Niclosamide

Last reviewed 2026-09-16. How this works.

Treating cancer Grade D

No human evidence of benefit, or none that survived testing.

This drug was made not to be taken up from the gut, and only early human studies of it exist.

Early-phase human work only, and it is poorly absorbed by design.

The claim

Niclosamide is a worming drug. The claim is that it also acts against cancer, usually argued from laboratory work on cell signalling.

What it does

The obstacle is built into the drug. Niclosamide was designed to act on parasites inside the gut and to stay there, which means it is poorly absorbed into the bloodstream on purpose. A compound that does not enter the blood in quantity has a long way to go before it reaches a tumour elsewhere in the body.

The evidence

A search of the trials registry returns three completed studies of niclosamide in cancer. Their primary outcomes are prevention of symptomatic infection, dose-limiting toxicity and adverse events. None measures what happened to a cancer.1

So the manuscript description of early-phase work only is borne out, and the register can now say what that means: the human studies asked whether it could be given, not whether it does anything.1

The harm

What has not been looked at

Nothing here says what niclosamide does to a person. Two of the three completed studies did measure toxicity and adverse events, but the register holds no result from them, so the absence is of a reported finding rather than of an attempt.

Scope: people with cancer.

Established by registry search, ClinicalTrials.gov, query intervention "niclosamide", condition "cancer", status completed, checked 2026-09-16. Result: toxicity and adverse events were measured in two studies; no result is recorded in this register.1

What is absent

No completed trial of niclosamide in cancer has measured survival, recurrence or tumour response. The three that exist measured infection, toxicity and adverse events.

Scope: any tumour type.

Established by registry search, ClinicalTrials.gov, query intervention "niclosamide", condition "cancer", status completed, checked 2026-09-16. Result: three completed studies, none with a cancer outcome endpoint.1

What to ask

Is there any human evidence that niclosamide reaches a tumour in a person?

The dish-to-person gap

The register records no human pharmacokinetic measurement for niclosamide. This matters more here than on most pages, because the drug was designed to stay in the gut rather than enter the bloodstream.

When to call your team — no warning signs are recorded

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.

The register records no warning signs for this entry because no human safety study has been found to base them on. Signs must never be written from what is typical of a drug class.

Interactions

No interaction between niclosamide and a cancer treatment class has been studied in a person. An empty list means nobody has looked.

Nobody having looked is not the same as nothing being there. That is what this line says and all it says.

The whole interaction list, and how to turn it into a pharmacy review request.

Find a trial instead

A recruiting trial is the same decision with monitoring attached.

Search ClinicalTrials.gov for recruiting studies of niclosamide

A trial is the same decision with monitoring, a known preparation and a doctor who knows what else you are taking.

References

  1. A search of the ClinicalTrials.gov registry for completed studies of niclosamide in cancer, run on 16 September 2026. Three completed studies returned.

    https://clinicaltrials.gov/api/v2/studies?query.intr=nicl…

    How this was reached: search run during build, via a search run during the content build; not a source the manuscript carries.