6,8-二硫辛酸，简称硫辛酸（lipoic acid），为含硫八碳脂酸，在6、8位上有二硫键相连（C6和C8上的氢原子被二硫键取代），有氧化、还原二型。6、8位上巯基脱氢为氧化型硫辛酸（两个硫原子通过二硫键相连），加氢变成还原型称为二氢硫辛酸（二硫键还原为巯基）。硫辛酸虽然不属于维生素，但其可作为辅酶参与机体内物质代谢过程中酰基转移，起到递氢和转移酰基的作用(即作为氢载体和酰基载体)，具有与维生素相似的功能（类维生素），因此也被生物化学教材列入维生素中讲述。

Lipoic acid (LA), also known as α-lipoic acid and alpha lipoic acid (ALA) and thioctic acid is an organosulfur compound derived from octanoic acid. ALA is made in animals normally, and is essential for aerobic metabolism. It is also manufactured and is available as a dietary supplement in some countries where it is marketed as an antioxidant, and is available as a pharmaceutical drug in other countries.