Milk thistle is a plant that comes from the same group of flowers as the daisy. 

What is milk thistle?

Milk thistle is also called holy thistle, Marian thistle, Mary thistle, St. Mary thistle, Our Lady's thistle, wild artichoke, Mariendistel (German), and Chardon-Marie (French). 

The medicinal compound in milk thistle is silymarin, an extract of milk thistle seeds. It is an antioxidant that protects against cell damage.

Silymarin contains 4 compounds, including silybin (the most active), isosilybin, silychristin, and silydianin. Most research has studied silymarin or its major compound silybin, instead of the plant as a whole.

Silymarin has been used in some herbal remedies throughout Europe for thousands of years.

How you take it

Milk thistle supplements are made from the ripe seed, which is the fruit of the plant. They are available as capsules, tablets, powder, and liquid extract.

Powdered milk thistle can be made into a tea. The usual daily dose ranges from 140 to 400 milligrams of silymarin, usually divided into 2 or 3 doses.

Research into milk thistle and cancer

Research in the laboratory has shown that milk thistle might help to treat some liver diseases, such as hepatitis and cirrhosis. 

Researchers have tested silymarin and silybin in the laboratory in cancer cells. They found that silybin:

• may help the chemotherapy drugs cisplatin and doxorubicin work better against ovarian cancer and breast cancer cells
• may directly destroy prostate, breast, and cervical cancer cells
• may slow down prostate cancer cell growth

Research has also looked at silybin in animal cancers. Studies seem to show that it can:

• reduce the side effects of chemotherapy
• make some chemotherapy drugs work better
• stop or slow the growth of some cancer cells
• block tumours from starting or continuing to grow
• help to repair liver tissue
• reduce bowel cancer cell growth in mice

A few trials have looked at milk thistle for people with cancer.

A small randomised clinical trial in children with acute lymphoblastic leukemia found that silymarin reduced the harmful effects of chemotherapy on the liver without stopping the treatment from working. Larger trials are needed.

Some clinical trials have studied milk thistle or silymarin as a treatment for hepatitis, liver cirrhosis, or bile duct disorders. These trials have had mixed results.

In one trial for patients with chronic hepatitis having treatment to boost their immune system, people taking silymarin had fewer symptoms and a better quality of life.

The effects of silymarin in some early studies suggest that it might be helpful in preventing liver inflammation or liver cancer. No published clinical trials have looked at silymarin for preventing or treating cancer in humans. 

A number of companies on the internet claim that milk thistle can help to detoxify and protect your liver. They also claim that it can help to protect other organs of the body such as the gallbladder and spleen.

Some claim that it slows the growth of some types of cancer, including breast and prostate cancer. Although it is possible that milk thistle may play a part in treating liver disease and some types of cancer there is currently no evidence for this.

We need a lot more research with reliable clinical trials before we can be sure that milk thistle will play any part in treating or preventing cancers.

Side effects of milk thistle

Although taking milk thistle is generally considered safe, we would recommend anyone thinking of taking it to talk to their doctor first. Women who are pregnant or breastfeeding should not take this herb.

When taken as recommended, people report very few bad side effects from milk thistle or silymarin.

Several large, carefully designed studies in patients with liver disorders have found that taking silymarin may rarely cause diarrhoea, feeling sick, heartburn, or stomach upset.

At high doses (more than 1,500 milligrams a day) it can cause mild allergic reactions.

We don't yet have enough research to know whether milk thistle may affect cancer treatments and make them more or less effective.

Generally antioxidant supplements are not recommended during chemotherapy or radiotherapy treatment, because they may block some of the cancer killing effects of these treatments.