i found this on webmd
Milk thistle "cleanses" and therefore fortifies the liver, a vital organ responsible for processing everything from nutrients in foods to medications and chemical pollutants. The herb partly accomplishes this task by preventing a reduction in concentrations of glutathione, an amino-acidlike compound that is critical to neutralizing toxins. Some studies indicate that milk thistle can actually increase glutathione levels by as much as 35%.
In addition, silymarin alters the outer membrane of liver cells in such a way that toxins are barred from entering. It even encourages the formation of new, healthier liver cells to replace old, damaged ones. Various types of liver damage benefit from these remarkable properties, and the herb has repeatedly demonstrated its power to do so.
In the largest trial to date specifically studying the use of milk thistle for chronic liver disease, more than 2,500 people suffering from this condition showed improvement in their liver function tests when taking the extract. Many also reported that they felt better. People with an acute form of liver disease--acute viral hepatitis--also stand to benefit from milk thistle. In a double-blind study reported in 1998, the extract improved the results of liver function tests, speeded recovery, and reduced the length of hospital stays as compared with a placebo.
Guidelines for Use
Don't expect teas made with milk thistle to improve any medical condition. Unfortunately, they contain only trace amounts of silymarin, which dissolves poorly in water. Also avoid products containing milk thistle leaf; they have no therapeutic value.
There are products now available in which silymarin is chemically bound to phosphatidylcholine, a key component of lecithin (a natural fat). These preparations may in fact be better absorbed and more effective than regular milk thistle.
Herbal blends that go by such names as "liver complex" and "lipotropic factors" combine milk thistle with other herbs and nutrients. Popular ingredients include dandelion, methionine, inositol, and choline, compounds believed to enhance liver function and increase the flow of fats and bile from the liver and gallbladder.
Take milk thistle 30 minutes before meals for optimal effectiveness.
Keep in mind that it will take at least a week or two before you notice milk thistle's beneficial effects. For chronic conditions, you will need to take the herb for quite some time