Fenbendazole Case Reports: Clinical Evidence Review

This post reviews published case reports documenting clinical observations associated with fenbendazole use in human cancer patients. It covers the Stanford University case series by Chiang et al. (2021), and the subsequently retracted case series by Makis, Baghli, and Martinez (2025). Each set of cases is presented with its appropriate evidentiary context.

Overview

Case reports occupy a defined position in the hierarchy of clinical evidence. They cannot establish causality, cannot control for confounding, and cannot be generalized to populations. Their value lies in hypothesis generation — identifying signals that may warrant formal investigation — and in documenting unusual observations that accumulate over time into a pattern. The case reports reviewed here describe patients with advanced, often treatment-refractory cancers who self-initiated fenbendazole as part of their regimen and experienced outcomes that their treating physicians considered noteworthy enough to document.

Two published case series are reviewed. The first, by Chiang et al. (2021), was published in Clinical Oncology Case Reports and emerged from the Department of Medicine at Stanford University Medical Center and the University of Washington. It documents three patients with genitourinary malignancies. This paper has not been retracted and represents peer-reviewed documentation from a major academic medical center.

The second, by Makis, Baghli, and Martinez (2025), was published in Case Reports in Oncology (Karger) and documented three patients with advanced breast cancer, prostate cancer, and melanoma. This paper was retracted in January 2026 due to an undisclosed conflict of interest by the first author. The clinical details it describes are summarized here with appropriate caveats. In all cases discussed, concurrent standard-of-care therapies were present and likely contributed to outcomes; the contribution of fenbendazole specifically cannot be isolated.

Stanford Case Series — Chiang et al. 2021

Chiang et al. documented three patients at Stanford University Medical Center and the University of Washington who self-administered fenbendazole-containing regimens and experienced favorable outcomes relative to their oncologic history. All three had genitourinary malignancies. The authors explicitly acknowledged that confounding by concurrent therapies could not be excluded and called for formal clinical trials to evaluate FBZ in cancer.

Case 1: Metastatic Renal Cell Carcinoma (mRCC)

A 63-year-old male with metastatic clear cell renal cell carcinoma was treated sequentially with pazopanib, then cabozantinib, both of which failed to control disease. He received only three doses of nivolumab (240 mg per dose), which was discontinued due to immune-related adverse events. At this point — with multi-line treatment failure — the patient self-initiated fenbendazole 1 g orally three times per week without additional prescribed systemic therapy.

Interval magnetic resonance imaging demonstrated near-complete resolution of the left renal mass and decreased signal in pancreatic and bone lesions. The patient maintained this response for approximately 10 months on fenbendazole monotherapy alone, without resuming any systemic oncologic treatment. The authors noted that nivolumab, even at limited doses, can produce durable responses in mRCC via immune memory mechanisms; its potential residual contribution to the observed response cannot be excluded.

Case 2: Metastatic Urothelial Carcinoma — Progressive Aortocaval Node

A 72-year-old male with metastatic urothelial carcinoma of the urethra had achieved a near-complete response to first-line gemcitabine and cisplatin chemotherapy. Surveillance imaging subsequently identified progressive growth of an aortocaval lymph node measuring 2.0 cm × 1.5 cm. The patient declined further chemotherapy and self-initiated a supplement-based regimen: fenbendazole 1 g orally three times per week, vitamin E 800 mg/day, curcumin 600 mg/day, and CBD oil.

Serial imaging over nine months showed progressive decrease in the aortocaval node from 2.0 × 1.5 cm to 0.5 × 0.5 cm. At nine months this represented a complete radiographic response. The fenbendazole plus vitamin E combination used in this case is the same pairing hypothesized in the Gao et al. 2008 JAALAS study (discussed in detail in Post 3 of this series). No concurrent cytotoxic or immunologic treatment was administered during the self-medication period.

