The key is to have milk thistle as the main ingredient so that you get a minimum of 650 - 1000 mg of it a day (depending on your size and needs). At one point I had read a report indicating that a minimum of 650 mg was needed for real effectiveness in serious liver damage. My husband is very large, so he took 1000 mg a day. I would buy the more expensive brands from the local health food store. My husband would by the cheaper brands at the homeopathic section of Long's drug store. It took about a month for the Body Odor , gray colour and yellow eyes to clear up. After three months he could tolerate small amounts of oil in his food without feeling like someone had kicked him in the side. After a year he could again slather his pasta in olive oil.
I like to have some dandeion and burdock root in my post flush formulas because they help to clean the liver, whereas milk thistle protects and regenerates.
Another good liver herb is the ayurvedic herb bhumyamalaki (phyllanthus niruri), also known as chanca piedra.
I've added a link to some further information on milk thistle dosages from clinical studies done in Europe.