Milk thistle (Silybum marianum) is a member of the sunflower family. It is well-known for its use in liver disease. The seeds of the milk thistle contain a compound called 'silymarin,' which is an antioxidant and is thought to help in protecting and regenerating the liver. Milk thistle has been used in humans, and has been recommended as part of the therapy for a number of liver conditions in dogs and cats, including toxicity (poisoning) from insecticides, pesticides, mushrooms, and some drugs.
Milk thistle seems to be a very safe supplement with no known toxicity, although long-term studies in cats and dogs have not been performed. Very large doses may cause loose stools. Milk thistle has not been shown to be a preventive for any specific condition, and is usually recommended by veterinarians as a daily supplement during times when the liver is actually under stress.