The following comes from
http://altmedicine.about.com
Side Effects and Safety Concerns:
Side effects may include indigestion, headache and itching. Rarely, milk thistle may result in heartburn, gas, diarrhea, joint pain and sexual dysfunction.
People with allergies to daisies, artichokes, kiwi, common thistle or plants in the aster family may also be allergic to milk thistle. There have been several reports of anaphylactic shock in people who have used milk thistle products.
The safety of milk thistle in pregnant or nursing women is unknown.
Theoretically, milk thistle may lower blood sugar levels, so it should be used with caution by people with diabetes, hypoglycemia and those taking medications or supplements that affect blood sugar levels.
There is a theoretical risk that milk thistle could have an estrogen-like effect, so people with hormone-sensitive conditions such as endometriosis, uterine fibroids or cancers of the breast, uterus and ovaries should avoid milk thistle, particularly the above ground parts of the plant.
Milk thistle may reduce the effectiveness of oral contraceptives. One constituent of milk thistle can inhibit an enzyme called beta-glucuronidase, which is involved in the activity of oral contraceptives.