Silymarin is unique to Milk thistle, the famous European herb that has been in use since biblical time.

Silymarin refers to Milk thistle flavonoids consisting of flavonolignans including silybinin, silychristin, silydianin, and isosilybin. Milk thistle fruits (seeds) are the main source of silymarin, which are the major secondary metabolites of Milk thistle. As the molecular structure of silybinin, one of the major silymarin components indicates, silymarin belongs to flavonoids, which consist of a diverse group of more than 2,000 phenolic phytochemicals with a common theme of C6-C3-C6 structure where C6 stands for a phenol ring. The majority of falvonoids and isoflavonoids including silymarin are anti-oxidants as well, and many of them are pigments offering a variety of colors to flowers and fruits, and some of them are functional part of plant defensive mechanisms.