[anonymous]

I took milk thistle once, years ago, I forget why now. Anyway, I felt nauseous 20 minutes later and vomited.

Hope this helps. Below is an excerpt from an informative blog, click on link for the full article and comments. http://skeptvet.com/Blog/2011/04/milk-thistle-in-dogs-and-cats/

As far as risks, there appear to be few. Nausea, diarrhea, and other gastrointestinal effects are sometimes seen, and allergic reactions have been reported in humans.
So overall, the in vitro and laboratory animal evidence indicates it is plausible that milk thistle extract might have beneficial effects, though harmful effects in some situations could be expected as well. In humans, the clinical trials show weak evidence for benefit in diabetics and inconsistent but generally negative evidence for benefit in alcoholic or hepatitis-associated liver disease. Very little experimental, and apparently no high quality or controlled clinical research exists in dogs and cats. What there is suggests a benefit is possible. But we must bear in mind that preliminary, low-quality trials of milk thistle in humans looked promising but were not supported by subsequent better quality trials.
A clinical trial comparing animals with naturally occurring liver disease treated identically except for receiving either milk thistle or a placebo would be quite useful. In the meantime, use of the compound is not unreasonable given the suggestive low-level evidence, but it is not much more than a hopeful shot in the dark at this point.