Milk Thistle

Background

• Milk thistle is a tall plant with large purple flowers that is native to Europe. It was introduced into North America by early colonists.
• The main constituent of milk thistle extract is silymarin, a mixture of compounds.
• Historically, milk thistle was used for liver disorders and to increase breast milk production.
• Milk thistle is promoted as a dietary supplement for liver disorders, diabetes, and other conditions.

How Much Do We Know?

• There isn’t enough high-quality evidence to allow definite conclusions to be reached about the effects of milk thistle on health conditions in people.

What Have We Learned?

• Results from a small number of studies in people show that milk thistle extracts may help to control blood sugar in people with type 2 diabetes. Most of this research was done in Middle Eastern countries, and it is unclear whether the same results would be seen in other parts of the world.
• Results from clinical trials of milk thistle for liver diseases, such as alcohol-related liver disease, hepatitis B and C, non-alcoholic fatty liver disease, and liver problems caused by cancer chemotherapy, low oxygen levels, or toxins have been conflicting or too limited to allow conclusions to be reached. Two studies funded by the National Center for Complementary and Integrative Health—one on hepatitis C and one on non-alcoholic steatohepatitis (a progressive form of fatty liver disease)—did not show benefits from supplementation with the milk thistle extract silymarin.
• It is unclear whether milk thistle has an effect on breast milk production.
• It’s not known whether milk thistle is helpful for other conditions.

What Do We Know About Safety?

• Milk thistle, when taken orally, appears to be well tolerated. The most common side effects are digestive symptoms such as bloating, nausea, and gas.
• Concerns have been raised about poor chemical and microbiological quality of milk thistle dietary supplements sold in the United States and other countries. Some products have been found to contain amounts of silymarin substantially different from what’s stated on the label or to be contaminated with pesticides, microorganisms, or mycotoxins (harmful substances produced by molds or other fungi).
• Milk thistle may cause allergic reactions, particularly among people who are allergic to related plants (for example, ragweed, chrysanthemum, marigold, and daisy).
• If you take any type of medicine, talk with your health care provider before using milk thistle or other herbal products; some herbs and medicines interact in harmful ways. 
• Little is known about whether it’s safe to use milk thistle during pregnancy or while breastfeeding.

Keep in Mind

• Take charge of your health—talk with your health care providers about any complementary health approaches you use. Together, you can make shared, well-informed decisions.

• Federal Regulation of Herbal Products
  • Depending on what’s in them, how they’re intended to be used, and how they’re administered (orally or topically), herbal products are regulated in a variety of ways. Many herbal products intended for oral use are marketed as dietary supplements. The rules for making and distributing dietary supplements are less strict than those for drugs.
  • Unlike drugs, dietary supplements are not approved by the U.S. Food and Drug Administration (FDA) before they are sold to the public. When public health concerns arise about the safety of a dietary supplement or an ingredient including an herb, the FDA can take action to protect the public. Manufacturers and distributors of supplements are responsible for evaluating the safety and labeling of their products before marketing to ensure that they meet all regulatory requirements. 

For More Information

• Using Dietary Supplements Wisely
• Know the Science: How Medications and Supplements Can Interact
• Know the Science: How To Make Sense of a Scientific Journal Article

Key References