right i couldn't find the website that told me about milk thistle nitrate poisoning but i found this very informative website which lists side effects and they state for liver support of alcohol and drug use, use 200mg tablets 2-3times daily.. and we're talking about 1000mg daily over a long period of time. therefore also the side effects would become more common.. take a look
http://www.umm.edu/altmed/ConsHerbs/MilkThistlech.html