Milk Thistle (Silymarin): The Number the Label Won't Tell You
Educational overview — not medical advice. Milk thistle is generally well tolerated (mild GI upset); the cautions that matter are a ragweed-family allergy and possible additive blood-sugar lowering with antidiabetic drugs — see does it work.
Milk thistle's active is silymarin — and most labels won't tell you how much is in the bottle. Silymarin is a flavonolignan complex (main component: silibinin) that the research doses in milligrams of silymarin, not seed weight. The trap: a quality extract states an extract mg and a silymarin % (~70–80%), so actual active = extract mg × %; cheap products headline "1,000 mg," "3,000 mg," "4:1," or "full-spectrum" with no %. Content varies ~2,800% between products (Fenclová 2019: 35–125% of label). Of 12 products we track, only 5 disclose a silymarin %. The evidence is modest: it reduces elevated ALT/AST, with no confirmed mortality benefit in cirrhosis.
As an Amazon Associate we earn from qualifying purchases. Ranked by disclosed silymarin, never commissions.
Start here
What milk thistle is (and what actually does the work)
Milk thistle (Silybum marianum) is sold for "liver support," and the compound behind the research is silymarin — a family of flavonolignans whose main component is silibinin (silybin). Silymarin is what the trials standardize on, and the pharmacopeial convention (USP / German Commission E) is an extract standardized to about 70–80% silymarin (composition per NIH LiverTox). So the first question isn't dose — it's whether the bottle even tells you its silymarin content, because the big milk-thistle milligram number on the front is not the active.
The disclosure moat, briefly
A quality product prints both an extract mg and a silymarin %, so you can compute the active with a single multiplication (extract mg × %). Cheap products headline a big raw number — "1,000 mg" softgels, "3,000 mg" 4:1 extracts, "full-spectrum" seed — with no silymarin %, so the active is unknowable. And it genuinely varies: independent testing (ConsumerLab) found silymarin per serving swinging ~2,800% (2 of 6 at 40–60% of label), and peer-reviewed testing found 35–125% of the declared amount (Fenclová 2019). That's why a true cost-per-active ranking is possible only for the disclosers.
The one-line takeaway Silymarin is the point, and it's computable only when a silymarin % is printed — yet of the 12 products we track, 5 disclose no percentage at all. So shop on disclosure first, not on the milligram number. And keep expectations modest: the evidence supports reducing elevated ALT/AST as an adjunct, not a "detox" or a cure.
Does it work? (the short version)
Modestly, and worth stating honestly. Lead with the mortality question: a Cochrane review found no significant all-cause mortality benefit in cirrhosis, and the liver-mortality signal was not significant in high-quality trials (Rambaldi 2007). Its best-supported effect is reducing elevated ALT/AST in fatty liver disease — ALT/AST down meaningfully across 26 trials (Li 2024), statistically real but of debated clinical relevance. A fasting-glucose drop in type 2 diabetes rests on low-quality trials (Voroneanu 2016). And the famous 93% Amanita mushroom-poisoning survival comes from an intravenous hospital drug (Legalon SIL), not an oral capsule (Mengs 2012) — see the evidence page.
Frequently asked questions
What is the active ingredient in milk thistle?
Silymarin — a flavonolignan complex, main component silibinin. Not the seed/extract weight on the label. Standardized extracts are ~70–80% silymarin; compute active as extract mg × %. Content varies ~2,800% (Fenclová 2019: 35–125% of label).
Does milk thistle work?
Modestly. Reduces elevated ALT/AST in NAFLD (Li 2024), clinical relevance debated. No confirmed cirrhosis-mortality benefit (Rambaldi 2007 Cochrane null). Low-quality glucose signal (Voroneanu 2016). The 93% Amanita figure is an IV drug (Mengs 2012), not a capsule.
How much should I take?
~200 mg silymarin 2–3×/day (~420 mg/day; range 140–800). Actual silymarin = extract mg × %. Most labels don't disclose the %, so you often can't compute the dose.
Is it safe?
Generally well tolerated (mild GI upset). Cautions: ragweed-family (Asteraceae) allergy (NCCIH/MedlinePlus) and possible additive blood-sugar lowering with antidiabetic drugs. Little clinically significant CYP450 interaction at typical doses.
Related guides
- Berberine — another metabolic-support botanical with a standardization and dosing gap
- NAC — the other "liver/antioxidant" supplement with a real IV-vs-oral distinction
- Curcumin — another poorly-absorbed plant extract where the form and % on the label matter
Sources
- Rambaldi A, et al. "Milk thistle for alcoholic and/or hepatitis B or C virus liver diseases." Cochrane Database Syst Rev. 2007. PMID: 17943794 (no confirmed mortality benefit).
- Li S, et al. "Administration of silymarin in NAFLD/NASH: A systematic review and meta-analysis." Ann Hepatol. 2024. PMID: 38579127 (26 RCTs; ALT/AST reduced).
- Voroneanu L, et al. "Silymarin in type 2 diabetes mellitus: a systematic review and meta-analysis of RCTs." J Diabetes Res. 2016. PMID: 27340676 (low-quality glucose signal).
- Mengs U, et al. "Legalon SIL: the antidote of choice in amatoxin poisoning." Curr Pharm Biotechnol. 2012. PMID: 22352731 (intravenous silibinin, not oral).
- Fenclová M, et al. "Poor chemical and microbiological quality of commercial milk thistle supplements." Sci Rep. 2019. PMID: 31366891 (content 35–125% of declared).
- NIH LiverTox, "Milk Thistle (Silymarin)." NBK548817 (composition; ~70–80% silymarin standardization). NCCIH/MedlinePlus for the Asteraceae allergy caution.
- Full product dataset: /milk-thistle/cost-by-brand.json (CC BY 4.0).