The supplement quality problem: what independent testing keeps finding

At a glance: what independent testing found

Each row is one study or government test. The details follow below.

Supplements are not approved before they are sold

In 1994, Congress passed the Dietary Supplement Health and Education Act (DSHEA), which created a separate set of rules for supplements. Under it, the FDA has no authority to approve dietary supplements before they are marketed, and it does not test them before they reach shoppers [1]. The company that makes or sells a supplement is responsible for making sure it is not adulterated or mislabeled [1].

Makers must follow current good manufacturing practices (cGMP), federal rules meant to protect a product's identity, purity, strength and composition [1]. But the FDA can act against an adulterated or mislabeled supplement only after it is already on the market [2]. In plain terms, the rules are real, but enforcement mostly happens after people have bought the product. The government's own records and independent lab studies show what slips through.

Hidden prescription drugs

The FDA keeps a public list of products it has found with hidden drug ingredients. When researchers at the California Department of Public Health reviewed its warnings from 2007 through 2016, they counted 776 adulterated supplements from 146 companies [3].

Most were sold for sexual enhancement (45.5%), weight loss (40.9%) or muscle building (11.9%), and 20.2% contained more than one unapproved drug. In 757 of the 776 products (97.6%), the drug was not listed on the label [3].

These were real drugs. The most common were sildenafil, a prescription drug ingredient, in sexual-enhancement products; sibutramine, which was pulled from the US market in 2010 over safety concerns, in 84.9% of the tainted weight-loss products; and synthetic steroids or steroid-like ingredients in 89.1% of the tainted muscle-building products [3][4].

A follow-up review covering 2007 through 2021 counted 1,068 unique adulterated products, some with as many as five drug ingredients in a single product [5]. The FDA's current Health Fraud Product Database says it includes only a small fraction of potentially hazardous products, and that consumers should still be cautious about products that are not on it [6].

Warnings do not always clear these products from sale. Researchers bought 27 recalled supplements a mean of 34 months after their FDA recall: 18 of them (66.7%) still contained a banned drug, and 6 contained additional banned ingredients the FDA had not reported [7]. In the 776-product review, fewer than half of the tainted products (46.4%) were associated with a voluntary recall [3].

Online selling is a major route. From 2014 to 2016, 38.6% of the tainted samples the FDA identified came from online sampling and 34.3% from examining international mail [3].

Large marketplaces are not immune. Between December 2019 and February 2020, the FDA bought 26 sexual-enhancement products on Amazon.com, all shipped through Amazon's fulfillment service on behalf of third-party sellers. Lab tests found undeclared sildenafil, tadalafil or vardenafil in all 26, and 25 of them were labeled as dietary supplements [4].

In its 2021 warning letter, the FDA wrote that Amazon had acknowledged its systems sometimes fail to detect products it tries to restrict, and that the agency was concerned Amazon's filters were inadequate [4]. In December 2023, the FDA sent Amazon another warning letter after test purchases of seven more products, six of them labeled as energy supplements, that contained undeclared tadalafil or sildenafil [8].

Hidden drugs matter because nobody can plan around an ingredient they do not know is there. The FDA warns that these particular drug ingredients can interact with nitrate medicines and lower blood pressure to dangerous levels [4].

Labels that do not match what is inside

Even without hidden drugs, what is on the label is often not what is in the bottle.

Researchers at the Uniformed Services University and the University of Mississippi bought 30 featured immune-support supplements from Amazon.com in May 2021 and analyzed them by liquid chromatography-mass spectrometry. Only 13 of the 30 had accurate labels. Of the 17 that did not, 13 listed ingredients the lab could not detect and 9 contained substances that were not on the label. None of the 30 carried a third-party certification seal [9].

A 2023 study in JAMA tested 25 melatonin gummy products bought online. Only 3 of the 25 were within 10% of the labeled amount. In the 24 that contained melatonin, the actual amount ranged from 74% to 347% of the label; the remaining product had no detectable melatonin but did contain 31.3 mg of CBD [10].

An earlier study of 31 melatonin supplements found content ranging from 83% below to 478% above the label, with differences of up to 465% between lots of the same product. More than 71% missed their label claim by more than 10%, and eight contained serotonin as a contaminant [11].

Vitamin D, one of the simplest supplements to make, showed the same pattern. When researchers tested five pills each from 15 over-the-counter vitamin D3 products, the pills held 52% to 135% of the expected dose, and only about a quarter of the bottles had all five pills meet United States Pharmacopeia (USP) standards. Pills from the one USP-verified maker were generally more accurate than the rest [12].

