Milk thistle (Silybum marianum) is one of the best-selling plants for the liver. Its extract, silymarin, has been tested in dozens of clinical trials in people with hepatitis, alcohol-related liver disease or fatty liver. What do these studies actually show? Here is what research establishes, what remains uncertain, studied doses, absorption and precautions.
Key takeaways
- Milk thistle supplies silymarin, a flavonolignan mixture extracted from its fruits, often called seeds.
- A Cochrane review of 18 trials found no mortality effect in alcohol-related or hepatitis-related liver diseases.
- In fatty liver, a meta-analysis observed lower transaminases, but biopsy trials remain mixed.
- Silymarin is well tolerated but poorly absorbed. EMA advises against use during pregnancy, breastfeeding and under 18.
What is milk thistle?
Milk thistle is a spiny plant in the Asteraceae family, native to the Mediterranean basin. It is recognised by large dark-green leaves marbled with white veins and purple flowers. According to Christian legend, the marbling represents drops of the Virgin Mary's milk, explaining its nickname “Our Lady's milk”.
The part used is the fruit, a small dry achene commonly called a seed. Consumed as food and a medicinal plant for centuries, milk thistle is now mainly sold as extracts concentrated in silymarin.
What is silymarin?
Silymarin is not one molecule, but a mixture of flavonolignans: silybin A and B, isosilybin A and B, silychristin and silydianin. Silybin, also called silibinin, is the most studied. Commercial extracts are obtained using solvents (ethanol, acetone, ethyl acetate…), concentrating these compounds far above their seed content.
In laboratory and animal studies, silymarin behaves as an antioxidant and modulates inflammation and fibrosis. These properties explain liver interest, but do not prove a human effect: only clinical trials can answer that.
What do liver studies show?
In brief: mixed results. The most rigorous trials show no clear benefit on important outcomes such as mortality or biopsy-assessed liver condition. Some blood-marker signals exist, but their significance remains to be confirmed.
Hepatitis and alcoholic liver disease: the Cochrane review
In 2007, Cochrane pooled 18 randomised trials involving 1,088 patients with alcoholic liver disease or hepatitis B or C. Milk thistle reduced neither overall mortality nor complications. Liver-related mortality was lower across all trials, but not in methodologically good-quality ones. The authors considered these results to question the plant's usefulness for these diseases.
A more recent US trial supports this. In 154 hepatitis C patients, 420 or 700mg silymarin three times daily for 24 weeks did not lower transaminases (ALT) more than placebo.
Fatty liver: marker signals
Non-alcoholic fatty liver is the most studied setting today. A 2021 meta-analysis of eight randomised trials observed a greater transaminase reduction with silymarin than placebo, independently of weight loss. Authors nevertheless highlighted uneven quality of included studies.
Biopsy trials are less clear. In Malaysia, 99 steatohepatitis (NASH) patients received 700mg three times daily or placebo for 48 weeks. The primary outcome, at least a 30% activity-score reduction, did not differ between groups (32.7% versus 26.0%). Fibrosis regressed in 22.4% of the silymarin group versus 6.0% with placebo, a secondary result requiring confirmation. A US trial in 78 patients showed no benefit.
What about healthy people?
No trial has shown that milk thistle makes a healthy liver function better. The European Medicines Agency (EMA) accepts traditional milk thistle medicines for relieving digestive complaints and supporting liver function. This use rests on long-standing use, not conclusive trials, and assumes a doctor has ruled out serious disease.
Milk thistle and blood glucose: a possibility requiring confirmation
A meta-analysis of five trials in 270 type 2 diabetes patients observed a fasting glucose reduction of around 27mg/dl and a glycated haemoglobin reduction of around one percentage point with silymarin. Authors judged evidence low-quality and made no recommendation. The US NIH complementary medicine centre notes that most studies come from the Middle East. Other ingredients are studied here, such as berberine and its glucose trials or gymnema and sugar cravings.
Milk thistle and cancer: where does research stand?
