebendazole, and the widely shared story of Joe Tippens. Tippens claimed fenbendazole helped him achieve remission from stage 4 small cell lung cancer when combined with curcumin and CBD-enriched cannabis oil.

However, Tippens’ narrative often omits a critical detail: he was also enrolled in a clinical trial for Keytruda, a PD-L1 checkpoint inhibitor immunotherapy known to produce complete responses in some patients. His outcome may have been driven by genetic factors such as microsatellite instability or a high mutational burden, which are strongly associated with favorable responses to immunotherapy. For instance, a study published in the New England Journal of Medicine reported a 100% complete response rate in similar cases treated with checkpoint inhibitors.

Fenbendazole belongs to the benzimidazole class of drugs, which includes mebendazole, another dewormer being explored for its anticancer properties. Unlike fenbendazole, mebendazole has undergone limited clinical trials and has shown promise in certain cancers, particularly glioblastoma. Research suggests that mebendazole disrupts cancer cell metabolism and interferes with microtubule formation which is essential for cell division.

While mebendazole has more documented evidence in human cancer care, its data remains preliminary, and further studies are needed to confirm its potential. Fenbendazole, in contrast, lacks clinical trials to support its use in humans.

Outcomes from animal studies: a mixed picture

Animal studies on fenbendazole present a complex and sometimes contradictory story. One study on mice found that cancers grew faster in animals treated with fenbendazole compared to untreated controls—nearly doubling the growth rate. 

Interestingly, in the same study, when combined with a mix of vitamins, fenbendazole slowed cancer growth compared to controls. However, the vitamin regimen was extensive and would be difficult to replicate in humans. Vitamins alone also showed anticancer effects, but the combination appeared to be more effective. These findings underscore the need for further research and a nuanced understanding of how fenbendazole interacts with other treatments.

Case reports and clinical observations: proceed with caution

In clinical practice, there are reports of fenbendazole causing spikes in inflammation, which can lead to cancer recurrence and more aggressive tumors.   

The potential for fenbendazole to lead to faster growth of cancers is particularly concerning for individuals with early-stage cancers or potentially curable diseases. One case report describes severe liver injury observed in a woman with small-cell lung cancer taking fenbendazole. When she stopped taking the drug, her liver injury resolved. Numerous clinicians CancerChocies has interviewed noted similar stories. The dosages in Facebook groups range from 200 mg to as much as 2,000 mg. 

Pharmacology researchers have detailed case reports and issues with the drug’s solubility, which can impact its absorption. Orally administered fenbendazole may not reach sufficient systemic levels to impact tumors.¹Nguyen J et al. Oral Fenbendazole for Cancer Therapy in Humans and Animals. Anticancer Research, 2024 Sep;44(9):3725-3735. Studies investigating ways to make the drug orally soluble are needed for fenbendazole to be an effective cancer drug for humans. 

While fenbendazole may have a role for advanced-stage patients with limited options, its risks warrant caution. Without proper monitoring and a comprehensive approach, the outcomes could be counterproductive, including risks of cancer progression, adverse interactions when combined with other treatments, and liver damage. 

Despite the enthusiasm surrounding fenbendazole, it’s critical to review the evidence and to work with practitioners who can monitor you before proceeding with repurposed drugs without sufficient evidence. Studies on fenbendazole in animals show almost no benefit when used alone, reinforcing the idea that single-treatment approaches are insufficient. Mebendazole, with a more robust evidence base, could be a better alternative for some cancers, although its use also requires further research and should be a part of a comprehensive protocol. 

Evidence-based integrative oncology

Both CancerChoices and Patient Led Oncology work to empower people with cancer to critically evaluate treatments, moving beyond anecdotal claims to make informed, science-based decisions. We urge you to explore integrative oncology approaches that prioritize both safety and effectiveness. And to work with trained practitioners who can monitor your care. To explore the evidence to date behind fenbendazole and other complementary therapies in cancer care, see CancerChoices’ Supplements and Therapies Database. To find integrative practitioners who are experts in a whole-person approach to cancer care, see CancerChoices’ page on Finding Practitioners in Integrative Oncology. For expert insights on options for stage 4 cancers, reach out to Patient Led Oncology.   

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