Case 3: T4 Bladder Urothelial Carcinoma (85% Squamous)

A 63-year-old female presented with T4 bladder urothelial carcinoma with squamous histology. She was treated with concurrent AMVAC chemotherapy (methotrexate, vinblastine, doxorubicin, and cisplatin) for six cycles while simultaneously self-administering fenbendazole 1 g orally three times per week. Follow-up computed tomography demonstrated no evidence of disease after completing treatment.

This case differs from the others in the series in that fenbendazole was administered concurrently with standard multiagent chemotherapy rather than as a substitute. Complete responses are possible in T4 bladder cancer with AMVAC; the contribution of fenbendazole to this outcome is uncertain. The case documents tolerability of the combination rather than establishing efficacy attributable to FBZ.

Makis, Baghli, and Martinez 2025 — Retracted Case Series

Makis et al. published a case series in Case Reports in Oncology (Karger) documenting three patients who self-administered fenbendazole as part of their cancer management. The paper was retracted on January 21, 2026, on the grounds that the first author, William Makis, had been offering paid consultation services related to fenbendazole-based cancer protocols through The Wellness Company at the time of manuscript submission — an undeclared conflict of interest. The editor concluded that this conflict “would have affected the interpretation of the work and recommendation.” The retraction notice stated that the author’s explanation was found unsatisfactory.

The retraction is based on conflict-of-interest grounds, not on identified data fabrication or scientific misconduct in the case descriptions themselves. The clinical events described may have occurred. However, the undisclosed commercial interest creates legitimate uncertainty about whether case selection, dose framing, or outcome reporting was influenced by promotional motivations. These cases should be treated as unverified clinical observations rather than peer-reviewed documentation.

Retracted Case 1: Stage IV Metastatic Breast Cancer (ER+ HER2-)

The patient was described as a woman with stage IV metastatic ER-positive, HER2-negative breast cancer with confirmed bone metastases to the spine. Her standard-of-care regimen included fulvestrant injections (a pure estrogen receptor antagonist/degrader) and targeted radiation to two spinal metastases in January 2022. She added fenbendazole 222 mg/day continuously to her regimen, along with vitamin D3 5,000 IU/day and a standard multivitamin.

The tumor marker CA 27.29 decreased from 316 to 36.6 (within normal range) over approximately eight months. A PET scan in April 2022 confirmed complete metabolic remission. The patient remained in remission for approximately three years at the time of the 2025 publication. A transient elevation of ALT and AST was noted in July 2022 and resolved spontaneously without FBZ dose adjustment; fulvestrant was considered a possible contributor. The authors interpreted the outcome as consistent with FBZ contribution, but fulvestrant alone can produce dramatic and durable responses in ER+ metastatic disease, and the concurrent spinal radiation may have reduced systemic tumor burden.

Retracted Case 2: Stage IV Metastatic Prostate Cancer (Bone and Lymph Node Metastases)

A male patient with stage IV metastatic prostate cancer involving bone and lymph node metastases was described. His standard-of-care regimen included androgen deprivation therapy (ADT) consisting of relugolix (Orgovix, a GnRH receptor antagonist) plus apalutamide (Erleada, an androgen receptor inhibitor), and denosumab (Xgeva) for bone health. Self-administered additions to this regimen included fenbendazole 222–444 mg/day (generally daily, with occasional dose reductions), vitamin D3 5,000–10,000 IU/day plus K2 and magnesium, melatonin 10–40 mg/day, berberine, curcumin, artemisinin, and cimetidine.

At one year, bone lesion regression and lymph node resolution were documented. At two years, significant further bone lesion regression was noted. PSA became undetectable (below 0.05 ng/mL) and remained undetectable for more than two years. Near-complete response was documented on PSMA-PET/CT at 26 months. The ADT regimen (relugolix plus apalutamide) represents potent standard-of-care therapy; many patients achieve sustained PSA suppression on this combination alone. The contribution of fenbendazole or any of the supplements to this outcome cannot be separated from the ADT effect.

