Wikipedia
Polysaccharide-K (Krestin, PSK) is a protein-bound polysaccharide, which is used as an anticancer immunologic adjuvant in some countries.
PSK is isolated from the fruitbody of Trametes versicolor. Preliminary evidence indicates PSK has anticancer activity in vitro, in vivo and in human clinical trials. Preliminary research has also demonstrated that PSK may inhibit various cancer onset mechanisms. Preliminary evidence indicates PSK may have use as an adjuvant in the treatment of gastric, esophageal, colorectal, breast and lung cancers. Human clinical trials suggest PSK may affect cancer recurrence when used as an adjuvant, and basic research has demonstrated it inhibited certain human cancer cell lines in vitro.
The MD Anderson Cancer Center reported that it is a "promising candidate for chemoprevention due to the multiple effects on the malignant process, limited side effects and safety of daily oral doses for extended periods of time." The Therapeutic Goods Administration in Australia reported that the WHO has only eight records of adverse effects with PSK and none reported for PSP.