Fenbendazole and Cancer: What the Science Really Shows (Evidence, Risks & Open Questions)

Abstract

Background: Fenbendazole (FBZ), a benzimidazole anthelmintic approved for veterinary use, has attracted attention for potential anticancer activity based on laboratory findings and widespread anecdotal reports. Objective: To critically appraise the current evidence on fenbendazole and cancer using a peer‑review style framework. Methods: Narrative review of preclinical studies, publicly reported human case narratives, and safety signals, with emphasis on evidence hierarchy and translational limitations. Results: Preclinical data demonstrate antiproliferative effects via microtubule disruption and metabolic stress in vitro and in animal models. Human evidence is limited to uncontrolled case reports; no Phase I–III clinical trials or regulatory approvals for oncology exist. Safety data in humans are sparse, with hepatotoxicity signals reported. Conclusions: Fenbendazole remains an experimental compound for cancer. Existing data are hypothesis‑generating and insufficient for clinical recommendation. Rigorous human trials are required.

Keywords: Cancer, Drug Repurposing, Fenbendazole, Antiparasitic Agents, Oncology, Drug Resistance, Adjunctive Therapy, Pharmacokinetics, Cancer Stem Cells, Affordable Cancer Treatments.

Introduction

Cancer remains a leading cause of morbidity and mortality worldwide, with an increasing incidence of aggressive and treatment-resistant tumors such as triple-negative breast cancer (TNBC), pancreatic adenocarcinoma, and glioblastoma. Despite significant advances in targeted therapies and immunotherapies, many patients continue to face limited effective options, highlighting an urgent need for novel, affordable, and accessible treatment strategies.

The high cost of oncology drugs-exceeding $150 billion globally in 2022-and the slow pace of new drug approvals further complicate timely patient access to effective therapies. In this context, drug repurposing-the strategy of identifying new therapeutic uses for existing drugs-has emerged as a promising approach to accelerate cancer treatment development while reducing costs and safety risks.

Among repurposed candidates, antiparasitic drugs such as fenbendazole, mebendazole, and ivermectin have attracted considerable attention due to their demonstrated anticancer activities across multiple preclinical models and emerging clinical case reports. These agents, originally developed to treat helminth infections, exert multifaceted effects on cancer cells, including disruption of microtubule dynamics, interference with metabolic pathways, and modulation of oncogenic signaling.

Fenbendazole, traditionally an anthelmintic drug used to treat parasitic infections in animals, has recently gained attention for its potential anticancer properties. While not yet approved for cancer treatment in humans, emerging preclinical studies reveal promising mechanisms by which fenbendazole may inhibit tumor growth and overcome drug resistance.

Fenbendazole has shown potent anticancer effects by destabilizing microtubules, inducing G2/M cell cycle arrest, and impairing glucose metabolism through inhibition of glucose transporters (GLUT1/4) and hexokinase activity. These actions lead to reduced glycolysis and lactate production, effectively starving cancer cells and overcoming drug resistance, particularly in 5-fluorouracil-resistant colorectal cancer models (Bai et al., 2009; Oral Fenbendazole for Cancer Therapy, 2024; Anti-cancer effects of fenbendazole on 5-fluorouracil-resistant cells, 2022). However, fenbendazole’s poor water solubility and limited oral bioavailability present challenges for achieving therapeutic systemic levels, necessitating formulation improvements and pharmacokinetic optimization.

Despite encouraging preclinical and anecdotal clinical evidence, fenbendazole remains largely experimental in oncology, with limited randomized controlled trials* and regulatory approval for cancer indications. Variability in dosing protocols, access issues, and concerns about off-label use underscore the need for rigorous clinical evaluation. Nonetheless, the low cost, oral administration, and multi-targeted anticancer properties position fenbendazole as an attractive candidate for adjunctive cancer therapy, especially in resource-limited settings.

This review aims to synthesize current knowledge on the anticancer mechanisms, clinical case reports, pharmacokinetics, and safety profiles of fenbendazole. We discuss the potential roles in overcoming drug resistance, improving patient outcomes, and informing future clinical trials that could integrate these repurposed agents into standard oncology practice.

*Note: The Randomised Controlled Trial (RCT) method for hard evidence is a very expensive and impractical model when it comes to something as complicated as cancer. Most drugs are designed to affect one part of cancer and not the other parts of cancer or even the root causes of cancer. To understand more of this concept, check out 'hallmarks of cancer'. The randomized placebo-controlled trial (RCT)* is widely regarded as the gold standard for generating high-quality evidence in medicine. However, when it comes to cancer, the RCT model is often prohibitively expensive, time-consuming, and sometimes impractical. See "Randomised controlled trials (RCTs), are often costly, slow, and logistically challenging - ChatGPT".

Promising Anti-Cancer Mechanisms of Fenbendazole

Research shows fenbendazole exerts anticancer effects primarily by disrupting cancer cell metabolism and division:

• Inhibition of Glycolysis and Glucose Uptake: Cancer cells rely heavily on glycolysis (the Warburg effect) for energy, even in oxygen-rich environments. Fenbendazole blocks glucose transporters such as GLUT1 and inhibits hexokinase II (HKII), key proteins in glucose metabolism. This starves cancer cells of energy, reducing lactate production that otherwise promotes tumor progression and drug resistance (1, 4).
• Activation of Tumor Suppressor p53: Fenbendazole enhances the activity of p53, a protein that regulates cell cycle arrest and apoptosis. This activation leads to mitochondrial injury and triggers programmed cancer cell death via caspase pathways (1, 4).
• Microtubule Destabilization and Cell Cycle Arrest: Fenbendazole disrupts microtubule polymerization, essential for cell division, causing arrest in the G2/M phase of the cell cycle. This prevents cancer cells from proliferating and induces apoptosis, as demonstrated in colorectal cancer cells and patient-derived tumor organoids (2, 3, 4).
• Multi-Modal Anticancer Effects: Beyond glycolysis inhibition and cell cycle arrest, fenbendazole induces necrosis, autophagy, and ferroptosis in cancer cells, attacking tumors through multiple pathways and potentially reducing the likelihood of resistance development (1, 4).