Retracted Case 3: Stage IV Melanoma (Recurrent, BRAF V600 Mutation)

The third patient was described as having recurrent stage IV melanoma with a confirmed BRAF V600 mutation. The patient self-initiated fenbendazole 222–444 mg/day in December 2023 during a treatment-free window — a period when neither immunotherapy nor chemotherapy was being administered. Surgery was performed for ureteral obstruction. Circulating tumor DNA (ctDNA) assessment showed evidence of initial progression followed by apparent remission. Complete remission was documented at 11 months.

The authors cited Pombinho et al.’s finding that benzimidazoles activate p53 and suppress Mdm2 and MdmX in melanoma cell lines as a mechanistic rationale for FBZ activity in BRAF-mutant melanoma. This preclinical observation is plausible, but spontaneous remission in advanced melanoma, while rare, has been documented. Without a controlled comparison, causal attribution to fenbendazole is not possible. This case is nonetheless notable if verified, as it describes a treatment-free period where no standard therapy was being administered when the apparent remission occurred.

Comparison of All Reported Cases

⚠ = Data from retracted paper; not peer-verified; exercise appropriate caution when interpreting.

Mechanism of Action

The mechanisms proposed to explain the clinical observations span several well-studied cellular pathways. Fenbendazole binds to beta-tubulin at the colchicine binding site with moderate affinity, disrupting mitotic spindle formation, inducing G2/M cell cycle arrest, and ultimately triggering apoptosis. This mechanism, documented by Kumar et al. (2018) in multiple cancer cell lines, is analogous to established cancer drugs such as vinca alkaloids and taxanes, though FBZ binds with lower affinity and may be better tolerated.

At the level of protein homeostasis, fenbendazole impairs the ubiquitin-proteasome degradation pathway, causing accumulation of p53, cyclin B1, and IkB-alpha. This results in endoplasmic reticulum stress, ROS generation, cytochrome c release, and mitochondrially-mediated apoptosis. Pombinho et al. (2019) showed that benzimidazoles including FBZ specifically downregulate Mdm2 and MdmX — the principal negative regulators of p53 — restoring p53 activity in cell lines where it remains structurally wild-type, including melanoma and breast cancer cell lines.

Fenbendazole also disrupts cancer cell glucose metabolism by downregulating GLUT1 and GLUT4 transporters and hexokinase II, reducing glucose uptake in cancer cells with high glycolytic dependency. A potential immune-relevant mechanism was identified by Jung et al. (2023): FBZ upregulates PD-L1 expression on tumor cells, which may increase sensitivity to concurrent PD-1/PD-L1 checkpoint inhibitors. This is potentially relevant to Case 1 in the Stanford series, where the patient had received nivolumab.

In 5-fluorouracil-resistant colorectal cancer cells, Park et al. (2022) demonstrated that FBZ activates ferroptosis — a form of iron-dependent oxidative cell death — augmenting apoptosis through mechanisms partially independent of p53. This suggests FBZ may retain activity in chemotherapy-resistant cells through alternative death pathways.

Dosing Details

A notable difference exists between the doses used in the two case series. Chiang et al. (2021) patients used fenbendazole 1 g orally three times per week (approximately 428 mg/day averaged over a week). Makis et al. (2025) patients used 222–444 mg/day continuously, corresponding to one or two standard Panacur veterinary sachets daily. Neither dose has been studied in formal pharmacokinetic trials in humans. The 222 mg dose derives from the standard single-dose sachet used in veterinary practice; human oral bioavailability at this dose is unknown.

Across all six cases, fenbendazole appeared clinically tolerable. The only documented biochemical adverse event was a transient ALT/AST elevation in Makis Case 1, which resolved without FBZ discontinuation. An independent case report by Shimizu et al. (2021) documented hepatotoxicity in an NSCLC patient who self-administered FBZ while on pembrolizumab, with liver injury resolving on FBZ cessation. Clinically significant hepatotoxicity appears uncommon but has been observed, and liver function monitoring is advisable.

