The short answer
PSK, a turkey tail extract, is an approved add-on to chemotherapy in Japan. In the US no mushroom product is approved or established as cancer treatment, and supplements can interact with care.
Polysaccharide-K (PSK), from turkey tail, is an approved cancer product in Japan and has been given alongside chemotherapy since the mid-1970s, mainly after surgery for gastric or colorectal cancer.
Japanese meta-analyses of randomised trials report modest survival gains when PSK is added to chemotherapy, including a hazard ratio of 0.88 in pooled gastric cancer data.
That evidence describes a regulated, standardised extract used as an addition to chemotherapy, not capsules bought online.
NCI states the FDA has not approved turkey tail, PSK or reishi as a treatment for cancer or any other condition; in the US they are sold as dietary supplements.
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The full explanation.
The claim
Reishi, turkey tail, chaga, maitake and lion's mane are sold as capsules, powders and coffee blends. The claims attached are big: they boost immunity, shrink tumors, or treat cancer. Prices are high, and the language borrows heavily from real research.
Why the idea appeals
The appeal here rests on something real, which is what makes it tricky. Mushroom compounds have been studied seriously for decades. Some are used in cancer care in other countries. And several cancer drugs did start out in nature. So when a seller says "there is research behind this", they are not always lying. The gap is between research existing and a product being proven.
What is actually used, and where
Polysaccharide-K, known as PSK or krestin, is an extract of turkey tail (Trametes versicolor). In Japan it is an approved product used in cancer treatment. It has been given alongside chemotherapy since the mid-1970s, usually after surgery for gastric or colorectal cancer. Meta-analyses of randomised trials there have reported modest survival benefits when PSK is added to chemotherapy. One pooled gastric cancer analysis found around a 12% relative reduction in the risk of death.
That is a real finding. It is also narrower than it sounds. It describes a standardised pharmaceutical-grade extract. It was given as an addition to chemotherapy, in specific cancers, in a healthcare system that regulates it as a drug. It says nothing about capsules bought online.
In the United States, the picture is different. NCI states that the FDA has not approved turkey tail, PSK or reishi as a treatment for cancer or any other condition. They are sold as dietary supplements. That means they are not reviewed for effectiveness before sale. NCI also notes that batches and brands may not be the same as one another. Reishi is used in China to support the immune system of people having chemotherapy or radiotherapy. The trials behind that use are small, and they often measure immune markers rather than survival or symptoms.
The risks worth knowing
Mushroom supplements are not automatically gentle. Reishi is the best-documented example. It can raise bleeding risk alongside anticoagulants, the drugs that thin the blood. Its polysaccharides block liver enzymes, including CYP3A, that many chemotherapy drugs depend on. It can boost immune response, which matters if you take immunosuppressants. And rare cases of liver toxicity have been reported. Reishi spore powder can also raise the tumor marker CA72-4. That can muddy the results your team is using to track you. Supplements are not tested batch by batch, so what is in the jar may not match the label.
None of that means a mushroom supplement will harm you. It means your oncology team and pharmacist cannot assess the interaction unless they know what you are taking. Bring the label to your appointment.
What this does not mean
It does not mean mushroom research is quackery. PSK is genuine pharmacology, and it remains under study. It does not mean culinary mushrooms are a problem. Eating them is fine, and they are a perfectly good food. And it does not mean anyone who has taken a supplement has made a mistake.
What it does mean is this. No mushroom product is an established cancer treatment in the United States. And nothing here should displace treatment that is.
The bottom line
Turkey tail's PSK has a real but limited role as an add-on to chemotherapy in Japan. No mushroom product has been shown to treat cancer on its own. None is approved for cancer in the US. If you want to try one, the sensible route is through your care team. They can check it against your drugs and your blood counts before you spend anything.
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Words to know
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Common questions
Is turkey tail an approved cancer treatment?
In Japan, PSK, an extract of turkey tail, is an approved product used in cancer treatment and given with chemotherapy. In the United States, NCI states the FDA has not approved turkey tail or PSK as a treatment for cancer or any other condition; they are sold as dietary supplements.
Are mushroom supplements safe to take alongside chemotherapy?
Not automatically. Reishi can increase bleeding risk, can inhibit liver enzymes such as CYP3A that many chemotherapy drugs rely on, and can enhance immune response, which matters with immunosuppressants. Ask your oncology team or pharmacist and show them the label before you start.
Is the supplement I can buy the same as the PSK used in Japan?
Almost certainly not. PSK is a standardised, pharmaceutical-grade extract regulated as a drug. NCI notes that in the US supplements are not reviewed for effectiveness before sale and that batches and brands may not be the same as one another.
What about reishi for immune support?
Reishi is used in China to support the immune system of people having chemotherapy or radiotherapy, but the trials are small and often measure immune markers rather than survival or symptoms. The FDA has not approved it as a cancer treatment.
Are mushrooms in food a problem?
No. Eating culinary mushrooms is fine and they are a perfectly good food. The questions on this page are about concentrated extracts and supplements taken at high doses.
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Written by: Cancer ExplainedSources last checked: 2026-07-30 what this meansLast updated: 2026-08-10Next planned review: 2027-01-30
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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