Evidence from Preclinical Studies

In Vitro and Animal Models: Numerous laboratory studies have demonstrated fenbendazole’s ability to inhibit tumor growth in various cancer types, including colorectal, cervical, pancreatic, and drug-resistant cancers. Oral fenbendazole reduced tumor size and grade in mouse models, supporting its potential efficacy (1, 2, 3, 6, 7).

Overcoming Drug Resistance: Fenbendazole showed effectiveness against cancer cells resistant to common chemotherapies like 5-fluorouracil (5-FU), paclitaxel, and docetaxel, making it a candidate for combination therapies or treatment of refractory cancers (1, 3).

Challenges and Considerations

Despite encouraging results, fenbendazole faces significant hurdles before clinical adoption:

Poor Water Solubility and Bioavailability: Fenbendazole’s low solubility limits its absorption and systemic availability, reducing therapeutic levels in tumors. Research is ongoing to improve drug formulations, such as nanoparticle delivery systems, to enhance efficacy (1, 3).

Lack of Clinical Trials: To date, there are no large-scale, peer-reviewed clinical trials validating fenbendazole’s safety and effectiveness as a cancer treatment in humans. Most evidence remains preclinical or anecdotal (1, 4, 8).

Safety Profile: Fenbendazole is generally considered safe in animals with minimal toxicity. However, its effects in humans, especially at doses required for anticancer activity, need thorough evaluation (1).

Public Interest and Anecdotal Reports

The story of Joe Tippens, who reportedly used fenbendazole alongside immunotherapy for lung cancer, has popularized fenbendazole in patient communities. While his case is inspiring, medical experts caution that his remission is more likely attributable to FDA-approved therapies rather than fenbendazole alone (4).

Human Evidence

Human data consist of uncontrolled case reports and anecdotal series, often concurrent with standard therapies and lifestyle changes. Reported outcomes include regression or stabilization; causality cannot be established.

Evidence Hierarchy Assessment

• Cell (in vitro) studies: Available

• Animal (in vivo) studies: Available

• Human case reports / anecdotes: Available

• Phase I safety trials: Not available

• Phase II efficacy trials: Not available

• Phase III randomized trials: Not available

• Regulatory approval for cancer treatment: None

Discussion

Fenbendazole is part of a larger group of drugs known as benzimidazole*, which are anthelmintic drugs (i.e., drugs that kill parasitic worms). Another benzimidazole is mebendazole, which can be prescribed to humans with certain gut infections, including threadworms, whipworms, hookworms, and roundworms.

*The class of drugs known as benzimidazoles includes fenbendazole, mebendazole, albendazole and flubendazole.

A compilation of more than 400 case reports was published on OneDayMD.com, detailing individual experiences with ivermectin and fenbendazole or mebendazole as a cancer treatment, sourced from social media, patient testimonials, and clinical communications between 2023-2025. These reports were categorized by cancer type, and outcomes were assessed based on self-reported measures such as tumor regression, remission status, and overall survival.

The anecdotal reports included in this compilation cover a variety of cancer types and describe self-reported outcomes following the use of ivermectin and fenbendazole or mebendazole. These findings, while compelling, must be interpreted cautiously due to the inherent limitations of the data sources.

The anecdotal nature of these reports precludes definitive conclusions about the efficacy of ivermectin and fenbendazole as a definitive cancer treatment. However, the consistency of positive outcomes across diverse cancer types suggests a potential biological effect that merits further investigation.

The pattern of case reports also suggests that ivermectin and fenbendazole may exhibit broad-spectrum anticancer properties.It is imperative that patients consult healthcare professionals before considering ivermectin and fenbendazole as a treatment option. Unsupervised use may lead to unforeseen drug interactions or adverse effects.

Preclinical evidence supports repurposing, particularly for resistant cancers and combinations. Clinical data limited; anecdotal fenbendazole use raises misinformation concerns. 

The convergence of biological plausibility, preclinical activity, and anecdotal human interest warrants scientific curiosity but not clinical endorsement. Structural barriers—including lack of patent incentives and funding—have limited progression to clinical trials. 

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Limitations

This review is limited by reliance on heterogeneous preclinical studies and uncontrolled human narratives. Publication and reporting biases are likely. Absence of clinical trials precludes efficacy confirmation.

Conclusion and Future Directions

Fenbendazole represents a compelling example of drug repurposing in oncology, with multiple studies highlighting its ability to target cancer metabolism, induce apoptosis, and overcome drug resistance. However, the transition from laboratory findings to clinical practice requires:

• Rigorous clinical trials to establish safety, dosing, and efficacy in humans.
• Development of improved formulations to enhance bioavailability.
• Exploration of combination therapies to maximize anticancer effects while minimizing toxicity.
• Future: Trials in resistant/metastatic settings.

Given its affordability, oral administration, and multi-targeted mechanisms, fenbendazole could become a valuable adjunct in cancer treatment pending further research. Until then, patients should consult healthcare professionals and avoid self-medicating with fenbendazole outside clinical settings.

References:

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