Safety and side effects

Taken in appropriate doses, oral use of milk thistle appears to be safe.

Milk thistle can cause:

• Gastrointestinal issues, such as diarrhea, constipation, nausea, vomiting and abdominal bloating
• Itchiness
• Headache

If you have diabetes, use milk thistle with caution, since the supplement might lower blood sugar. There is also concern that milk thistle might affect estrogen levels. If you have breast cancer, uterine cancer, ovarian cancer, endometriosis or uterine fibroids, consider avoiding milk thistle.

Milk thistle can cause an allergic reaction, including a severe, potentially life-threatening allergic reaction (anaphylaxis). An allergic reaction is more common in people who are allergic to other plants in the Asteraceae family, such as ragweed, daisies, marigolds and chrysanthemums.