Milk thistle is a member of the aster family that is native to southern Europe and Asia and now naturalized throughout much of the world. Like most thistles, milk thistle is characterized by sharp spines or thorns that surround the flower head. The herb is grown as an ornamental specimen as well as a commercial crop. Although other parts of the plant may be used, milk thistle is primarily cultivated and harvested for its seeds.
Milk thistle seeds contain several active chemicals, including a group of closely related flavonolignans. Collectively known as silymarin, the group consists of silybin A, silybin B, isosilybin A, isosilybin B, silychristin, isosilychristin and silydianin. The raw seeds contain about 6% of this flavonolignan complex, while the liquid extract prepared from the seeds contains as much as 80%.