Milk thistle was the very first thing I tried when diagnosed with hep C in '93. I worked in a building with someone who swore by it, and gave me a lot of literature. There were dietary restrictrions, as well, such as drinking only DISTILLED water, no carbonated drinks, no caffeine, and other things I've since forgotten. I tried it for about 6 weeks and got a sore behind; it was very irritating. So I gave up on it, but did run into the same person around '95 and she told me she was clear of hep C. So who knows?
Just bear in mind with any health food supplements, they are not regulated. The quality and quantity are virtually unknown and vary from one brand to another.
I don't see that milk thistle causes any harm, but you might want to research it thoroughly before trying it.