A compound in Milk Thistle seed dubbed silymarin has been shown to protect the liver from a variety of harmful substances. Silymarin is the key constituent that exerts a protective effect against substances potentially harmful to the liver. It prevents toxins from penetrating the interior of liver cells while promoting the growth of healthy new cells to repair liver damage.

Other compounds in the seed have a similarly renewing effect on the kidneys, and may be beneficial for the prostate gland, while inhibiting the growth of certain prostate tumor cells. The seeds also have antioxidant properties, mopping up damaging free radicals.