The dried fruit of milk thistle contain a flavonoid complex known as silymarin. This constituent is responsible for the medical benefits of the plant. 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.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.