Ergothioneine is a naturally occurring amino acid derivative containing sulfur. It is synthesized primarily by certain fungi and mycobacteria, and is accumulated in plants through root uptake. In humans, ergothioneine is obtained exclusively through diet, with mushrooms being the richest natural source. Unlike many other antioxidants, ergothioneine has its own specific transporter (OCTN1) in the human body, allowing it to be actively taken up into cells and concentrated in tissues most susceptible to oxidative stress, such as the liver, kidneys, eyes, and skin.

Ergothioneine is recognized as a potent and stable antioxidant with unique properties. It exhibits strong free radical scavenging activity, protects mitochondria from oxidative damage, and has been shown to extend cellular healthspan in preclinical studies. Unlike glutathione, ergothioneine does not readily autoxidize and remains stable under physiological conditions, providing sustained protection.