Milk thistle is a plant that is native to Europe and was brought to North America by early colonists. Milk thistle is now found throughout the eastern United States, California, and South America. The plant grows up to 2 meters high and has large, bright purple flowers.

Milk thistle gets its name from the milky sap that comes out of the leaves when they are broken. The leaves also have unique white markings that, according to legend, were the Virgin Mary's milk. The above ground parts and seeds are used to make medicine. The seeds are more commonly used.

Milk thistle is taken by mouth most often for liver disorders, including liver damage caused by chemicals, alcohol, and chemotherapy, as well as liver damage caused by Amanita phalloides (death cap) mushroom poisoning, jaundice, chronic inflammatory liver disease, cirrhosis of the liver, and chronic hepatitis.

Milk thistle is also taken by mouth for loss of appetite, heartburn (dyspepsia), gallbladder complaints, enlarged prostate (benign prostatic hyperplasia), a blood disorder called beta-thalassemia, and infertility.

Some people take milk thistle by mouth for diabetes, kidney damage caused by diabetes, hangover, diseases of the spleen, prostate cancer, inflammation in the lungs and chest, malaria, depression, uterine complaints, increasing breast milk flow, allergy symptoms, starting menstrual flow, obsessive-compulsive disorder (OCD), Alzheimer's disease, Parkinson's disease, multiple sclerosis, high cholesterol, and menopausal symptoms.

People apply milk thistle to the skin for skin toxicity caused by radiation.