Milk thistle: what the research does and doesn't show

The evidence doesn't support several of the ideas that circulate about milk thistle: that it reliably prevents a hangover, that it causes constipation, or that a cup of milk thistle tea delivers the same thing as the extract used in research. What the reviewed safety literature does say: GI upset (diarrhea, not constipation) is the most common side effect, one small trial found no anomalies in pregnancy, and several commonly searched questions — bloating, kidney health outside diabetic kidney disease, parasites — simply haven't been tested.

What the evidence doesn't support

It doesn't support milk thistle as a hangover preventive or cure — no clinical trial has tested that use. It doesn't support the idea that milk thistle typically causes constipation; the GI effect that shows up in the safety research is diarrhea, not constipation. It doesn't support treating milk thistle tea as equivalent to the standardized extract used in the trials cited across this site — tea is a far lower, unstandardized dose. None of it describes a specific brand's dose or product, and nothing here is a claim about any product.

What the trials found

A 2019 safety review found silymarin generally well tolerated in humans, even at high doses (up to 2,100 mg/day) for 24 weeks, with gastrointestinal discomfort — mainly nausea and diarrhea — as the main limiting effect. The same review found one clinical trial reporting no anomalies with silymarin use during pregnancy, though it still recommends caution and further study given how little pregnancy-specific research exists. A separate NIH reference monograph (LactMed) adds detail on breastfeeding specifically: it found no valid clinical trial support for milk thistle boosting milk supply, and noted that silymarin's components are poorly absorbed and not excreted into breastmilk in measurable amounts, making it considered unlikely to affect a breastfed infant — but it also recorded that breastfeeding mothers taking milk thistle for milk supply occasionally reported side effects including weight gain, nausea, dry mouth and irritability. Neither source reports constipation as a common effect, and neither source tested bloating as an outcome at all.

Neither the 2019 review nor the LactMed monograph, nor any other trial we found, tested milk thistle for hangover prevention, general kidney health, or against parasites. The kidney research that does exist is specific to people with diagnosed diabetic kidney disease, covered in milk thistle benefits: what the research does and doesn't show, and doesn't extend to kidney health in people without that diagnosis.

Questions people ask

Can milk thistle cause diarrhea or constipation?

Diarrhea is one of the more commonly reported side effects in the safety research; constipation isn't reported as a common effect in the sources reviewed here. See milk thistle side effects for the full list.

Does milk thistle help a hangover?

No clinical trial has tested milk thistle for hangover prevention or recovery in otherwise healthy people who drink occasionally. The liver-disease research covered elsewhere on this site studied people with a diagnosed condition, not occasional drinkers, so it doesn't speak to this use.

Is milk thistle safe during pregnancy or breastfeeding?

One small trial found no anomalies with silymarin use in pregnancy, and an NIH reference source considers it unlikely to affect a breastfed infant based on how poorly its compounds are absorbed and transferred into milk — but both sources describe the overall safety data as limited, not as a green light. Ask a doctor before taking milk thistle if you're pregnant or nursing. See who should not take milk thistle.

Does milk thistle cause bloating or weight gain?

Bloating hasn't been tested as an outcome in the research we reviewed. Weight gain was occasionally reported by breastfeeding mothers taking milk thistle for milk supply in one reference source, but that's a small, specific population, not a general finding — see does milk thistle help with weight for the larger pooled data on weight, which found no significant change either way.

What is milk thistle tea good for?

None of the research cited across this site was conducted using tea. Tea steeps a small, unstandardized amount of the plant material, while the trials used a standardized extract at specific milligram doses of silymarin — so none of those findings transfer directly to a cup of tea. See how to choose a good milk thistle supplement for how format affects dose.

Is milk thistle good for kidneys?

The kidney research that exists is limited to people with diagnosed diabetic kidney disease, and even there, two trials reached opposite conclusions. See milk thistle benefits: what the research does and doesn't show for both trials.

Is milk thistle safe for kids or dogs?

The research reviewed across this site is in adults. There's no established pediatric dosing, so a child's use should be a doctor's call rather than a scaled-down adult dose, and dosing for a dog or other pet is a question for a veterinarian, not this review.

When to talk to a doctor

Milk thistle is not a treatment for any disease covered above. If you're pregnant, nursing, considering it for a child, or take any prescription medication, talk to a doctor before adding it — see milk thistle and medications: what to check.

Related reading: milk thistle side effects, who should not take milk thistle, and what is milk thistle.