Ergothioneine is a naturally-occurring amino acid that is found in some foods, such as black beans, kidney beans, oat bran, and oyster mushrooms, and is also made by some bacteria and fungi. It was discovered in 1909 and named after the ergot fungus from which it was first purified. Ergothioneine cannot be synthesized by the human body, so it must be obtained from outside sources. Ergothioneine is a potent antioxidant that accumulates in areas of the body that experience large amounts of oxidative stress, such as the eyes and liver. Oxidative stress is an imbalance between the production of free radicals and the ability of the body to counteract or detoxify their harmful effects through neutralization by antioxidants.

Topical application of ergothioneine is thought to protect the skin from oxidative stress that can be induced by environmental factors such as UV radiation, pollution, cigarette smoking, etc. Free radicals damage important cellular components, such as the cell membrane, DNA, and cellular proteins like collagen. Damaged collagen is a major factor that contributes to the appearance of aged skin. Therefore, by using topical antioxidants like ergothioneine, the skin will be better protected from free radicals.