Milk Thistle Benefits, Potential Side Effects, and Cautions

This fact sheet provides basic information about milk thistle—common names, what the science says, potential side effects and cautions, and resources for more information.

Common Names—milk thistle, Mary thistle, holy thistle. Milk thistle is sometimes called silymarin, which is actually a mixture of the herb’s active components, including silybinin (also called silibinin or silybin).
Latin Name—Silybum marianum

Milk thistle is a flowering herb native to the Mediterranean region. It has been used for thousands of years as a remedy for a variety of ailments, and historically was thought to have protective effects on the liver and improve its function.

Today, its primary folk uses include liver disorders such as cirrhosis and chronic hepatitis and gallbladder disorders. Other folk uses include lowering cholesterol levels, reducing insulin resistance in people who have both type 2 diabetes and cirrhosis, and reducing the growth of breast, cervical, and prostate cancer cells.

Silymarin, which can be extracted from the seeds (fruit) of the milk thistle plant, is believed to be the biologically active part of the herb. The seeds are used to prepare capsules, extracts, powders, and tinctures.

What the Science Says about Milk Thistle

Previous laboratory studies suggested that milk thistle may benefit the liver by protecting and promoting the growth of liver cells, fighting oxidation (a chemical process that can damage cells), and inhibiting inflammation.

However, results from small clinical trials of milk thistle for liver diseases have been mixed, and two rigorously designed studies found no benefit.

A 2012 clinical trial, co-funded by NCCAM and the National Institute of Diabetes and Digestive and Kidney Diseases, showed that two higher-than-usual doses of silymarin were no better than placebo for chronic hepatitis C in people who had not responded to standard antiviral treatment.
The 2008 Hepatitis C Antiviral Long-Term Treatment Against Cirrhosis (HALT-C) study, sponsored by the National Institutes of Health (NIH), found that hepatitis C patients who used silymarin had fewer and milder symptoms of liver disease and somewhat better quality of life but no change in virus activity or liver inflammation.

Milk Thistle Side Effects and Cautions

In clinical trials, milk thistle appears to be well tolerated in recommended doses. Occasionally, people report various gastrointestinal side effects.
Milk thistle can produce allergic reactions, which tend to be more common among people who are allergic to plants in the same family (for example, ragweed, chrysanthemum, marigold, and daisy).
Milk thistle may lower blood sugar levels. People with diabetes or hypoglycemia, or people taking drugs or supplements that affect blood sugar levels, should use caution.
Tell all your health care providers about any complementary health practices you use. Give them a full picture of what you do to manage your health. This will help ensure coordinated and safe care.

Sources
Agency for Healthcare Research and Quality. Milk Thistle: Effects on Liver Disease and Cirrhosis and Clinical Adverse Effects. Evidence Report/ Technology Assessment no. 21. Rockville, MD: Agency for Healthcare Research and Quality; 2000. AHRQ publication no. 01-E025.
Fried MW, Navarro VJ, Afdhal N, et al. Effect of silymarin (milk thistle) on liver disease in patients with chronic hepatitis C who failed interferon therapy: a randomized, placebo-controlled trial. JAMA. 2012;308(3):274-282.
Ladas E, Kroli DJ, Kelly KM. Milk thistle (Silybum marianum). In: Coates P, Blackman M, Cragg G, et al., eds. Encyclopedia of Dietary Supplements. New York, NY: Marcel Dekker; 2005:467-482.
Milk thistle. Natural Medicines Comprehensive Database. Accessed at naturaldatabase.com on October 7, 2009.
Milk thistle (Silybum marianum), silymarin. Natural Standard Database Web site. Accessed at naturalstandard.com on October 7, 2009.
Milk thistle fruit. In: Blumenthal M, Goldberg A, Brinckman J, eds. Herbal Medicine: Expanded Commission E Monographs. Newton, MA: Lippincott Williams & Wilkins; 2000:257-263.
National Center for Complementary and Alternative Medicine. Hepatitis C: A Focus on Herbal Supplements. National Center for Complementary and Alternative Medicine Web site. Accessed at nccam.nih.gov/health/hepatitisc/hepatitiscfacts.htm on July 18, 2012.
Rambaldi A, Jacobs BP, Gluud C. Milk thistle for alcoholic and/or hepatitis B or C virus liver diseases. Cochrane Database of Systematic Reviews. 2007;(4):CD003620.
Seeff LB, Curto TM, Szabo G, et al. Herbal product use by persons enrolled in the hepatitis C antiviral long-term treatment against cirrhosis (HALT-C) trial. Hepatology. 2008;47(2):605-612.