ZHICHENG BIO
Silymarin 80%: the number is meaningless until you know which method produced it
Why an 80% silymarin certificate and a 55% silymarin certificate can describe the same drum — and how to tell which one you are holding.
The short answer
An "80% silymarin" figure cannot have been produced by the European Pharmacopoeia assay, because that assay's own definition places standardised refined milk thistle extract in a band of 30% to 65%. The Ph. Eur. method sums six specific flavonolignan peaks by liquid chromatography; a non-specific UV/VIS absorbance method counts everything in the extract that absorbs near the measuring wavelength. The two numbers are not interconvertible by any fixed factor, and a buyer who compares one supplier's UV figure against another's LC figure is not comparing quality — they are comparing methods.
- What the pharmacopoeia allows
- Ph. Eur. monograph 01/2008:2071, Milk thistle dry extract, refined and standardised: "The nominal content of silymarin is within the range 30 per cent m/m to 65 per cent m/m (dried extract)", with content 90–110% of the nominal figure on the label
- What gets summed
- Six peaks and no others: silicristin, silidianin, silibinin A, silibinin B, isosilibinin A, isosilibinin B — all C25H22O10, Mr 482.4, all expressed as silibinin
- The conditions
- LC (2.2.29), C18 125 mm × 4 mm, 5 µm; phosphoric acid / methanol / water gradient; 0.8 mL/min; detection at 288 nm; silibinin B elutes at about 30 min; resolution ≥ 1.8 between silibinin A and B
- The composition gates
- The monograph also fixes the profile: silicristin + silidianin 20–45%, silibinin A + B 40–65%, isosilibinin A + B 10–20%, each with reference to total silymarin. A batch can hit the assay and still fail these
- What has actually been measured
- A 2023 review of commercial milk thistle materials reports silymarin content ranging from 17.21% to 63.96% across tested extracts (Metabolites 2023;13(3):440, PMID 36984880)
- What this does not say
- Nothing here says a UV figure is fraudulent or that an 80% material is worthless. It says the two numbers answer different questions, and that only one of them is the pharmacopoeial one
The two numbers, side by side
The Ph. Eur. assay is a subtraction exercise disguised as an addition. You inject the extract, you locate six named peaks against a milk thistle standardised dry extract CRS, and you add up those six areas. Anything else in the chromatogram — taxifolin, other flavonoids, co-extracted phenolics, degradation products — contributes nothing to the number, because it is not one of the six. That is the whole point of a specific method: it can only count what it can name.
A UV/VIS absorbance method does the opposite. You dissolve the extract, you read absorbance at a chosen wavelength, and you convert to concentration against a silibinin calibration curve. Everything in the solution that absorbs at that wavelength contributes. In a plant extract that is a long list, because the phenolic chromophore is common and the flavonolignan absorbance maximum sits in a crowded region — the Ph. Eur. reads its chromatogram at 288 nm, and a great many co-extracted phenolics absorb there too.
So the UV figure is bounded below by the LC figure and can exceed it by a lot. How much depends on the extract's own impurity profile, which is exactly the thing you were trying to measure. There is no conversion factor. A supplier who tells you "our 80% is equivalent to 55% by HPLC" is telling you the result for one batch, not a rule.
Why 65% is a ceiling worth remembering
The 30–65% band in monograph 2071 is not an arbitrary convention. It describes the range that a refined, standardised extract of Silybum marianum fruit actually occupies when the six flavonolignans are counted individually. Push a purification further and you stop having a standardised extract and start having isolated silibinin, which is a different article with a different specification.
The independent evidence lands in the same place. The 2023 Metabolites review, pulling together analyses of commercial milk thistle materials, reports measured silymarin content spanning 17.21% to 63.96%. The top of the observed range and the top of the pharmacopoeial band are within a percentage point of each other, from two entirely separate directions.
Which is why an 80% figure should stop a buyer rather than impress one. It is not a better extract measured the same way; it is a different measurement. The same review cites an HPTLC survey in which only 10 of 21 finished supplements fully complied with their label claims, nine lacked the taxifolin position altogether, and two showed none of the milk thistle fingerprint at all — so the gap between what a certificate says and what is in the drum is not hypothetical in this material.
