Turkey Tail Mushroom: The Most Clinically Studied Mushroom in the World

Turkey tail (Trametes versicolor) is the most clinically studied mushroom in the world. Its primary active compound, PSK (polysaccharide-K, also called Krestin), has been the subject of over 400 clinical trials — primarily in Japan, where it has been approved as a cancer adjunct therapy since 1977.

Here is the complete evidence review.

PSK: The Most Studied Mushroom Compound

PSK is a protein-bound polysaccharide extracted from turkey tail. It was developed as a pharmaceutical product in Japan in the 1970s and has been covered by Japan's national health insurance since 1977.

The clinical evidence for PSK is primarily in oncology — specifically as an adjunct to chemotherapy and radiation for gastric, colorectal, and breast cancers.

The Japanese Cancer Trials

Gastric cancer: A meta-analysis of 8 RCTs involving 8,009 patients found PSK supplementation alongside chemotherapy significantly improved 5-year survival rates compared to chemotherapy alone. The effect was consistent across multiple chemotherapy regimens.

Colorectal cancer: Multiple RCTs have found PSK improves disease-free survival in colorectal cancer patients receiving adjuvant chemotherapy.

Breast cancer: A 2012 NIH-funded trial (the first FDA-approved clinical trial of a mushroom supplement in the US) found turkey tail extract improved immune function markers in breast cancer patients after conventional treatment.

The Mechanism

PSK activates two key components of the immune system:

Natural killer (NK) cells: NK cells identify and destroy cancer cells and virus-infected cells without requiring prior sensitization. PSK significantly increases NK cell activity.

T-lymphocytes: Specifically CD8+ cytotoxic T-cells, which are the primary immune cells responsible for destroying cancer cells. PSK increases both the number and activity of CD8+ T-cells.

Chemotherapy and radiation suppress the immune system — which is one reason cancer can recur after treatment. PSK helps restore immune function during and after treatment.

PSP: The Chinese Counterpart

PSP (polysaccharide-peptide) is a similar compound found in turkey tail that has been studied primarily in China. It has similar immunomodulatory properties to PSK and has been used as a cancer adjunct therapy in China since the 1980s.

The Prebiotic Effect

A 2014 study published in PLOS ONE found turkey tail extract significantly increased populations of beneficial gut bacteria (Lactobacillus, Bifidobacterium) while reducing potentially harmful bacteria. This prebiotic effect adds another dimension to turkey tail's health benefits beyond immune modulation.

Quality Considerations

PSK and PSP are found in the fruiting body of turkey tail, not in mycelium. Products that use mycelium-on-grain will have dramatically lower PSK/PSP content.

This article is for informational purposes only and does not constitute medical advice.