Rare side effects of milk thistle include diarrhea, gas, bloating, nausea and upset stomach, according to WebMD. Studies indicate it can be combined with other herbs and taken safely for up to 41 months in tea or as a supplement.
Due to its estrogen-like attributes, women who have endometriosis or fibroid tumors should avoid milk thistle. Individuals with ragweed allergies should also avoid milk thistle, as it is a relative of the ragweed and daisy family.
Patients have used the herb as a natural supplement to treat a variety of liver problems, including hepatitis, cirrhosis, jaundice and gallbladder disorders, explains WebMD. Silymarin, the main ingredient in milk thistle, is an antioxidant and anti-inflammatory.