The arithmetic a buyer should do before signing
Take a 1,000 kg order. Quoted as "80% silymarin", you believe you are buying 800 kg of flavonolignan. If the material assays 55% by the Ph. Eur. six-peak sum — a wholly ordinary result for a good commercial extract — you have 550 kg. The 250 kg difference is not adulteration; it is other plant matter that a non-specific method counted as silymarin.
Now price it. Whatever your per-kilogram number is, divide it by the LC assay rather than the UV assay and you get the real cost per kilogram of flavonolignan. Do that for every quotation in front of you and the ranking usually changes, because the suppliers quoting UV figures are the ones whose apparent price per kilogram of active is lowest and whose real price is not.
The second calculation is the profile. Monograph 2071 requires silibinin A + B to be 40–65% of total silymarin. If your formulation's rationale is built on silibinin specifically, a batch sitting at the 40% end delivers a third less silibinin than one at 60%, at identical total silymarin. The assay figure alone will never show you this. The percentage breakdown will, and the monograph obliges a compliant supplier to have it.
What to write into the specification
Name the method, not just the number. "Silymarin ≥ 55.0%, by HPLC, sum of silicristin, silidianin, silibinin A, silibinin B, isosilibinin A and isosilibinin B, expressed as silibinin, per Ph. Eur. 2071, on the dried basis" is unambiguous and cannot be quietly satisfied with an absorbance reading. "Silymarin 80%" is not a specification; it is a hope.
Ask for the chromatogram alongside the certificate, and check that six peaks are identified and integrated. Ask for the composition breakdown as percentages of total silymarin, because the monograph requires it and the numbers reveal both the raw material and the process. Ask for loss on drying — 2071 sets a maximum of 5.0%, and a wet extract inflates every other figure on the page.
And fix the reference standard. The Ph. Eur. assay is run against milk thistle standardised dry extract CRS, not against a silibinin solution; a laboratory using neat silibinin as the standard will not reproduce the monograph result. If two certificates disagree, the reference standard is the first place to look — before anyone accuses anyone of anything.
The related trap for anyone buying a plant extract on an assay figure is the water-of-crystallisation question, which is set out in the quercetin assay-basis guide.
Frequently asked
Is 80% silymarin better than 65%?
Not necessarily — and probably not comparable. The European Pharmacopoeia monograph 01/2008:2071 defines standardised refined milk thistle extract as containing a nominal 30% to 65% silymarin, measured as the sum of six named flavonolignan peaks by liquid chromatography. A figure above that band was produced by a different method, most often a non-specific UV/VIS absorbance reading that counts other phenolics as silymarin. The two numbers are not interconvertible by any fixed factor.
Which six compounds does the pharmacopoeial assay count?
Silicristin, silidianin, silibinin A, silibinin B, isosilibinin A and isosilibinin B, all expressed as silibinin (C25H22O10, Mr 482.4). The monograph also constrains the proportions: silicristin plus silidianin 20-45% of total silymarin, silibinin A plus B 40-65%, isosilibinin A plus B 10-20%. Taxifolin, which is present in the extract and often discussed alongside silymarin, is not one of the six.
What silymarin content do commercial extracts actually contain?
A 2023 review in Metabolites (13(3):440, PMID 36984880) reports measured silymarin content across tested commercial extracts ranging from 17.21% to 63.96%. The same review cites an HPTLC survey in which 10 of 21 finished supplements fully complied with their label claims, nine lacked the taxifolin position, and two showed none of the milk thistle characteristics.
How do I write a milk thistle specification that cannot be gamed?
State the assay method inside the specification line: silymarin content by HPLC as the sum of the six Ph. Eur. flavonolignans, expressed as silibinin, per monograph 2071, on the dried basis, against milk thistle standardised dry extract CRS. Add the composition percentages, loss on drying (maximum 5.0% under the monograph), and require the chromatogram with the certificate.
This page describes raw-material sourcing and specification. It is not a health claim and not medical advice. Last updated 2026-08-07.