Laboratory studies of silymarin and cancer cells are numerous, but none establishes an anticancer effect in humans. Clinical research mainly concerns liver tolerance during chemotherapy. In a pilot trial in 50 children with acute lymphoblastic leukaemia and liver injury, milk thistle did not change transaminases at 28 days. A favourable trend appeared at 56 days, but no more. Any use during cancer treatment must be discussed with the oncology team.
Is silymarin well absorbed?
No; this is a major limitation. Silymarin dissolves poorly in water, crosses the intestinal wall poorly and is rapidly transformed by the body. In three volunteers taking 600mg extract, flavonolignans were eliminated within hours. At peak, the free form represented only around 17% of the circulating total; the rest was already bound to elimination molecules. Formulation techniques aim to improve absorption, but clinical value remains poorly documented.
Which form and dose?
The table gives EMA doses for traditional medicines and doses used in recent trials.
| Form | Reference dose | Note |
|---|---|---|
| Tea from crushed fruits | 3 to 5 g, 2 to 3 times daily | Silymarin poorly soluble in water |
| Fruit powder | 300 to 600mg, 2 to 3 times daily | Less concentrated than an extract |
| Dry extract | 200 to 1,000mg daily depending on extract | Check silymarin content |
| Silymarin in trials | 420 to 700mg, 3 times daily | Benefit not demonstrated |
Recent trials therefore tested high doses, 1.26 to 2.1 g silymarin daily, without clear benefit. It is also combined with other plants in supplements. For differences between capsules, softgels and tablets, see our dedicated article.
Adverse effects, contraindications and interactions
Milk thistle is generally well tolerated. Doses of 700mg three times daily for 24 to 48 weeks caused no more adverse effects than placebo. Reported effects are mainly digestive (bloating, nausea, diarrhoea). Headaches and allergic reactions are possible.
- Allergy: avoid with an Asteraceae allergy, such as ragweed, chrysanthemum, marigold or daisy.
- Pregnancy, breastfeeding, under 18: safety is not established and EMA advises against use.
- Diabetes: a glucose effect cannot be excluded. Discuss it with your doctor if taking treatment.
- Medicines: few documented interactions and no major effect on liver enzymes metabolising medicines. Nevertheless, caution with narrow-therapeutic-index medicines such as oral anticoagulants, thyroid hormones or immunosuppressants.
Yellowing skin or eyes, dark urine or pale stools require prompt medical attention.
What can a milk thistle supplement claim?
No milk thistle health claim is authorised by the EU. A supplement therefore cannot claim a liver effect from this plant. The only authorised claim mentioning the liver concerns a nutrient: “choline contributes to the maintenance of normal liver function”, for food supplying at least 82.5mg choline per 100 g, 100 ml or serving. This nutrient is presented in our article on choline.
Frequently asked questions
Is milk thistle dangerous for the liver?
The NIH LiverTox database, recording liver injuries from medicines and herbs, does not associate it with any liver injury case. Risk relates more to product quality: analyses found silymarin content differing from labels and even contaminants (pesticides, microbes, mould toxins). Choose an extract stating its content.
How long can milk thistle be taken?
Daily use for 24 to 48 weeks was well tolerated in trials. EMA nevertheless recommends consulting if digestive symptoms persist beyond two weeks because they may have another cause. Prolonged use deserves medical advice.
Should it be taken morning or evening?
No study has compared intake times. Depending on preparation, the EMA monograph specifies two to four daily doses, before meals for tea, powder and some extracts. Clinical trials divided the dose into three intakes.
Is milk thistle tea useful?
Tea is among EMA-recognised traditional uses, at 3 to 5 g crushed fruit per cup. Since silymarin dissolves poorly in water, an infusion probably extracts little. No clinical trial has evaluated the tea.
Does milk thistle lower cholesterol?
The data do not support that. In the diabetes meta-analysis, silymarin had no lipid-profile effect, including cholesterol and triglycerides.
Does milk thistle help with weight loss?
There is no basis to claim this: trials measured liver markers, not weight changes. In fatty liver, the transaminase reduction with silymarin was independent of weight loss.
