FROM A PREVIOUS THREAD
This is from the National Institute of Health.
You can read the report and make your own mind. Their conclusion is as follows:
Conclusions. Milk thistle's efficacy is not established. Published evidence is clouded by poor design and reporting. Possible benefit has been shown most frequently, but inconsistently, for aminotransferases, but laboratory tests are the most common outcome measure studied. Survival and other clinical outcomes have been studied less, with mixed results. Future research should include definition of multifactorial mechanisms of action, well-designed clinical trials, and clarification of adverse effects.
Adverse effects associated with oral ingestion of milk thistle include gastrointestinal problems (e.g., nausea, diarrhea, dyspepsia, flatulence, abdominal bloating, abdominal fullness or pain, anorexia, and changes in bowel habits), headache, skin reactions (pruritus, rash, urticaria, and eczema), neuropsychological events (e.g., asthenia, malaise, and insomnia), arthralgia, rhinoconjunctivitis, impotence, and anaphylaxis. However, causality is rarely addressed in available reports. For randomized trials reporting adverse effects, incidence was approximately equal in milk thistle and control groups.
Hope it helps,