Remedies › Fenbendazole

Fenbendazole (veterinary dewormer)

Also called fenben, Panacur, Safe-Guard, Hoe 881, methyl benzimidazole carbamate.

Fenbendazole is a benzimidazole dewormer sold for dogs, horses and livestock under names such as Panacur and Safe-Guard. It is not approved for human use in the US or the EU. It became a social-media cancer remedy after a 2016 personal story from a man with lung cancer who took it while in an immunotherapy trial, and a wave of self-treatment followed, notably among lung cancer patients in South Korea in 2019 and 2020.

What it has been studied for

Beyond Google: what a search won’t show you

Places a normal search rarely reaches: a retraction notice, a case series outside PubMed, the FDA's two adverse-event databases, trial registries and a patent office. Each item links to its primary record.

Retracted paper
The only PubMed case series of fenbendazole remissions was retracted
published May 2025, retracted January 2026
The 2025 Case Reports in Oncology series (three advanced cancers, two complete remissions) is still widely shared. The publisher and editor retracted it after concerns that the first author offered services related to the study's topic when it was submitted, which was not declared; the editor found the author's responses unsatisfactory.
Case series outside PubMed
Stanford and University of Washington case series: three genitourinary cancers
February 2021
Published in Clinical Oncology: Case Reports, a journal PubMed does not index. Three patients had complete responses while taking 1 g three times a week: one had also just received nivolumab, one received chemotherapy at the same time, and one took fenbendazole with vitamin E, curcumin and CBD after earlier chemotherapy and immunotherapy. Three cases with other treatment cannot show cause.
FDA adverse-event reports (drug database)
99 reports in the FDA's human drug database name fenbendazole
2017 to 2026, 43 of them received in 2025
Fenbendazole is a veterinary drug, so reports land in the drug database (FAERS), not the supplement one. Of 99 reports listing it: 54 serious, 22 with a hospital stay, 4 flagged as deaths (fenbendazole was a suspect drug in 2, one a fatal acute liver failure alongside ivermectin and a leukemia drug), 26 name it as the main suspect, 54 also list ivermectin, and 3 list fenbendazole products alone. Liver injury appears in 8 reports (two pairs look like duplicates); 6 reports of a dangerous heart rhythm appear to describe one man also taking hydroxychloroquine and ivermectin. In 6 reports the reporter coded the drug as effective for an unapproved use, all with ivermectin too. Reports do not prove cause.
FDA adverse-event reports (supplement database)
2 reports in the FDA's food and supplement database (CAERS)
2025 and 2026
One names a fenbendazole product alone (a 68-year-old with dizziness and urinary and E. coli infections, marked serious); the other a gut-flush product with ivermectin and fenbendazole (dizziness and headache, emergency-room visit). Neither records a hospital stay.
No registered trial
ClinicalTrials.gov has no trial testing fenbendazole in people
searched 2026-10-28
The registry search returns 9 studies for the word, none testing fenbendazole: five are trials of oxfendazole, a sister drug that the body turns partly into fenbendazole, given to people for worm infections (one NIH-sponsored safety study has posted results); the rest match on other words.
Same-class trial stopped
Swedish mebendazole cancer trial stopped for lack of effect
2018 to 2019
Repos Pharma tested mebendazole, the human drug closest to fenbendazole, in gastrointestinal cancers and cancers of unknown origin. It enrolled 11 people and was terminated for lack of effect; no results were posted. A trial of a related drug, not of fenbendazole.
Patent
WO2008036747: anthelmintic benzimidazoles for immune modulation, including neoplasia
2008
Filed by J. M. Cummins. Claims that benzimidazole dewormers, fenbendazole among them, can modulate immunity in animals with infections, autoimmune or inflammatory disease, or neoplasia, and can boost vaccines. A patent is a legal claim, not proof.
Research funding
No NIH grant names fenbendazole for cancer
searched 2026-10-28
NIH RePORTER lists 7 projects matching fenbendazole. The cancer ones fund mebendazole for ovarian cancer at the University of Oklahoma (2022 to 2024, about $160,000 to $181,000 a year). Fenbendazole's own grants were a pheasant food-safety residue study (2011) and pinworm and autoimmunity research (1999 to 2001).

