The dried fruit of milk thistle contain a flavonoid complex known as silymarin. This constituent is responsible for the medical benefits of the plant.¹ Silymarin is made up of three parts: silibinin, silidianin, and silicristin. Silibinin is the most active and is largely responsible for the benefits attributed to silymarin.²

Milk thistle extract may protect the cells of the liver by blocking the entrance of harmful toxins and helping remove these toxins from the liver cells.^{3, 4} As with other bioflavonoids, silymarin is a powerful antioxidant.⁵ Silymarin has also been shown to regenerate injured liver cells.⁶ Recent studies have shown that silymarin has the ability to block fibrosis, a process that contributes to the eventual development of cirrhosis in people with inflammatory liver conditions secondary to diseases such as alcohol abuse or hepatitis.⁷

Milk thistle extract is most commonly recommended to counteract the harmful actions of alcohol on the liver. Double-blind trials indicate that it helps the liver return to a healthy state once a person stops drinking.^{8, 9} Some trials suggest it may improve quality of life and even life expectancy in people with liver cirrhosis.^{10, 11} However, another trial found no effect in cirrhosis patients.¹² Milk thistle alters bile makeup, thereby potentially reducing risk of gallstones.¹³ However, this needs to be verified by human clinical trials. Milk thistle extract has been shown to protect the liver from the potentially damaging effect of drugs used to treat schizophrenia and other forms of psychosis.¹⁴ However, one trial found that it did not protect the liver from the potentially harmful effects of the drug Cognex (tacrine hydrochloride) used to treat early-stage Alzheimer’s disease. ¹⁵