水飞蓟素（英语：silymarin）是抗氧化剂，从一种名为乳蓟（Milk Thistle）植物提炼而成。水飞蓟素能稳定肝细胞膜，维持肝细胞之完整性，使毒素无法穿透破坏肝脏，并能加速合成肝脏细胞的DNA（去氧核糖核酸），可预防肝硬化、脂肪肝、胆管炎、牛皮癣等症，同时具有肝癌、前列腺癌、乳癌及子宫颈癌细胞的生长及分化抑制作用。是目前世界上所发现最具肝疾疗效的类黄酮。

Silibinin (INN), also known as silybin (both from Silybum, the generic name of the plant from which it is extracted), is the major active constituent of silymarin, a standardized extract of the milk thistle seeds, containing a mixture of flavonolignans consisting of silibinin, isosilibinin, silicristin, silidianin and others. Silibinin itself is mixture of two diastereomers, silybin A and silybin B, in approximately equimolar ratio.