Sources searched on 2026-10-28: ClinicalTrials.gov, Europe PMC preprints, Europe PMC patents, FDA CAERS (openFDA), FDA FAERS (openFDA), NIH RePORTER, Publisher retraction notices. Where we look

Safety notes

Fenbendazole is not approved for human use by the FDA or the European Medicines Agency, and its pharmacokinetics and safety in people are not well documented. Source
Six published case reports describe drug-induced liver injury in people who took it, including one confirmed by liver biopsy as severe; in five, liver tests returned to normal after stopping. Source
Liver injury from fenbendazole can look like the immune hepatitis caused by checkpoint-inhibitor cancer drugs, which can lead to unnecessary steroids or paused treatment if the care team is not told. Source
Taken together with ivermectin, as many online protocols suggest, it was linked to severe liver injury rated highly probable for the two drugs. Source
In human liver cells it is broken down mainly by the enzymes CYP2C19 and CYP2J2, so medicines that use or block those enzymes could change its levels. No human interaction study exists. Source
In rats, high doses acted as a liver tumor promoter after exposure to a cancer-causing chemical. Source
The FDA's drug adverse-event database holds 98 reports (count on 2026-10-28) naming fenbendazole, 8 of them describing liver injury. Reports cannot prove that fenbendazole caused anything.

Possible links

Where one finding points to others. These are leads for research, not conclusions.

Other conditions

Lung fibrosis 6 papers Includes one mouse study in which it reduced drug-induced lung scarring; the rest are mixed veterinary papers. Not yet read by us. See on PubMed
Viral infections 17 papers Lab studies, for example on a cattle herpes virus in cell culture. Not yet read by us. See on PubMed
Immune effects 50 papers Mostly lab-animal studies of how routine pinworm treatment changes immune and bone-marrow results. Not yet read by us. See on PubMed

Same mechanism

Disrupting microtubules (benzimidazole drug class) 142 papers Fenbendazole binds tubulin moderately in human cancer cells, the same target as the human dewormers mebendazole and albendazole and, by a different route, the vinca and taxane chemotherapy drugs. See on PubMed Source
Blocking sugar uptake 7 papers Lab work found it reduced glucose uptake, GLUT transporters and hexokinase II in cancer cells. See on PubMed Source
Mebendazole, the human drug in the same class 227 papers 227 PubMed papers on mebendazole and cancer, 6 tagged as clinical trials; ClinicalTrials.gov lists 11 mebendazole cancer trials, none with posted results, and a Swedish trial (NCT03628079) was stopped for lack of effect after 11 patients. Its evidence is not counted for fenbendazole. See on PubMed
Albendazole, another human drug in the same class 463 papers 463 PubMed papers mention albendazole and cancer or tumors, 14 tagged as clinical trials; many may concern parasitic cysts rather than cancer. Not yet read by us. See on PubMed

Combinations

Fenbendazole with vitamins (the vitamin E in online protocols) 2 papers The idea traces to a 2008 mouse lymphoma study in which neither fenbendazole nor extra vitamins alone slowed tumors, but the two together did (PMID 19049251). See on PubMed
Fenbendazole with ivermectin 3 papers Two are 2026 case reports: a short-lived melanoma response and a severe liver injury. The third is an older review. See on PubMed
Fenbendazole with chemotherapy or radiation 10 papers Lab and mouse work; a Yale mouse study found it did not add to the effect of radiation or docetaxel. See on PubMed
Fenbendazole with curcumin or CBD 1 papers The only paper is on honey bees. No published study has tested the curcumin and CBD parts of the online protocol with fenbendazole in cancer. See on PubMed
Fenbendazole with dichloroacetate (DCA) 2 papers One lab and one mouse study of lung cancer cells from the same Hanoi group. See on PubMed

Who does the research

M. Shibutani 6 papers Tokyo University of Agriculture and Technology (veterinary pathology); rat liver studies in which fenbendazole is one of the chemicals tested as a liver tumor promoter.
K. Mitsumori 6 papers Same Tokyo group; the same rat liver tumor-promotion work.
B. X. Hoang and T. Q. Nguyen 3 papers University of Science and Technology of Hanoi and the University of Pittsburgh School of Pharmacy; the 2024 review favoring fenbendazole and the lab and mouse studies with DCA.
S. Rockwell 2 papers Yale School of Medicine (therapeutic radiology), funded by the National Cancer Institute; found no effect on mouse breast tumor growth or on radiation response.

