The Reishi mushroom is a shiny little fungus that grows on rotten wood. For 2,000 years the Chinese have used Reishi in medicine, making it one of the oldest medicinally used mushrooms. Because of Reishi’s reputed health benefits and reputed absence of side effects, it has attained a reputation in the East as the ultimate herbal substance, to the degree of being called “the Elixir of Eternal Life.”