Milk thistle and NAC: what is the connection?
Both are often combined in liver formulas but are very different substances. NAC is a precursor to glutathione, the main antioxidant molecule our cells make. Learn more in our articles on N-acetylcysteine and glutathione and its absorption.
Conclusion
Milk thistle is well tolerated and extensively studied, but benefit evidence remains thin. Cochrane shows no mortality effect in hepatitis and alcoholic liver diseases. In fatty liver, silymarin lowers transaminases in some trials without clear lesion improvement. Absorption is poor and no health claim is authorised for the plant. If considering it, choose an extract stating silymarin content and discuss it with your doctor, especially with liver disease, diabetes or medication.
Sources
- Rambaldi A, Jacobs BP, Gluud C (2007). Milk thistle for alcoholic and/or hepatitis B or C virus liver diseases. Cochrane Database of Systematic Reviews, (4), CD003620. doi:10.1002/14651858.CD003620.pub3
- Fried MW et al. (2012). Effect of silymarin (milk thistle) on liver disease in patients with chronic hepatitis C unsuccessfully treated with interferon therapy: a randomized controlled trial. JAMA, 308(3), 274-282. doi:10.1001/jama.2012.8265
- Kalopitas G et al. (2021). Impact of Silymarin in individuals with nonalcoholic fatty liver disease: A systematic review and meta-analysis. Nutrition, 83, 111092. doi:10.1016/j.nut.2020.111092
- Wah Kheong C, Nik Mustapha NR, Mahadeva S (2017). A Randomized Trial of Silymarin for the Treatment of Nonalcoholic Steatohepatitis. Clinical Gastroenterology and Hepatology, 15(12), 1940-1949.e8. doi:10.1016/j.cgh.2017.04.016
- Navarro VJ et al. (2019). Silymarin in non-cirrhotics with non-alcoholic steatohepatitis: A randomized, double-blind, placebo controlled trial. PLoS One, 14(9), e0221683. doi:10.1371/journal.pone.0221683
- Voroneanu L et al. (2016). Silymarin in Type 2 Diabetes Mellitus: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. Journal of Diabetes Research, 2016, 5147468. doi:10.1155/2016/5147468
- Ladas EJ et al. (2010). A randomized, controlled, double-blind, pilot study of milk thistle for the treatment of hepatotoxicity in childhood acute lymphoblastic leukemia (ALL). Cancer, 116(2), 506-513. doi:10.1002/cncr.24723
- Wen Z et al. (2008). Pharmacokinetics and metabolic profile of free, conjugated, and total silymarin flavonolignans in human plasma after oral administration of milk thistle extract. Drug Metabolism and Disposition, 36(1), 65-72. doi:10.1124/dmd.107.017566
- Javed S, Kohli K, Ali M (2011). Reassessing bioavailability of silymarin. Alternative Medicine Review, 16(3), 239-249. PubMed 21951025
- Soleimani V et al. (2019). Safety and toxicity of silymarin, the major constituent of milk thistle extract: An updated review. Phytotherapy Research, 33(6), 1627-1638. doi:10.1002/ptr.6361
- European Medicines Agency (2018). European Union herbal monograph on Silybum marianum (L.) Gaertn., fructus (EMA/HMPC/294187/2013). ema.europa.eu
- National Center for Complementary and Integrative Health (2025). Milk Thistle: Usefulness and Safety. NIH. nccih.nih.gov
- LiverTox (regularly updated). Milk Thistle. LiverTox: Clinical and Research Information on Drug-Induced Liver Injury, National Institute of Diabetes and Digestive and Kidney Diseases. PubMed 31644124
- European Commission (2012). Regulation (EU) No 432/2012 of 16 May 2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children's development and health. Official Journal of the European Union, L 136, p. 1 (consolidated version). eur-lex.europa.eu
Food supplements do not replace a varied, balanced diet or a healthy lifestyle. Ask a healthcare professional for advice if you are taking medication, pregnant or breastfeeding.
