Milk Thistle is contra-indicated during pregnancy, otherwise it is safe for all body types. You don't wnat to take it for more than a month at a time, however, as it really does "rearrange" liver cells. It cleanses on a cellular level and too much of that is not good.
On the plus side, if you were to eat poisoned mushrooms, which attack the liver, a large dose of Milk Thistle would save you. All the alcohol you've ever drank, drugs, pot, whatever, all get cleansed out of the liver thanks to Milk Thistle.
When taking herbs it's best to take at least two at a time as they act on different things and react synergistically together. So i fyou want to do liver maintenence, get an herbal complex for the liver that includes things like chinese gentien, bupluerum, ginger, dandelion, B vitamins, amino acids, etc. Lecithin is also great.