Milk thistle and the liver: evidence, safety and what to ask first

Milk thistle is a plant from the same family of flowers as the daisy. Cancer Research UK explains that people have used its fruit and seeds for hundreds of years as a herbal remedy for some liver problems. Its scientific name is Silybum marianum, and other names include lady’s thistle, St Mary’s thistle and holy thistle.

The compound of interest is silymarin, an extract of the seeds, which the charity describes as an antioxidant that protects against cell damage. Silymarin contains at least four major compounds, of which silybin is the most active. Supplements come as capsules, tablets, powder and liquid extract, and the powder can be made into a tea.

Online marketing often claims that milk thistle detoxifies and protects the liver, gallbladder and spleen. Cancer Research UK notes that companies make these claims but states that it is only possible milk thistle may play a part in treating liver disease and some cancers, and that a lot more research with larger numbers of people is needed first.

The charity describes laboratory research suggesting a possible role in some liver diseases such as hepatitis and cirrhosis, plus a few small trials in people with cancer. In children with acute lymphoblastic leukaemia, small studies of around 80 children each looked at silymarin alongside chemotherapy and reported improvements in some liver and kidney results and reduced heart toxicity from doxorubicin, with larger studies still needed. A trial of a silymarin skin gel during breast radiotherapy reported less severe radiation skin reaction.

These findings are specialist, small and preliminary. None of them amounts to evidence that a supplement bought from a shop will protect an ordinary liver or reverse disease, and the charity is clear that milk thistle is not a treatment for cancer.

Cancer Research UK says milk thistle is generally considered safe but you should check with your doctor first. Rarely it can cause diarrhoea, feeling sick and heartburn. People with diabetes are advised to consult a doctor, and people with liver problems should do the same because high doses can raise levels of bilirubin and liver enzymes, which are the markers doctors use to monitor liver health.

The NHS adds general rules for herbal medicines. They are not recommended for children, should not be taken in pregnancy or breastfeeding, or if you have kidney or liver problems, or if you take certain medicines, including statins, antidepressants, diabetes medicines, blood pressure tablets, warfarin and hormonal contraception. Always check with a pharmacist first.

Cancer Research UK also notes that antioxidant supplements are generally not recommended during chemotherapy or radiotherapy because they may block some of the cancer-killing effects of treatment. Anyone having cancer treatment should ask their cancer team before taking milk thistle.

Supplements are not a substitute for diagnosis. Taking alcohol-related liver disease as one example, the NHS says the aim of treatment is usually to help you stop drinking completely, with support from a GP or specialist alcohol service, and that the liver can often recover if you stop drinking in the early stages. Later stages are harder to treat, and a badly damaged liver may need hospital care or a transplant assessment.

The NHS advises seeing a GP if you have symptoms of liver disease, such as persistent tiredness, loss of appetite, weight loss or yellowing of the skin or whites of the eyes. It says to call 999 or go to A&E if you vomit blood, have black sticky poo, or feel suddenly confused or drowsy.

If you already have a diagnosed liver condition, tell your specialist or GP before starting any supplement. Blood tests used to monitor the liver can be affected, and your team will want to know about anything that might change the results or add strain to the liver.

A short conversation with a pharmacist or GP can settle most concerns. The NHS says to check with a pharmacist before taking a herbal medicine if you are already on other medicines, including those bought without a prescription.

Useful questions include whether the product is compatible with your current medicines, whether it could affect liver blood tests, whether it is suitable alongside any cancer treatment, and whether it is registered with a traditional herbal registration (THR) mark. Cancer Research UK also notes that a doctor knows your individual situation and can give advice that general information cannot.