What is known about the safety and efficacy of fenbendazole for treating cancer in humans?

Fenbendazole and Cancer: Current Evidence

Fenbendazole is NOT approved by the FDA or EMA for human use, and there are NO clinical trials demonstrating safety or efficacy in humans with cancer. While preclinical studies show potential anticancer mechanisms, the absence of human data, combined with documented cases of severe liver injury, means this veterinary drug cannot be recommended for cancer treatment outside of properly designed clinical trials. 1, 2

Regulatory Status and Human Safety Concerns

• Fenbendazole is approved only for veterinary antiparasitic use, not for human cancer treatment by any regulatory authority. 1
• The first documented case of severe drug-induced liver injury (hepatocellular pattern) occurred in a 67-year-old woman who self-administered fenbendazole, with liver function tests normalizing only after 3 months of drug cessation. 2
• Pharmacokinetics and safety profiles in humans remain poorly documented in medical literature, as no formal human trials have been conducted. 1
• Social media has popularized fenbendazole's purported anticancer effects, leading to increasing unsupervised self-administration despite the lack of human evidence. 2

Preclinical Mechanisms (Laboratory Studies Only)

The following mechanisms have been observed only in cell culture and animal models, NOT in human cancer patients:

• Fenbendazole acts as a moderate microtubule destabilizing agent with moderate affinity for mammalian tubulin, causing mitotic arrest at micromolar concentrations. 3
• The drug inhibits glycolysis by blocking glucose uptake, down-regulating GLUT transporters, and inhibiting hexokinase II (HK II), a key glycolytic enzyme. 1, 3
• Fenbendazole induces oxidative stress and enhances apoptosis in cancer cells through multiple cellular pathways. 1
• Mitochondrial translocation of p53 occurs with fenbendazole treatment in experimental models. 3
• Proteasomal function impairment contributes to growth-inhibitory activity. 3

Critical Limitations of Preclinical Data

• In vivo mouse lymphoma studies showed NO anticancer effects, with rapid weight loss and tumor growth comparable to controls despite in vitro activity. 4
• Fenbendazole-treated tumors showed increased PD-L1 and CD86 expression and high percentages of immunosuppressive M2 macrophages, suggesting potential immune-related complications in the tumor microenvironment. 4
• The "starry sky pattern" observed in treated tumor tissues indicated actively proliferating cancer, contradicting in vitro results. 4
• Differential effects across cancer types: Triple-negative breast cancer cells (MDA-MB-231) showed greater vulnerability than luminal breast cancer cells (MCF-7), indicating unpredictable responses. 5

Standard-of-Care Cancer Treatments (What Should Be Used Instead)

For breast cancer risk reduction and treatment:

• Tamoxifen (20 mg daily for 5 years) reduces invasive ER-positive breast cancer risk in high-risk premenopausal and postmenopausal women. 6
• Raloxifene (60 mg daily for 5 years) reduces invasive ER-positive breast cancer risk in postmenopausal women. 6
• Anastrozole reduces breast cancer risk in postmenopausal high-risk women with proven efficacy (HR 0.42 for ER-positive tumors). 6

For metastatic melanoma:

• Dacarbazine remains standard chemotherapy when modern therapies are unavailable, with response rates of 10-20% and median survival of 6-9 months. 6, 7
• Immunotherapy (ipilimumab plus nivolumab, pembrolizumab) or targeted therapy (dabrafenib plus trametinib for BRAF V600 mutations) are first-line treatments. 7

For castration-resistant prostate cancer:

• Enzalutamide and apalutamide demonstrate proven metastasis-free survival benefits in non-metastatic CRPC. 6

Clinical Bottom Line

Patients inquiring about fenbendazole should be counseled about:

• The complete absence of human clinical trial data demonstrating safety or efficacy. 1
• Documented hepatotoxicity risk requiring months for recovery. 2
• Contradictory preclinical results showing in vitro activity but in vivo failure with potential immunosuppressive effects. 4
• Availability of FDA-approved, evidence-based cancer treatments with established safety profiles and proven survival benefits. 6, 7

The only appropriate use of fenbendazole in cancer would be within IRB-approved clinical trials designed to establish proper dosing, pharmacokinetics, safety profiles, and efficacy in humans. 1