Milk thistle helps to protect the liver cells and stimulates growth of new liver cells. It is not generally considered a liver cleanser though. There are other herbs such as burdock root, dandlion root, oregon grape and bhumyamalaki (phyllanthus niruri) that have a more cleansing effect and are great when combined with milk thistle. They won't replace a Liver Flush but can help significantly with detoxing your liver. My husband's liver went from 20% functioning to 80% functioning (per lab tests) in three months on 1000mg of milk thistle a day.
Take your milk thistle (and other liver herbs)in between meals so that it doesn't dampen your digestive fire.
Perhaps you'll feel more comfortable about doing a Liver Flush after you've done a course of liver herbs. I recommend Andreas Moritz 's flush method.