Milk thistle, or Silybum marianum, is native to southern Europe and southern Russia, and is common throughout North and South America, South Australia and Central Europe. Milk thistle is a biennial plant up to 10 feet in height, easily recognized by its brilliant reddish-purple flowers and large prickly leaves. The plant, most notably its seeds, contains a complex of therapeutic compounds known collectively as silymarin. Milk thistle has been employed as a medicine for at least 2000 years, primarily for ailments of the liver.