There is lots of conflicting info on this subject all over the boards. From what I've read, milk thistle is best used after your cycle to rejuvenate/regenerate liver cells. And NAC or ALA or some anti-oxidant is best used during the cycle. I've heard milk thistle can limit the effectiveness of the PH/AAS, but never read any definitive study.
Because the stuff is so cheap, I still use Milk thistle throughout any methyl cycle & continue thru PCT. I would rather have even a minute amount of liver protection, than a probably unnoticeable amount of increased gains.