Side effects of milk thistle

Updated July 18, 2017

Milk thistle, or Silybum marianum, which is a member of the sunflower family Asteraceae, is a plant with reddish-purple flowers and brown fruit that can be found in the Mediterranean. The plant's name comes from the milk-coloured liquid produced when the leaves are crushed, and the active ingredient is the flavonoid mixture silymarin, which contains silidianin, silicristin and silibinin; also, it contains anti-inflammatory and antioxidant properties. For centuries, milk thistle has been used to treat liver and gallbladder disorders and today is sold in health food stores in tablets, capsules, extracts and tinctures. Whilst usually well-tolerated, milk thistle supplements may cause side effects.

Stomach upset

Milk thistle can be taken safely in recommended doses for up to six years, states However, common effects that are reported tend to involve digestive disturbance. For example, after you start taking milk thistle you may experience an upset stomach or heartburn with stomach pain, diarrhoea and bloating, states the National Centre for Complementary and Alternative Medicine. Other side effects that may present are nausea with or without vomiting, excess gas, and a drop in appetite. These side effects likely will subside after the body gets used to taking the supplement. You should contact your doctor, however, if such symptoms persist or increase in severity.

Glucose changes stresses that taking milk thistle may affect serum glucose levels; specifically, this supplement may lower your blood sugar levels. Therefore, it is important that patients with diabetes as well as those prone to hypoglycaemia consult their doctor when considering milk thistle. It may be the case that you can still take milk thistle but with periodic monitoring of your blood sugar to ensure a safe level is being maintained.


As with any ingested substance, hypersensitivity to an ingredient of that substance is possible, and milk thistle is no exception. People who are allergic to plants in the aster family, artichokes, kiwi or to any of milk thistle's components - silibinin, silychistin, silydianin, silymonin, siliandrin - may have allergic reactions to milk thistle. In cases where you experience allergic reactions to any of the above, such as a rash or hives, then you should avoid taking milk thistle in any form, warns


It is imperative that patients be aware of the fact that active ingredients in milk thistle can interact with certain medications, as emphasised by the University of Maryland Medical Centre. For example, milk thistle ingredients can interfere with fexofenadine, an allergy drug; statins, or cholesterol medicine; phenytoin, an anti-seizure drug; halothane, an anesthetic; and some cancer medications. Drug complications may occur if milk thistle is taken also with anti-psychotics, anti-anxiety drugs and blood-thinning medications. If you are on any of these medications, discuss your drug regime and the possible effects of milk thistle with your GP before taking this supplement.

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About the Author

A.M. Tacon is an associate professor of health at Texas Tech University. Her research interests include psychosocial factors in cancer, complementary therapies and stress reduction in individuals with cancer. Dr. Tacon runs mindfulness-based stress reduction programs for women with breast cancer, which is based on various forms of mindfulness meditation.