Guide
Milk thistle and medications: what to check
The human interaction evidence for milk thistle is small and inconsistent — a real, genotype-dependent effect on one blood-pressure drug in one trial, no effect on four major enzymes in another.
Milk thistle's evidence on drug interactions is small and inconsistent: one human trial found a real, genotype-dependent effect on a blood-pressure drug, while a separate human study found no significant effect on the same and other liver enzymes that process many medications. Overall, reviews describe milk thistle as having low interaction potential — but "low" isn't "none," so ask a doctor before combining it with any prescription medication, especially one with a narrow safe-dose range.
What the research found
A 2009 randomized crossover trial gave 12 healthy men 140 mg of silymarin three times a day (420 mg/day) for 14 days, then measured its effect on losartan, a blood-pressure medication processed by the liver enzyme CYP2C9. The men were split by genotype: in those with the more common CYP2C9*1/*1 genotype, silymarin significantly increased losartan's blood levels; in those with the CYP2C9*1/*3 genotype, it didn't. Silymarin decreased levels of losartan's active breakdown product in both genotypes. That's a real, measured interaction — but a specific one, tied to one drug, one enzyme, and genotype.
A separate 2014 study gave 9 healthy volunteers a standardized milk thistle extract three times a day for 14 days and measured its effect on four major liver enzymes (CYP1A2, CYP2C9, CYP2D6 and CYP3A4/5) using probe drugs for each. It found no significant effect on any of the four — including CYP2C9, the same enzyme implicated in the losartan trial above. The two studies don't agree, which is itself the honest finding: milk thistle's effect on drug-metabolizing enzymes looks real in at least one case but isn't consistent or predictable across studies.
A 2019 safety review summarizing this literature describes silymarin as having low overall drug-interaction potential and no major effect on cytochrome P450 enzymes as a general matter, while still recommending caution with drugs that have a narrow safe-dose range. A reference database on milk thistle notes it may increase how quickly the body processes some drugs. Commonly repeated claims that milk thistle meaningfully affects warfarin through a different enzyme pathway (CYP3A4) aren't confirmed by the human data reviewed here.
Questions people ask
What medications should not be taken with milk thistle?
There's no complete list, because the human evidence is limited to a handful of small studies. The clearest documented case is losartan, where a 2009 trial found a genotype-dependent change in blood levels. Beyond that specific case, ask a doctor before combining milk thistle with any prescription medication, especially one with a narrow safe-dose range.
Does milk thistle interact with medications?
It can, though the evidence is inconsistent. One trial found a real effect on a blood-pressure drug in some people; a separate trial found no effect on four major drug-processing enzymes. Reviews describe the overall interaction potential as low but not zero.
Is milk thistle a blood thinner?
No human trial we found shows milk thistle acting as a blood thinner. The commonly repeated claim that it affects warfarin through CYP3A4 isn't confirmed by the human research reviewed here — but ask a doctor before combining any supplement with a blood-thinning medication, since the interaction evidence for milk thistle overall is limited and inconsistent.
Can I take milk thistle with metformin or other blood-sugar medication?
No human trial we found has directly tested milk thistle alongside metformin or similar medications. Because some research shows milk thistle can affect drug-processing enzymes in at least some people, ask a doctor before combining it with any prescription medication that manages blood sugar.
Does milk thistle affect thyroid medication or birth control?
No human trial we found has directly tested either combination. Ask a doctor before combining milk thistle with any prescription medication.
When to talk to a doctor
Ask a doctor before combining milk thistle with any prescription medication, and specifically flag it if you take losartan or another blood-pressure medication, or anything with a narrow safe-dose range. Milk thistle is not a substitute for a prescribed medication, and no dose or timing change should be made without medical guidance.
Related reading: who should not take milk thistle, milk thistle side effects, and how much milk thistle should you take a day.
Sources
- Effect of silymarin on the pharmacokinetics of losartan and its active metabolite E-3174 in healthy Chinese volunteers A 2009 trial found that 420 mg/day of silymarin for 14 days changed losartan blood levels in healthy volunteers, but the effect depended on the person's CYP2C9 genotype.
- The effects of milk thistle (Silybum marianum) on human cytochrome P450 activity A 2014 study found a standardized milk thistle extract, taken three times daily for 14 days, had no significant effect on four major drug-metabolizing liver enzymes in healthy volunteers.
- 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.