Milk thistle has been used medicinally for over 2,000 years, most commonly for the treatment of liver and gallbladder disorders. Silymarin comes from the seeds of milk thistle and is believed to be the active part of milk thistle. The terms "milk thistle" and "silymarin" are often used interchangeably. Milk thistle products are popular in Europe and the United States for various types of liver disease. Milk thistle has been well tolerated with mild side effects when used in a recommended amount and duration.