Milk thistle has been used for its purported medicinal properties for thousands of years, but its supplement form didn’t explode until recently. Milk thistle supplement sales in the U.S. reached $95 million in 2008, and its global market size skyrocketed to more than $1 billion by 2022, per one analysis.
While such growth is no doubt happy news for milk thistle supplement manufacturers and the people promoting it, some would-be users aren’t sure if there are real benefits to taking the product in any of its various forms.
Here are the advantages and downsides of taking milk thistle supplements, plus whether it is safe to do so.
What is milk thistle?
Native to Europe but common in the U.S. today as well, milk thistle is a flowering plant in the daisy (Asteraceae) family of flowers. This is the same plant family that has more than 30,000 other species including sunflowers, dahlias, chicory, lettuce, sage, marigolds, chrysanthemums, tarragon and, of course, daisies.
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As a plant, milk thistle has bright magenta or purple flowers surrounded by large, prickly leaves, “which expel a milky white sap when they are crushed, giving the plant its name,” explains Perri Halperin, a nutritional scientist and registered dietitian at All the Nutrition and the former clinical nutrition coordinator for the Mount Sinai Health System. This milky substance isn’t usually consumed, however, but one of the plant’s active ingredients, known as silymarin, is. It’s extracted from the plant’s seeds.
These and other extracts are manufactured into a dietary supplement that people take in pill, powder, tincture or liquid form, explains Amy Goodson, a nutritionist and registered dietitian at The Sports Nutrition Playbook.
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Is milk thistle good for you? Good for those with liver conditions?
Despite a variety of broad claims and purported health benefits, we actually know little about whether milk thistle can improve medical conditions or overall health. Only a handful of well-designed clinical studies have been conducted, notes the U.S. National Institutes of Health National Center for Complementary and Integrative Health (NCCIH).
Still, it’s believed that the antioxidant properties in milk thistle could help improve skin issues such as acne or dryness, may reduce cholesterol, and could also boost immune health, says Vanessa Rissetto, a New Jersey-based registered dietitian at Culina Health. “Emerging research suggests it might help manage blood sugar levels as well – though more studies are needed to confirm this,” says Goodson.
She says milk thistle is also often marketed to individuals seeking liver support, “such as those with liver conditions or with a history of heavy alcohol use.” This is because a 2017 meta-analysis shows a “minimal reduction” of liver disease in affected patients who took milk thistle. “It is thought to improve liver function by preventing toxins from attaching to the liver,” says Halperin. “This is, however, not proven.”
Halperin adds that some laboratory studies also suggest that the silymarin within milk thistle may have anti-cancer effects, such as slowing cancer cell division and reproduction and shortening the lifespan of cancer cells, “but more research is needed.”
One of the better understood and sometimes surprising uses of milk thistle is that it has been given as an emergency treatment for mushroom poisoning. This is because “some animal studies have found that milk thistle counteracts the toxic effects of poisonous mushrooms when given within 10 minutes of ingestion,” says Halperin.
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Is milk thistle safe?
While the NCCIH notes that milk thistle supplements are generally safe and well tolerated in recommended doses, “taking too much milk thistle can cause side effects like bloating, gas, diarrhea or nausea,” cautions Goodson.
Rissetto adds that diabetics should exercise caution when taking milk thistle, “since it’s known to affect blood sugar.” Milk thistle should also be avoided “if you are allergic to ragweed, chrysanthemums, marigolds, chamomile or daisies,” says Halperin.
And the supplement may also interact with medications metabolized by the liver, “such as certain blood thinners or cholesterol-lowering drugs,” says Goodson, “so individuals should consult their healthcare provider before using it.”
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