Milk thistle is a flowering plant in the Aster family, used in traditional medicine as a liver tonic. Milk thistle generally supports the immune system through its powerful antioxidant, free-radical scavenging action, its ability to preserve the supply of another important antioxidant, glutathione, as well as direct effects on immune cells.It’s helpful for healing after drug therapy, vaccinations, and infections such as canine parvovirus.