Written by Julia Green, Alexander Kalisch and the CAM-Cancer Consortium.
Updated January 29, 2015

Milk thistle (Silybum marianum)

Is it safe?

Adverse events

Orally milk thistle is usually well-tolerated 2,3,30. The rate of adverse events of milk thistle was low in clinical trials and in one randomized controlled trial equal to the placebo group 9-11,31-36. Described adverse effects were difficult to distinguish from symptoms of the underlying condition. In rare cases there were complaints of intestinal symptoms, headache and dizziness 2,10,31,33. In a phase I trial in which high doses of silybin phytosome (up to 20g) were given mild transient elevation of bilirubin, and the liver enzymes aspartate transaminase and alanine transaminase (AST and ALT) were reported 36. One participant receiving high dose silybin phytosome who subsequently had surgery developed a thrombo-embolism: this could have been due to the surgery 10.


Milk thistle has been assessed as likely safe when used orally and appropriately 2. There is insufficient information to assess safety in pregnancy and lactation 2.


Milk thistle extract might work as an inhibitor of CYP2C9 2,6,36. There is contradictory evidence about the effect of milk thistle on CYP3A4 2,29,35. A phase II clinical trial of 600mg silymarin in cancer patients found no in vivo effect on irinotecan, CYP34A or UGT1A1 pharmacokinetics 35.


In an animal experiment and in an experiment with an oestrogen responsive breast cancer cell line (MCF-7) a tumour growth was seen when adding silymarin 3. This raises the theoretical risk silymarin could have an oestrogen-like action in women with oestrogen-responsive breast cancer. However the more commonly used milk thistle seed extracts are not known to have oestrogenic effects in vivo 2 ,8. Cautions have been recommended in patients with a known hypersensitivity to plants in the Asteraceae family 2,8,12.

Citation Julia Green, Alexander Kalisch, CAM-Cancer Consortium. Milk thistle (Silybum marianum) [online document]. http://ws.cam-cancer.org/The-Summaries/Herbal-products/Milk-thistle-Silybum-marianum. January 29, 2015.


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