What is it?
An extract of the fruits of milk thistle, Silybum marianum, a thistle in the daisy family (Asteraceae). The small, hard fruits, often called seeds, contain silymarin, a mix of plant compounds called flavonolignans, of which silybin (silibinin) is the main one. Milk thistle fruit is best known as a dietary supplement and a traditional European herbal medicine. In cosmetics the extract is used as a skin-conditioning ingredient.
Key information
- Milk thistle fruit contains 1.3 to 3 percent flavonolignans, known together as silymarin, and 20 to 30 percent fatty oil.
- In 2-year feeding studies by the U.S. National Toxicology Program, milk thistle extract showed no evidence of causing cancer in rats or mice.
- The European Medicines Agency lists allergy to plants in the daisy family (Asteraceae) as a reason not to use milk thistle medicines.
- We found no safety review of milk thistle fruit extract by the Cosmetic Ingredient Review or the EU's Scientific Committee on Consumer Safety.
More information
In the European Union, milk thistle fruit preparations have been sold as herbal medicines for digestive and liver complaints for more than 30 years, and the mature fruit is described in the European Pharmacopoeia, which requires at least 1.5 percent silymarin. In 2018 the European Medicines Agency’s herbal committee (HMPC) published an assessment of milk thistle fruit taken by mouth.
Its review of the toxicology was reassuring. In 2-year feeding studies by the U.S. National Toxicology Program, rats and mice given diets containing up to 5 percent milk thistle extract showed no evidence of cancer. Some bacterial tests for genetic damage were positive and others negative; the committee called the findings inconsistent and sporadic and their relevance to people doubtful. In clinical trials involving more than 4,000 patients, side effects were infrequent and generally mild, mostly digestive upsets, with slight allergic skin reactions such as itching and rash in a few cases. No studies of skin tolerance were available to the committee.
The largest skin data set we found is an observational study of 4,230 people aged 12 to 25 with acne-prone skin who used a skin care product containing milk thistle fruit extract twice daily for 8 to 12 weeks. Dermatologists rated tolerance as good or very good in about 95 percent of them. The study was run by the product’s manufacturer. The extract has not been reviewed by the Cosmetic Ingredient Review (CIR) or the EU’s Scientific Committee on Consumer Safety, and its EU cosmetic ingredient listing carries no restrictions.
The main caution is allergy. The National Center for Complementary and Integrative Health notes that milk thistle may cause allergic reactions, particularly in people allergic to related plants such as ragweed, chrysanthemum, marigold and daisy. On labels, look for Silybum Marianum Fruit Extract.
Sources: European Medicines Agency: Assessment report on Silybum marianum (L.) Gaertn., fructus (2018); NIH National Center for Complementary and Integrative Health: Milk Thistle; Journal of Cosmetic Dermatology (2023): Tolerability and effectiveness of a dermocosmetic product containing Silybum marianum fruit extract in adolescents and young adults with acne-prone skin; European Commission CosIng: Silybum Marianum Fruit Extract
