The importance of milk thistle has been recognized by medicine for more than 2,000 years. Used since Greco-Roman times as an herbal remedy for a variety of ailments, milk thistle is a member of the aster family (asteraceae or compositae), which also includes daisies, artichokes, sunflowers, and echinacea. It is an annual herb native to the Mediterranean region, and is commonly found growing wild in a variety of settings, including roadsides. Historically, milk thistle has commonly been used to treat various digestive and liver ailments.

In the late 19th and early 20th centuries physicians in the United States used milk thistle seeds to relieve congestion of the liver, spleen, and kidneys. Today, scientific studies suggest that active substances in milk thistle protect the liver from damage caused by viruses, toxins, alcohol, and certain drugs such as acetaminophen.