Retraction Analysis and Scientific Integrity

Understanding the nature and scope of the Makis et al. retraction is important for accurately interpreting the evidentiary landscape. The retraction was issued on grounds of undisclosed conflict of interest, not data fabrication, protocol violations, or falsification of clinical records. The distinction is meaningful: it implies that the clinical events described may have occurred, but that the framing, interpretation, and case selection may have been influenced by the first author’s financial interest in promoting fenbendazole-based protocols.

The retraction statement, as issued by Karger, explicitly stated that the undisclosed conflict “would have affected the interpretation of the work and recommendation.” This language implies concern about the commercial context shaping how the authors presented and contextualised their findings — not necessarily that the patients or outcomes were fabricated. Nonetheless, without independent verification, these cases cannot be relied upon as objective scientific documentation. Readers and clinicians should weight them accordingly: as potentially real but unverified clinical observations, not as evidence for efficacy.

The broader scientific integrity concern is that self-administered regimens documented by practitioners with financial interests in those regimens are inherently subject to selection bias and interpretation bias. Cases with unfavorable outcomes are less likely to be reported. Even assuming the three Makis cases occurred as described, they represent a convenience sample from an unknown total population of patients who self-administered FBZ, many of whom may have had neutral or negative outcomes.

What the Evidence Does and Does Not Show

Taken together, the published case reports suggest that tumor regression temporally associated with fenbendazole self-administration has been observed in multiple cancer types across at least two independent reporting groups (Stanford 2021 and, before retraction, Makis 2025). The preclinical mechanistic literature provides plausible biological rationale. These observations are sufficient to justify formal clinical investigation.

However, the case reports do not establish that fenbendazole caused the observed responses. Every case involved concurrent medications or therapies (nivolumab, fulvestrant, ADT, chemotherapy) that could independently explain partial or complete responses. Case 2 in the Stanford series is the strongest argument for a possible FBZ contribution, given that disease was progressing on surveillance (without active cytotoxic therapy) and then regressed during the FBZ-plus-supplement period. Even here, curcumin, vitamin E, and CBD were also present.

A 2024 review identified FBZ’s pharmacokinetics and safety profile as sufficient to justify proceeding to Phase I clinical trials in humans. Son et al. (2020) reviewed benzimidazole anthelmintics broadly as drug-repurposing candidates, and Sharma et al. (2024) reviewed their specific application in hematological malignancies. No Phase I, II, or III trials have been completed as of March 2026. The Chiang et al. paper explicitly recommended formal clinical trials as the appropriate next step, a call that remains unmet.

Important Considerations

Confounding by concurrent therapies. In every case reviewed, standard-of-care treatment was present either concurrently or as a recent prior treatment. In the Stanford RCC case, nivolumab is known to produce durable responses in mRCC even at limited doses via immune memory. In the Makis breast cancer case, fulvestrant is highly active in ER+ disease and the spinal radiation directly targeted metastatic lesions. In the Makis prostate case, relugolix plus apalutamide is a potent ADT doublet. The claim that fenbendazole contributed independently to any of these outcomes is not supported by these observations alone.

Hepatotoxicity risk. Shimizu et al. (2021) documented drug-induced liver injury in an NSCLC patient on pembrolizumab who self-administered FBZ. Liver function tests should be assessed at baseline and monitored periodically in any individual using FBZ. The risk may be elevated in patients concurrently receiving checkpoint inhibitors or other hepatically metabolized agents.

Regulatory status. Fenbendazole is a veterinary antiparasitic and is not approved for human use in any jurisdiction. Its use in cancer patients constitutes off-label, investigational self-administration without established dosing, safety, or efficacy data. Physicians are not generally able to prescribe it for oncologic indications.