Milk Thistle will defiantly aid in the detoxification of your liver. However, just like a seat-belt only aids in your safety. If you're reckless, your chance of injury increases.
Studies on this herb have been done for over 20 years, and it's widely used to treat things like cirrhosis, and hepatitis caused by alcohol.
Silymarin, one of Milk Thistle's most active ingredients is responsible for production of new cells in the liver, and increasing Glutatione, which is responsible for many things in the body. One being detoxifying harmful substances.
Make sure that when you buy some, you get something that's standardized. GNC sells a great standardized formula.(Shame-less GNC plug. Yes I work there).
Just because a bottle says Milk Thistle, doesn't mean that it has the amount of active constituents needed to work properly. That goes with all herbs. Many companies just include the whole plant, when sometimes only less then 10% does any good.
There are also products called "Liver Support" that you can get. They're usually pretty good, but don't include much Milk Thistle.