Silymarin: Side Effects and Contraindications

Silymarin, the flavonolignan complex extracted from milk thistle seeds, is one of the best-selling herbal products in Europe. Gym-goers often reach for it as "liver protection", and it is frequently described as practically free of side effects. Our editorial team looked at what clinical trials and regulatory assessments actually say about its tolerability, interactions and the groups who should be cautious.

What silymarin is

Milk thistle (Silybum marianum) is a member of the daisy family, recognisable by the white veins on its spiny leaves. The fruits, commonly called seeds, contain silymarin, a mixture of compounds including silybin (also known as silibinin), isosilybin, silychristin and silydianin. Silybin is generally considered the most biologically active component.

Silymarin has poor water solubility and limited oral absorption. Manufacturers have tried to improve this with phospholipid complexes and other formulations, which means that milligram amounts on different labels are not always comparable. Standardised extracts typically state a percentage of silymarin, often calculated as silybin.

In the UK, milk thistle is sold both as a food supplement and, for some products, as a traditional herbal medicine registered by the MHRA under the Traditional Herbal Registration scheme. Registered products carry a THR mark and state indications based on traditional use, not on proven clinical efficacy.

The European Medicines Agency's Committee on Herbal Medicinal Products has published a monograph on milk thistle fruit, which is a useful reference for safety information. Intravenous silibinin is used in some European countries for death cap mushroom poisoning, but that is a hospital treatment unrelated to oral supplements.

Common and rare side effects

Overall, silymarin has a favourable tolerability profile in clinical trials. In the large randomised trial of patients with chronic hepatitis C by Fried and colleagues (2012), adverse events with doses of 420 mg and 700 mg three times daily were similar to placebo. A phase I study by Hawke and colleagues also found high doses to be generally well tolerated over short periods.

The most commonly reported side effects are digestive:

• mild laxative effect or loose stools;
• bloating, flatulence and abdominal discomfort;
• nausea or a feeling of fullness;
• headache, reported occasionally.

Allergic reactions are uncommon but can be significant. Because milk thistle belongs to the Asteraceae family, people allergic to ragweed, chrysanthemums, marigolds or daisies are more likely to react. Reactions range from rash and itching to, very rarely, anaphylaxis.

Reviews such as Soleimani and colleagues (2019) conclude that serious toxicity in humans is rare at commonly used intakes. However, the quality of over-the-counter products varies, and some adverse event reports may reflect contamination or adulteration rather than silymarin itself.

Contraindications and at-risk groups

The main absolute contraindication is known hypersensitivity to milk thistle or other Asteraceae plants. Beyond that, several groups should only use silymarin after medical advice.

Some laboratory studies suggest milk thistle extracts may have mild oestrogenic effects. The clinical relevance is unclear, but people with breast, uterine or ovarian cancer, endometriosis or fibroids are usually advised to be cautious.

The most important practical risk is indirect. Symptoms such as jaundice, dark urine, persistent upper abdominal pain or markedly raised liver enzymes require urgent assessment. Taking a herbal product instead of seeing a doctor can delay the diagnosis of serious conditions, including drug-induced liver injury.

Drug interactions

Laboratory experiments show that silymarin components can inhibit several cytochrome P450 enzymes and drug transporters. In humans, most interaction studies at typical doses have found small or clinically insignificant effects, but there are exceptions and uncertainties.

Medicines with a narrow therapeutic window deserve particular care. These include warfarin, some anti-epileptic drugs, ciclosporin and tacrolimus, and certain chemotherapy agents. Even modest changes in their blood levels can matter.

Because silymarin may slightly lower blood glucose, people taking metformin, sulfonylureas or insulin should monitor their glucose when starting it. Interactions with drugs metabolised by CYP2C9, such as some angiotensin receptor blockers, have also been studied, with variable results.

If you take any regular prescription medicine, tell your GP or pharmacist about milk thistle. In the UK, community pharmacists can check interactions quickly, and the Yellow Card scheme allows anyone to report suspected adverse reactions to herbal products.

Does it really protect the liver?

Silymarin has antioxidant and anti-inflammatory activity in laboratory models, which underpins its reputation. Clinical results are less convincing. A Cochrane review by Rambaldi and colleagues (2007) found no significant effect on mortality or complications in alcoholic or viral hepatitis when only high-quality trials were considered.

The Fried trial in hepatitis C patients showed no meaningful improvement in liver enzymes compared with placebo. Earlier reviews, including Saller and colleagues (2001) and Jacobs and colleagues (2002), described a generally safe compound but inconsistent efficacy.

For strength athletes, the issue is often oral anabolic steroids or other hepatotoxic substances. There is no reliable evidence that silymarin prevents liver injury caused by these drugs. Relying on it as a "shield" may create false security around substances that carry serious, sometimes irreversible risks to the liver, heart and endocrine system.

Research on non-alcoholic fatty liver disease continues, and some trials show improvements in certain markers. These findings are preliminary, and weight management, alcohol reduction and physical activity remain the cornerstones of treatment.

Conclusions of the editorial team

Silymarin is generally well tolerated, with mild digestive complaints as the most common side effects. Allergy in people sensitive to the daisy family is the main contraindication.

Caution is warranted in pregnancy, childhood, hormone-sensitive conditions and with narrow-therapeutic-window medicines. Discussing use with a pharmacist is simple and sensible.

Its liver-protective reputation outpaces the clinical evidence. It cannot neutralise the harm of hepatotoxic drugs, and it should never replace medical evaluation of liver symptoms.

For related reading, our editorial team suggests our articles on TUDCA and its side effects, interpreting liver function tests, and drug-induced liver injury in sport.

References

1. 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;308(3):274–282.
2. Rambaldi A, Jacobs BP, Gluud C. Milk thistle for alcoholic and/or hepatitis B or C virus liver diseases. Cochrane Database Syst Rev. 2007;(4):CD003620.
3. Saller R, Meier R, Brignoli R. The use of silymarin in the treatment of liver diseases. Drugs. 2001;61(14):2035–2063.
4. Jacobs BP, Dennehy C, Ramirez G, et al. Milk thistle for the treatment of liver disease: a systematic review and meta-analysis. Am J Med. 2002;113(6):506–515.
5. Soleimani V, Delghandi PS, Moallem SA, Karimi G. Safety and toxicity of silymarin, the major constituent of milk thistle extract: an updated review. Phytother Res. 2019;33(6):1627–1638.
6. Hawke RL, Schrieber SJ, Soule TA, et al. Silymarin ascending multiple oral dosing phase I study in noncirrhotic patients with chronic hepatitis C. J Clin Pharmacol. 2010;50(4):434–449.
7. European Medicines Agency, Committee on Herbal Medicinal Products. European Union herbal monograph on Silybum marianum (L.) Gaertn., fructus. London: EMA; 2018.