Who pays for it

US National Cancer Institute (NIH) 6 papers
Czech Ministry of Education, Youth and Sports 2 papers
National Natural Science Foundation of China 1 papers

Funding is listed only where PubMed records it; most fenbendazole cancer papers list none. Three of the six NCI-funded papers are mouse studies of fenbendazole itself (two found no tumor effect, one found an effect only together with extra vitamins); the other three are about mebendazole or the drug class.

What people report

We read public first-person posts and counted every outcome, good or bad. 40 reports, read on 2026-10-28; 4 mention side effects.

18Helped
8No effect
2Got worse
5Mixed
7Too early

Why these counts prove nothing on their own: people who post are not a random sample, most take other treatments at the same time, and nobody checks the diagnosis. We show them because they tell researchers what to test next.

Read all 40 reports

Helped Tumor markers had been rising; about three weeks after starting, CA15-3 fell from 55 to 45, and a later test showed a further small fall. HealthUnlocked, 2019-09 (Still on Ibrance and fulvestrant; markers only, no scan yet; the poster is unsure fenbendazole caused it.)
Helped Reports a tumor marker falling from 109 to 32 over about a year of three-days-on, four-days-off use. HealthUnlocked, 2020 (On letrozole throughout; a later update describes markers falling again while she was off fenbendazole.)
Mixed After four weeks, a PET scan showed disease down about half but also a new bone metastasis; she cannot tell what is working. HealthUnlocked, 2019 (Started letrozole months earlier and takes several other supplements.)
No effect PSA rose from undetectable to 0.26 over nine months on the standard schedule; he concludes it has not worked for him. HealthUnlocked, 2020-03 (Stopped Lupron two months after starting fenbendazole, which can let PSA rise on its own.)
No effect Taking it since the previous December with no change in his rising PSA trend. HealthUnlocked, 2020-03 (PSA only; other treatment not described.)
Too early Took it for one month before starting chemotherapy; does not know whether it helped, and says it eased his constipation. HealthUnlocked, 2020-03 (Only one month before chemotherapy; no cancer result attributable.)
Helped With no other treatment, PSA fell from 2.27 to 1.5 over six months on high doses; liver tests stayed normal. HealthUnlocked, 2019-11 (PSA only, no repeat scan; other supplements added along the way. He later switched to mebendazole; a follow-up thread shows PSA 1.2 by mid-2020.)
Mixed Her husband's PSA fell from 13.4 to 10.1 in the first months, then kept rising slowly; he stopped when the bottle ran out. HealthUnlocked, 2019-11 (Caregiver report; on abiraterone at the same time; liver tests reported normal.)
Worse Her husband took about 1 g a day, several times the usual dose, and developed worsening cramps and diarrhoea, milder at a lower dose. Side effects: stomach cramps, diarrhoea. HealthUnlocked, 2019-11 (Caregiver report; no cancer result reported; dose far above what forums suggest.)
No effect Her husband took the usual small dose for 15 weeks; his last two PSA tests rose, so they stopped. No side effects. HealthUnlocked, 2019-11 (Caregiver report; PSA only.)
No effect Took the standard schedule and saw no result; in a later thread he says it never lowered his PSA. HealthUnlocked, 2019-11 (PSA only; other treatment not described.)
No effect Took it for over a year with no discernible effect and no noticeable side effects. HealthUnlocked, 2022-01 (No numbers given.)
Mixed PSA rose to 1.4, then fell to 0.5, and some small metastases left scan reports; he calls it neutral and still cannot tell. Side effects: brief diarrhoea, raised liver enzymes (ALT/AST), back to normal in two months. HealthUnlocked, 2022-01 (On Lupron throughout; later Provenge immunotherapy and radiation.)
Helped Took it alongside Eligard and responded very well, with lymph nodes clearing; he says he cannot credit fenbendazole. No side effects. HealthUnlocked, 2022 (Hormone therapy alone often produces large responses.)
Too early Took 444 mg a day for two weeks before a biopsy that found a small amount of cancer; no follow-up given. HealthUnlocked, 2023-12 (Two weeks; no earlier PSA for comparison.)
Helped Reports PSA falling from 143 to 26 in two months and to zero for the following four months, with no side effects. HealthUnlocked, 2024 (Does not say whether hormone therapy, standard at this stage, was also given; the post includes a phone number and promotional wording.)
