Milk Thistle

Is milk thistle safe?

For most adults, yes. Milk thistle is one of the few herbal liver supplements that is not itself a cause of liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary, and in trials even very high doses caused no more side effects than placebo. The catch is the product rather than the plant. Independent testing over a decade has found that many bottles contain much less of the active ingredient than they suggest, and some contain mould toxins and pesticide residues.

It also does less than its marketing claims. NCCIH, part of the US National Institutes of Health, says there is not enough good evidence to draw firm conclusions about any health effect.

Does milk thistle harm or help the liver?

LiverToxLiverToxA free US National Institutes of Health database that summarises what is known about liver injury caused by medicines, herbs and supplements.More in the glossary, the US government database on drug-induced liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary, says milk thistle has not been implicated in causing liver injury and is still used widely by people with acute and chronic liver disease. That puts it in a different class from turmeric and green tea extract, which both have documented cases of serious liver damage.

The evidence for benefit is thin. The best US trial, published in JAMA in 2012, gave 154 people with hepatitis C either 420 mg or 700 mg of silymarin three times a day, or placebo, for 24 weeks. Only two people in each group reached the target fall in liver enzymes, and there was no difference in virus levels or quality of life. The side effects were comparable with placebo, which is reassuring for safety even though the result was disappointing.

Side effects and allergy

The most common side effects are digestive: bloating, nausea and gas. The one to know about is allergy. Milk thistle belongs to the daisy family, and NCCIH warns that people allergic to related plants such as ragweed, chrysanthemums, marigolds and daisies are more likely to react. If you have hay fever from ragweed, ask a doctor or pharmacist before trying it.

Little is known about safety in pregnancy or breastfeeding, even though milk thistle has a long folk history as a way to increase breast milk.

Interactions

Silymarin inhibits some liver enzymes in laboratory studies, which raised worries about drug interactions. A 2019 review of the human data concluded that these effects are unlikely to matter in clinical practice. There are two exceptions worth knowing.

• Losartan: in a 2009 study of 12 healthy men, 140 mg of silymarin three times a day for two weeks raised blood levels of losartan and lowered its active metabolite, depending on the person's genes.
• Diabetes drugs: a 2016 meta-analysis of five small trials found that silymarin lowered fasting blood sugar by about 27 mg/dL and HbA1c by about 1 point. The authors called the evidence low quality, but anyone on insulin or other glucose-lowering drugs should watch for low blood sugar.

What is actually in the capsule?

This is where milk thistle loses most of its score. Silymarin is usually sold as an extract "standardised to 80%", and the numbers often do not hold up.

ConsumerLab attributes part of the shortfall to suppliers certifying extracts with older methods that are not specific to silymarin and overstate it. The contamination findings matter more for safety. A 2015 study screening for 57 mould toxins in herbal supplements found the highest levels in milk thistle products, up to 37 mg per kg in total. Most of the toxins came from Fusarium and Alternaria moulds.

ConsumerLab notes that most clinical studies used about 200 mg of silymarin two or three times a day, so a product with 17 mg a day is unlikely to do much.

How to choose and use it

1. Check the silymarin amount in milligrams on the label, not just the extract weight.
2. Pick a third-party testedThird-party testingTesting of a supplement by an independent lab to check that it contains what the label says and is free of set contaminants.More in the glossary product (USP Verified, NSF or a ConsumerLab approval), which at least checks what is in the capsule.
3. Skip it if you react to ragweed or other daisy-family plants.
4. Do not use it instead of medical care for hepatitis, fatty liver or cirrhosis. If you take NAC or other liver supplements as well, tell your doctor.

How we scored it

Organ toxicity (88) and dose safety (85) are strong: no liver injuryDrug-induced liver injuryLiver damage caused by a medicine, herbal product or supplement. It is usually rare and unpredictable, but it can be serious.More in the glossary cases in LiverToxLiverToxA free US National Institutes of Health database that summarises what is known about liver injury caused by medicines, herbs and supplements.More in the glossary and placebo-like side effects at up to 2,100 mg of silymarin a day. Regulatory actions (85) reflect a legal supplement with only a few old manufacturing recalls, and interactions (78) a small number of modest ones. Product quality (40) is the weak point, with repeated tests finding low silymarin and contamination.