A supplement label is a promise written in milligrams. Independent testing programmes, academic laboratories and regulators have been checking that promise for twenty years, and the results are remarkably consistent: a large minority of products do not contain what the label says. Sometimes there is too little, sometimes there is far too much, and occasionally there is something that should not be there at all. This is not an argument against supplements. It is an argument for understanding how a number on a label is produced — and for knowing which questions separate a verified product from a hopeful one.
How often does it happen? The published numbers
The most rigorous dataset comes from the US National Institutes of Health Dietary Supplement Ingredient Database, which analysed adult multivitamin/mineral products in accredited laboratories. Measured content ranged widely against the label: on average, products contained more than declared for most vitamins (overages of 10–50% for some water-soluble vitamins) but the spread between individual products was very large, with some delivering well under the declared amount (Andrews et al. 2017). A 2013 study of over-the-counter vitamin D found individual pills ranging from 9% to 146% of the labelled dose, with compounded products showing even wider variation (LeBlanc et al. 2013). In 2022, a study of immune-support supplements found that 17 of 30 products had inaccurate labels — 13 were missing listed ingredients and 9 contained ingredients not declared (Crawford et al. 2022). And in 2023, an analysis of sports supplements containing botanical stimulants found that only 11% contained an accurate quantity of the labelled ingredient, with actual amounts ranging from 0% to 334% of the claim (Cohen et al. 2023). The pattern recurs across categories: herbal products, probiotics, fish oils and protein powders all show the same distribution — a majority within tolerance, a persistent tail that is not.
Why it happens: six mechanisms, none of them mysterious
1. No input testing. Good Manufacturing Practice for supplements (US 21 CFR Part 111; EU food-hygiene and HACCP rules) requires that each incoming ingredient be tested for identity and, where relevant, potency before use. A manufacturer that trusts the supplier’s paperwork instead inherits every error upstream. 2. Overage used as a blunt instrument. Vitamins degrade over shelf-life, so formulators add extra at manufacture so that the product still meets label at expiry. Done properly, overage is calculated from real stability data. Done carelessly, it produces the 146% pill — and for nutrients with a narrow safety margin (vitamin A, vitamin D, selenium, iron) a large uncontrolled overage is a safety issue, not a bonus. 3. Poor blend uniformity. When a few milligrams of an active ingredient are mixed into kilograms of filler, inadequate mixing means one capsule gets double and the next gets half. 4. Degradation. Heat, light, moisture and oxygen destroy vitamins, probiotics and omega-3s during transport and storage; a product that met label at the factory may not meet it on the shelf. 5. Wrong analytical method. Some laboratories measure “total” content with a method that cannot distinguish active from degraded forms; the number looks fine while the product does not work. 6. Economic adulteration. The cheapest way to “meet” a label is to substitute or dilute a costly ingredient — a topic that deserves its own article on identity testing.
The difference between a label claim and a specification
Regulators in most jurisdictions allow a tolerance around the declared value — the EU’s guidance on nutrient tolerances for food supplements, for example, accepts roughly −20% to +50% of the declared value for vitamins and minerals, with tighter limits where safety requires. That range exists because natural variation and analytical uncertainty are real. But a tolerance only means something if the product is actually tested against it, batch by batch, at release and through shelf-life. The document that records this is the certificate of analysis (how to read one column by column): for each active ingredient it should list the specification (the acceptable range), the method, and the result for that specific lot. A product with a label but no certificate has a promise but no evidence.
Who checks, and what the seals mean
Because pre-market approval does not exist for supplements, verification has been filled by three kinds of actors. Pharmacopoeial programmes test products against public standards — the United States Pharmacopeia’s verification scheme being the best known (USP standards vs USP Verified explained). Independent testing organisations buy products at retail and publish results. Accredited third-party laboratories (ISO/IEC 17025) perform batch testing under contract to manufacturers who choose to pay for it. A seal is only as good as the programme behind it: the questions are whether testing is batch-specific, whether the laboratory is accredited, whether the method is validated for that matrix, and whether results are available for your lot rather than a lot from last year.
A buyer’s five-point check
1. Ask for the lot-specific certificate of analysis. A serious manufacturer will provide it, or publish it through a batch-verification page. If the answer is “we don’t share that,” you have your answer. 2. Look for an accredited laboratory name. ISO/IEC 17025 accreditation means the laboratory’s methods and competence have been audited. 3. Check that the result is a number, not a tick. “Conforms” is weaker than “102.3% of label (spec 95–125%)”. 4. Check the test covers identity and potency. Identity confirms it is the right molecule; potency confirms how much. Both are needed. 5. Check the expiry date assumes your storage. Heat-sensitive products (fish oil, probiotics, liquid vitamins) should state storage conditions and, ideally, have real-time stability data behind them (what GMP actually requires).
The clinical bottom line
Across independent surveys, somewhere between one in six and one in two supplement products fail to match their label within tolerance, through a handful of well-understood manufacturing failures rather than any mystery. The remedy is not suspicion of all supplements but a documentary habit: a lot-specific certificate of analysis from an accredited laboratory, stating identity and quantified potency, is the only evidence that the milligrams on the label are the milligrams in the bottle.
Primary sources
- Andrews KW, Roseland JM, Gusev PA, et al. Analytical ingredient content and variability of adult multivitamin/mineral products: national estimates for the Dietary Supplement Ingredient Database. Am J Clin Nutr. 2017;105(2):526–539. doi:10.3945/ajcn.116.134544
- LeBlanc ES, Perrin N, Johnson JD Jr, Ballatore A, Hillier T. Over-the-counter and compounded vitamin D: is potency what we expect? JAMA Intern Med. 2013;173(7):585–586. doi:10.1001/jamainternmed.2013.3812
- Crawford C, Avula B, Lindsey AT, et al. Analysis of select dietary supplement products marketed to support or boost the immune system. JAMA Netw Open. 2022;5(8):e2226040. doi:10.1001/jamanetworkopen.2022.26040
- Cohen PA, Avula B, Wang YH, et al. Presence and quantity of botanical ingredients with purported performance-enhancing properties in sports supplements. JAMA Netw Open. 2023;6(7):e2323879. doi:10.1001/jamanetworkopen.2023.23879
- US Food and Drug Administration. 21 CFR Part 111 — Current Good Manufacturing Practice in manufacturing, packaging, labeling, or holding operations for dietary supplements. ecfr.gov
- European Commission. Guidance document for competent authorities on the setting of tolerances for nutrient values declared on a label. December 2012. food.ec.europa.eu
Educational information on supplement quality and consumer protection, not medical advice.
Was this article helpful?
Related Coverage
Editorial standards. This article was produced under the GMJ News editorial process, with oversight by the GMJ Editorial Board. Our editorial process. Spotted an error? Contact the editorial team.