No effect Took 222 mg rising to 880 mg a day for several months; PSA kept rising and a PET scan was unclear. HealthUnlocked, 2024 (PSA only.)
Helped PSA fell from 8 to 5 after 12 weeks; after he later stopped for two months, PSA was unchanged. HealthUnlocked, 2020-03 (PSA had already fallen from 12 to 8 before he started; PSA only.)
Helped After 50 days, PSA fell from 73 to 49, the large tumor on MRI shrank to under half its size, and urinary symptoms cleared. HealthUnlocked, 2020-03 (Short follow-up with no later update found; diet and DMSO also used.)
Helped Her partner's PSA fell from 300 to 3 in about four and a half months. HealthUnlocked, 2024-10 (Hormone therapy and enzalutamide started at the same time and usually lower PSA sharply.)
Too early Taking up to 2,000 mg a day with ivermectin; notices better urine flow and says time will tell. HealthUnlocked, 2023 (No test results given.)
Helped PSA fell from 6.3 to 1.9 three weeks after starting, and later to 0.6. HealthUnlocked, 2020-03 (Started Lupron shortly before; the poster agrees the fall could be the Lupron.)
Helped After about a year with half-dose Xtandi, PSA was 0.00 and he felt better; he does not know if fenbendazole was the cause. HealthUnlocked, 2020 (On enzalutamide throughout.)
Helped PSA had risen to 5.7; one week after starting fenbendazole it was 3.1, which surprised his oncologist. Inspire, 2022-05 (Only one week; on Lupron and Xtandi; he says he does not know the cause.)
Mixed After 4.5 years, PSA has not fallen (8 to 10), but a 2023 MRI showed no lesions where two earlier MRIs did. Inspire, 2023-11 (Different MRI methods across scans; PSA unchanged.)
Helped PSA fell from 6.0 to 4.2; he says fenbendazole was the only change. Inspire, 2023-11 (PSA only; his doctor was not told.)
No effect About eight months on the usual schedule: lesions very slightly smaller on MRI, PSA unchanged; he thinks it had no significant effect either way. Inspire, 2024-05 (Low-grade disease that often changes little.)
Helped Tumor markers halved each month after she started fenbendazole and then Xeloda. Inspire, 2019-08 (Chemotherapy started at nearly the same time; scan result not posted.)
Mixed Scans after several months showed stable disease, but her tumor marker kept rising; no side effects. Inspire, 2020-01 (On chemotherapy at the same time; lobular cancer is hard to see on scans.)
Too early In remission after surgery and chemotherapy; started it to prevent recurrence; no follow-up results posted. Inspire, 2019-06 (Already in remission when starting.)
Helped CA125 fell from about 1,200 to 19 and tumors shrank by half or more, which made surgery possible; no side effects in five months. Inspire, 2019-08 (Chemotherapy and bevacizumab at the same time.)
Helped PSA fell 30% after four weeks of the protocol; he then started hormone therapy. Inspire, 2019-07 (Four weeks; PSA only.)
Helped His wife's CA125 had been rising about 30% a month; after four weeks it was essentially unchanged. Inspire, 2019-09 (Caregiver report; four weeks; one marker reading.)
No effect After about two months she saw no change, good or bad, and paused because of stomach problems. Side effects: stomach problems. Inspire, 2019-12 (No test results given.)
Helped After chemotherapy plus eight weeks of daily fenbendazole, her PET scan was nearly clear and her surgeon found little disease. Inspire, 2020-02 (Standard chemotherapy at the same time.)
Too early Taking it daily with a targeted drug; a scan was pending. Inspire, 2024-12 (No result posted.)
Worse Took it to stay in remission; her liver enzymes rose and were normal again a month after stopping. Side effects: raised liver enzymes. Macmillan Online Community, 2026-01 (Liver result only; no cancer outcome; date approximate.)
Helped Tumors shrank and then stayed stable; he says he cannot tell whether fenbendazole, immunotherapy or the rest made the difference. Personal blog, 2025-11 (On immunotherapy; the site also asks readers for treatment donations.)
Too early Took it in the two months before surgery, which found clear lymph nodes; reports no ill effects. Inspire, 2025-01 (Surgery removed the tumor, so no effect of fenbendazole can be read.)
Too early Two weeks into the protocol before chemotherapy; feels better, but the tumor had not been re-imaged. Inspire, 2025-01 (Two weeks; no test results.)

Checked against PubMed on 2026-10-28.