Are shiitake mushroom supplements safe?
Shiitake mushroom have been used in traditional Chinese medicine and some parts of Asia as well as Russia. It is believed that Shiitake mushroom contains chemicals that might boost the immune system and improve circulation, but there are insufficient studies that may prove these claims. The published studies were done in animals rather than people. There are also no sufficient proof that shiitake mushroom supplements are safe. Supplements that are available on the market have not been tested for potency. It is LIKELY SAFE when consumed as a food but POSSIBLY UNSAFE in medicinal amount. It can cause various sickness, like stomach discomfort, blood abnormalities, and skin swelling (inflammation) and other adverse reactions. Although it promises a lot of benefits, it is better to have more studies/research on it and be safe rather than sorry.
As seen in Quora: https://www.quora.com/Are-shiitake-mushroom-supplements-safe/answer/Thomas-Vegky