Fenbendazole
Also called: fenben, Panacur, dog wormer.
Last reviewed 2026-09-16. How this works.
When to call your team — this entry is incomplete
Whatever you are taking or thinking of taking, the people treating you can only weigh it if they know about it. That is true when nothing has been written down here, and it is most true then.
A source for this part is missing. Harm has been recorded in people for this entry and the register cannot yet say what it looks like, because no source it holds describes that. What is written below is the gap itself, not a summary of the signs.
Liver injury in people taking fenbendazole on their own is reported in the professional-body notice cited on this page, but that notice points to the clinical literature in general without naming a case report. No signs are written here, because signs written from what is typical of a drug class would be a guess, and a guess is what this register exists to refuse. Until the underlying case reports are found, this part of the entry is unfinished rather than empty, and the absence of a list here is not a sign that there is nothing to watch for.2
This block is about what to watch for, and it is not all this page holds: an interaction recorded against a treatment class and what has been recorded about harm are below. Nothing in this block is an all-clear for a question it does not mention.
Treating cancer Grade D
No human evidence of benefit, or none that survived testing.
Harm has been recorded in people for this claim. The grade measures how strong the evidence for benefit is; it says nothing about the harm, and the two are read from different parts of this page.
This is a worming drug for animals, and no study has ever measured what it does in a person.
A dog wormer. Nobody has measured what it does in a person.
The harm
Cases of liver injury in people taking fenbendazole on their own have been reported. The professional-body notice recording this points to the clinical literature in general rather than to a named case report, so this page states that the reports exist and does not describe them.2
The same notice names the route by which this could affect treatment that is working. Fenbendazole is broken down through the cytochrome P450 pathway in the liver, and the notice states that its use may alter how established cancer therapies behave in the body. That is the most concrete reason on this page to tell an oncology team about it.2
There is a second problem the notice raises, and it is specific to how people obtain this. Veterinary formulations are what most people buy, and those lack regulatory oversight for human use: they may vary in concentration between batches or contain impurities. Whatever is known about the compound does not transfer to a particular tub of it.2
The notice also asks oncology teams to be proactive and non-judgemental when the subject comes up, which is worth knowing before deciding whether to mention it.2
What has not been looked at
No study has examined what fenbendazole does in a person with cancer. The absence runs in both directions: nobody has shown that it helps, and nobody has established what harm it may do either. What exists instead are scattered reports of liver injury.
Scope: any tumour type, any population.
Established by regulatory statement, ASCO Clinical Notice, May 2026, checked 2026-05-01. Result: no robust, peer-reviewed clinical evidence of benefit, and none establishing tolerability, for treating any human malignancy.2
The claim
Fenbendazole is a worming drug licensed for animals. The claim, which spread from a single widely shared personal account rather than from a study, is that it treats cancer in people.
What it does
Benzimidazole drugs bind tubulin and disrupt the scaffolding a cell uses to divide, which is the same broad target as several licensed cancer drugs. A plausible target is where a question starts. It is not evidence that anything happens in a person.
The evidence
There is none to summarise. This section of the book normally lists every human trial by name. For fenbendazole there is no trial to name, and that is the finding rather than a gap in the writing.1
The American Society of Clinical Oncology states that no robust, peer-reviewed clinical evidence demonstrates that fenbendazole is effective for treating any human cancer, and recommends against its use outside a clinical trial.2
The widely shared account that started this also records that the person involved was enrolled in a pembrolizumab trial at a major cancer centre at the same time, and was taking several other compounds. Every published version of the account records the trial.
What is absent
What has never been measured
No human pharmacokinetic study of fenbendazole has been published, so nobody knows what concentration it reaches in a person or whether it reaches one at all.
Scope: any route, any population.
Established by literature search, published literature, checked 2026-09. Result: no report of plasma concentration, absorption or bioavailability in a person.1
What to ask
Given that fenbendazole has never been measured in a person, what would you want to know from me if I were considering it or already taking it?
The dish-to-person gap
No human pharmacokinetic study of fenbendazole has been published. A search of the literature returned no report of plasma concentration, absorption or bioavailability in a person.1
Interactions
Fenbendazole is metabolised through the cytochrome P450 pathway in the liver, and a professional body states that its use may alter how established cancer treatments behave in the body. That is a named mechanism against a treatment class rather than a general caution. It has not been measured in a person, because no human pharmacokinetic study of fenbendazole exists to measure it against, which is an unusual and uncomfortable combination: a documented route to an interaction and no way to quantify it.2
Fenbendazole
Recorded as documented, and the source names no particular treatment.
Does fenbendazole going through the same liver enzymes as some cancer drugs matter for what I am on?
The whole interaction list, and how to turn it into a pharmacy review request.
Find a trial instead
The search below is expected to return nothing, and that is the point. You can run it yourself and see the absence, rather than taking this page’s word for it.
Search ClinicalTrials.gov for recruiting studies of fenbendazole OR febantel
A trial is the same decision with monitoring, a known preparation and a doctor who knows what else you are taking.
References
A search of ClinicalTrials.gov and of the published literature, conducted in September 2026
No persistent identifier recorded.
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 13 note 5.
ASCO Clinical Notice, May 2026, recommending against ivermectin and fenbendazole for cancer treatment outside of clinical trials. Published by the American Society of Clinical Oncology.
https://connection.asco.org/do/asco-clinical-notice-recom…
How this was reached: transcribed from chapter note, via docs/CHAPTER-NOTES.json, chapter 13 note 6.
Held against this source and not confirmed:
cases of liver injury associated with self-administered fenbendazole
The notice states these have been reported in the clinical literature but names no case report. The claim is used on this page, attributed to the notice, and is explicitly not used to write warning signs.