The U.S. Food and Drug Administration does not strictly regulate herbs and supplements. There is no guarantee of strength, purity or safety of products, and effects may vary. You should always read product labels. If you have a medical condition, or are taking other drugs, herbs, or supplements, you should speak with a qualified healthcare provider before starting a new therapy. Consult a healthcare provider immediately if you experience side effects.


Avoid in people with known allergy or sensitivity to milk thistle, its parts, or to members of its plant family (e.g. daisies, artichokes, common thistle, and kiwi). Anaphylactic shock (a severe allergic reaction) and rash from milk thistle has been reported in several people.

Side Effects and Warnings

Milk thistle is likely safe when taken in recommended doses for 4-6 years.

Milk thistle is possibly safe when taken in doses greater than recommended, or when taken for more than 4-6 years.

Milk thistle may cause allergic skin reactions, bloating, blood clots, collapse, constipation, decreased platelets, diarrhea, eczema, elevated liver enzymes, fever, gas, giddiness, headache, heart attack, heartburn, high bilirubin (a toxic substance) in the blood, hives, impotence, increased creatinine, increased lactate dehydrogenase level, infection, insomnia, irritability, itching, joint pain, liver damage, loss of appetite, nausea, non-specific muscle and joint effects, pounding heart, rash, severe allergic reactions, sexual dysfunction, stomach distress or pain, skin pigment lightening, skin reactions, sweating, taste changes, tremor, vomiting, and weakness.

Milk thistle may lower blood sugar levels. Caution is advised in people with diabetes or low blood sugar, and in those taking drugs, herbs, or supplements that affect blood sugar. Blood glucose levels may need to be monitored by a qualified healthcare professional, including a pharmacist, and medication adjustments may be necessary.

Use caution in combination with agents processed by the liver's "cytochrome P450" enzyme system.

Use caution in combination with agents that undergo a chemical process called glucuronidation.

Use cautiously in pregnant or breastfeeding women.

Avoid use of above-ground parts of the milk thistle plant in women with hormone-sensitive conditions, including breast, uterine, and ovarian cancer, endometriosis, and uterine fibroids.

Avoid in people with known allergy or sensitivity to milk thistle, its parts, or to members of its plant family.

Pregnancy and Breastfeeding

Use cautiously in pregnant or breastfeeding women due to a lack of sufficient evidence to support safe use.

Milk thistle has been used historically to improve flow of breast milk, and limited study of milk thistle in pregnant women reported a lack of side effects.

This evidence-based monograph was prepared by The Natural Standard Research Collaboration