From my reading of peer reviewed medical journal articles there is plenty of evidence that a variety of mushrooms have a positive effect on reducing tumor size and with Turkey Tail (also known as Trametes versicolor) mushroom there is excellent research on animals that it can kill prostate cancer stem cells. It has also been demonstrated that it has a role in combating breast cancer and colorectal cancer among others. As yet there are no large human drug trials with turkey tail . There have been studies on two polysaccharides, PSP and PSK, that have been purified from turkey tail in Japan. There was a breast cancer study examining the safety of various doses of turkey tail. They found that the largest dose they tested had no negative side effects. However, like all food and mushrooms, some people have upset stomach and loose bowel movements when taking mushrooms. As for me, I am very allergic to consuming raw mushrooms, but when cooked and or processed with heat I enjoy them in food and as tea / medicine. You may be interested in the article, Mycochemicals against Cancer Stem Cells which can be found on Google Scholar. One problem the drug companies have is that if Turkey tail was recognized as medicine, big pharma would not be able to make $$$$$$ on it because it is so widely available in markets and in the wild.