The Benefits and Side Effects of Milk Thistle

Milk thistle (tea, tea leaves, a flower and seeds)
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Milk thistle (Silybum marianum) is an herb said to have properties that promote liver health. The seeds contain silymarin, a group of compounds (including silybin, silydianin, and silychristin) said to have antioxidant and anti-inflammatory effects and protect liver cells. 

Why People Use It

While milk thistle is most often used for liver conditions, such as hepatitis and cirrhosis, the herb is also said to fight the following health issues:

Some proponents also claim that milk thistle can protect against certain forms of cancer, including breast cancer and prostate cancer.

Health Benefits

Here's a look at the science behind the potential health benefits of milk thistle:

Liver Disease: Some preliminary research suggests that silymarin may improve liver function by keeping toxic substances from binding to liver cells. However, studies on the milk thistle's effectiveness in treating liver disorders have yielded mixed results.

For example, most clinical evidence indicates that milk thistle neither improves liver function nor reduces the risk of mortality in people with alcoholic liver disease, hepatitis B, or hepatitis C, according to a report published in the American Journal of Gastroenterology in 2005.

Furthermore, some small studies have shown that milk thistle may improve liver function in people with cirrhosis, while other clinical trials have demonstrated that milk thistle may be of little or no benefit to people with this disease.

Hepatitis C: Milk thistle is sometimes used by people with chronic hepatitis C (a viral infection that can attack and damage the liver). A survey of 1,145 participants in the NIH-funded HALT-C (Hepatitis C Antiviral Long-Term Treatment Against Cirrhosis) trial found that 23 percent of participants were using herbal supplements, with milk thistle being by far the most common.

Examining the participants' medical and lifestyle histories, participants reported that milk thistle was associated with fewer and milder symptoms of liver disease and a somewhat better quality of life, however, there was no change in virus activity or level of inflammation in the liver. 

A 2012 trial published in JAMA examined the use of milk thistle (420 mg silymarin or 700 mg silymarin, taken three times per day) or a placebo for 24 weeks. At the end of the treatment period, researchers found that milk thistle was no better than a placebo in lowering blood levels of an enzyme that indicates liver damage. 

Another study, published in BioMed Research International, looked at previously published studies on silymarin for people with chronic hepatitis C infection. In their analysis, silymarin was not found to be better than a placebo in improving laboratory values (ALT and HCV RNA) or quality of life.

Diabetes: Several studies have shown that milk thistle may be beneficial for people with diabetes.

The most recent research on milk thistle and diabetes includes a study published in Phytomedicine in 2015. For the study, 40 people with diabetes were treated with either silymarin or a placebo for 45 days. At the study's end, members of the silymarin group showed a greater improvement in antioxidant capacity and a greater reduction in inflammation in comparison to those given the placebo.

According to the study's authors, these findings suggest that silymarin may benefit diabetes patients by reducing oxidative stress (a process known to play a major role in the development of diabetic complications).

Additionally, several small clinical trials conducted in recent years have found that milk thistle may aid in diabetes control by regulating blood sugar levels and preventing the progression of diabetes-related kidney damage.

Learn more about milk thistle and diabetes.

Seasonal Allergies: A small study published in Otolaryngology and Head and Neck Surgery in 2011 shows that silymarin may help treat seasonal allergies. In a clinical trial involving 94 people with seasonal allergies, researchers observed that those treated with silymarin for one month had a significantly greater improvement in the severity of their symptoms (compared to those given a placebo for one month).

Possible Side Effects

Milk thistle may trigger a number of adverse side effects, including nausea, diarrhea, abdominal bloating, and gas. It may cause headaches, indigestion, joint pain, and sexual dysfunction. Allergic reactions such as hives and difficulty breathing are possible. People with allergies to daisies, artichokes, kiwi, common thistle, or plants in the aster family may also be allergic to milk thistle.

A study published in the International Journal of Food Microbiology found that a high percentage of milk thistle supplement samples tested were contaminated with fungi. Whole seeds had the highest levels, followed by the herb (no live fungi were found in tea bags, liquid extracts, capsules, or soft gels). Fungi produce toxic compounds known as mycotoxins. 

Milk thistle may lower blood sugar levels, so it should be used with caution by people with diabetes and by anyone taking medications or supplements that affect blood sugar levels.

Since there is a theoretical risk that milk thistle could have an estrogen-like effect, people with hormone-sensitive conditions such as endometriosis, uterine fibroids, or cancers of the breast, uterus, or ovaries should avoid milk thistle. Milk thistle may theoretically reduce the effectiveness of oral contraceptives by inhibiting an enzyme called beta-glucuronidase.

Milk thistle can change the way your body metabolizes drugs in the liver and interact with medications.

Pregnant and breastfeeding women should avoid milk thistle. You can get tips on using supplements here, but keep in mind that the FDA doesn't regulate dietary supplements, so the purity and source can vary widely. if you're considering the use of milk thistle, be sure to talk with your healthcare provider first.

Where to Find It

Dietary supplements containing milk thistle are sold in many natural-foods stores, drugstores, and stores specializing in herbal products. You can also purchase milk thistle products online.

Bottom Line

While laboratory studies suggest that milk thistle has beneficial properties, the effectiveness of milk thistle for the treatment of liver and other conditions needs to be examined further in large, well-designed human trials. If you're interested in learning more about it, talk with your healthcare provider to determine if it's appropriate for you. 

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Article Sources
  • Ebrahimpour Koujan S, Gargari BP, Mobasseri M, Valizadeh H, Asghari-Jafarabadi M. Effects of Silybum marianum (L.) Gaertn. (silymarin) extract supplementation on antioxidant status and hs-CRP in patients with type 2 diabetes mellitus: a randomized, triple-blind, placebo-controlled clinical trial. Phytomedicine. 2015 Feb 15;22(2):290-6.
  • Fried MW, Navarro VJ, Afdhal N, et al. Effect of silymarin (milk thistle) on liver disease in patients with chronic hepatitis C unsuccessfully treated with interferon therapy: a randomized controlled trial. JAMA. 2012 Jul 18;308(3):274-82.
  • Rambaldi A, Jacobs BP, Iaquinto G, Gluud C. Milk thistle for alcoholic and/or hepatitis B or C liver diseases--a systematic cochrane hepato-biliary group review with meta-analyses of randomized clinical trials. Am J Gastroenterol. 2005 Nov;100(11):2583-91.
  • 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.
  • Yang Z, Zhuang L, Lu Y, Xu Q, Chen X. Effects and tolerance of silymarin (milk thistle) in chronic hepatitis C virus infection patients: a meta-analysis of randomized controlled trials. Biomed Res Int. 2014;2014:941085.