Guide
Milk thistle side effects
Human trials describe milk thistle as generally well tolerated, even at high doses, with GI upset as the main limiting effect. Rare severe allergic reactions are documented, especially in people allergic to related plants.
Milk thistle is generally well tolerated in the human trials that have tested it, including at doses well above what most labels sell. The most common side effects are gastrointestinal — nausea and diarrhea — and severe allergic reactions, while rare, have been reported and are worth knowing about before you start.
What the research found
A 2019 review of silymarin's safety and toxicity describes it as well tolerated in humans at typical doses, and even at a high dose of 700 mg three times a day (2,100 mg/day) for 24 weeks in the trials it covered. The gastrointestinal effects it reports — nausea and diarrhea — were the main limiting symptoms as the dose rose. It found silymarin has low potential for drug interactions overall, though it still recommends caution when combining it with drugs that have a narrow safe-dose range; see milk thistle and medications: what to check. That review also notes one clinical trial reported no anomalies with silymarin use during pregnancy, but the overall data is limited enough that it still says to ask a doctor before using milk thistle while pregnant.
A National Institutes of Health drug-and-lactation reference database lists mild side effects — diarrhea, headache and skin reactions — as the most commonly reported in adults, with severe allergic reactions and anaphylaxis reported rarely. It specifically flags people allergic to ragweed, daisies, artichokes, common thistle or kiwi — all members of the same plant family as milk thistle (Asteraceae/Compositae) — as having a higher chance of a cross-reaction. See who should not take milk thistle for more on that.
Questions people ask
What are the side effects of milk thistle?
The most commonly reported are gastrointestinal — nausea and diarrhea — along with headache and skin reactions in some people. A 2019 safety review found these mild and manageable even at doses well above what most labels sell.
Does milk thistle have any side effects?
Yes, though they're generally mild in the research: gastrointestinal upset (nausea, diarrhea), headache and skin reactions are the most commonly reported. Severe allergic reactions are rare but documented, particularly in people allergic to ragweed, daisies or other plants in the same family.
Can milk thistle cause an allergic reaction?
Yes, rarely, including severe reactions. People with a known allergy to ragweed, daisies, artichokes, common thistle or kiwi are at higher risk of a cross-reaction, since milk thistle is in the same plant family.
When to talk to a doctor
Stop taking milk thistle and seek medical attention if you notice signs of an allergic reaction — hives, swelling, or trouble breathing — especially if you have a known allergy to ragweed or other Asteraceae-family plants. Talk to your doctor before starting milk thistle if you're pregnant, nursing, or take any prescription medication, since the safety evidence in those situations is more limited than the general adult data above.
Related reading: who should not take milk thistle, milk thistle and medications: what to check, and how much milk thistle should you take a day.
Sources
- Safety and toxicity of silymarin, the major constituent of milk thistle extract: An updated review A 2019 safety review found silymarin generally well tolerated in humans, even at high doses for 24 weeks, with gastrointestinal upset as the main limiting side effect.
- Milk Thistle An NIH reference monograph reports no valid clinical trial support for milk thistle increasing milk supply, describes it as generally well tolerated with occasional GI/skin side effects, and flags rare but real allergic cross-reactivity with ragweed-family plants.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.