Reishi mushroom is a fungus. Some people describe it as "tough" and "woody" with a bitter taste. The above-ground part and portions of the below-ground parts are used as medicine.
Reishi mushroom is used for cancer, aging, boosting the immune system to prevent or treat infections, and for many other reasons, but there is no good scientific evidence to support these uses.
In combination with other herbs, reishi mushroom is used to treat prostate cancer. Also in combination with other ingredients, reishi mushroom may be used for viruses, such as the human papilloma virus (HPV), genital herpes, and cold sores.