Milk thistle may cause diarrhea, stomach upset, bloating, gas or nausea. It is generally safe to be taken for up to 41 months and causes few, if any, serious side effects, according to WebMD.
Milk thistle is touted as a natural remedy for liver-related problems, such as jaundice, hepatitis and cirrhosis, states WebMD. Some individuals also point to its use for lowering cholesterol, improving symptoms of type 2 diabetes, and reducing growth of cancer cells. As of 2015, there is insufficient medical research to determine if milk thistle is effective for liver problems, but early research suggests it may improve insulin resistance, which is characteristic of type 2 diabetes.
Women who are pregnant or breastfeeding should not take milk thistle, indicates WebMD. Others who should avoid taking milk thistle include those who are allergic to ragweed and related plants, those with a history of hormone-related cancers, and women with fibroids or endometriosis.
Milk thistle may potentially interact with certain drugs, and a doctor should be consulted before taking milk thistle together with drugs such as antipsychotics, birth control pills, hormone therapy, Dilantin or halothane, states University of Maryland Medical Center. Milk thistle may also interact with other drugs, such as allergy medication, cholesterol drugs, or other medications that are processed by the same liver enzymes.