Fish oil has the same issue. In a test of 32 fish oil supplements sold in New Zealand, only 3 contained at least the labeled amount of the omega-3s EPA and DHA, and 69% contained less than two-thirds of it [13]. A later industry-led study of 47 products in the same market found much better label compliance (91%), so results differ by method and by market [14].

The consistent lesson is that a label is a claim, and only testing shows whether a given bottle meets it.

Heavy metals and other contaminants

Some traditional formulas combine herbs with metals on purpose, a practice called rasa shastra, but metals turn up in other products too. A JAMA study bought 193 Ayurvedic products from websites and found detectable lead, mercury or arsenic in 20.7% of them, including 17.1% of the products made without metals on purpose [15].

US-made products were no better than Indian-made ones (21.7% vs 19.5%). Of the metal-containing products, 95% were sold by US websites, 75% claimed to follow good manufacturing practices, and every one exceeded at least one standard for acceptable daily intake of toxic metals [15].

A US Government Accountability Office (GAO) investigation had an accredited lab test 40 popular single-ingredient herbal supplements. It found trace amounts of at least one potentially hazardous contaminant in 37 of them: all 37 had trace lead, 32 also had mercury, 28 cadmium, 21 arsenic and 18 pesticide residues [16].

GAO noted that none of the amounts were considered an acute toxicity hazard and the metals did not exceed FDA or EPA rules for supplements, but the FDA agreed that 16 of the 40 products would violate US pesticide tolerances if its own testing confirmed the results [16]. The point is not alarm. It is that purity has to be tested, not assumed.

Live products bring their own problems. A working group of the European Society for Paediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) reviewed studies from around the world and found that misidentified strains, cultures that were no longer alive, counts that could not be verified and occasional contamination, sometimes with pathogens, are surprisingly common in commercial probiotics [17].

Copies and look-alikes in online marketplaces

Counterfeits are a known problem in online marketplaces in general. When GAO bought 47 items from third-party sellers on popular consumer websites, testing by the brand owners found that 20 were counterfeit, including all 13 cosmetics GAO bought [18]. Those items were not supplements, but GAO concluded that the growth of e-commerce has shifted counterfeit sales online, where fakes are sold alongside authentic products, and that counterfeit goods can threaten consumers' health and safety [18].

The FDA's experience with tainted supplements shows the same pattern of products coming back. After Amazon told the FDA it had restricted the 26 tainted products and their sellers, the agency bought one of the same products from the same seller a few weeks later, and it kept finding nearly identical products offered by the same third-party sellers [4]. Recalled products, as noted above, can stay on sale for years [7].

None of this means every marketplace listing is fake. It means that when many unrelated sellers can offer the same product, the shopper carries the burden of working out which bottle is genuine, fresh and properly stored. Fullscript, the platform our store runs on, makes the same point: it says that letting resellers ship supplements through a marketplace can lead to counterfeit products, expired supplements or questionable quality standards [19].

Hidden doses and weak formulas

A product can match its label exactly and still fall short of what the research used.

FDA labeling rules let a company group ingredients into a "proprietary blend" and list only the blend's total weight. Ingredients without an established Daily Value are listed in order by weight, with no individual amounts [20]. You can see what is in the blend, but not how much of each.

In a study of the 100 top-selling pre-workout supplements, products listed an average of 18.4 ingredients, and 44.3% of all ingredients sat inside proprietary blends with undisclosed amounts. Where amounts were shown, the average dose of beta-alanine was below the dose the research supports, and caffeine was near the low end of the effective range [21].

The chemical form matters too. In a comparison of four magnesium products, only about 4% of the magnesium in magnesium oxide was absorbed, while magnesium chloride, lactate and aspartate were absorbed significantly better [22]. In a deuterium-labeling study, natural vitamin E (RRR-alpha-tocopherol) showed roughly twice the availability of the synthetic form [23].

Curcumin, the much-studied compound in turmeric, is poorly absorbed, quickly metabolized and quickly eliminated, which is why researchers have tested ways around it, such as adding piperine or binding it to phospholipids [24]. The same "500 mg" on two labels can mean very different amounts that your body can actually use.

How to spot a better bottle

You cannot run a lab test at home, but you can tilt the odds in your favor.

• Look for an independent certification, such as USP Verified or NSF Certified for Sport, named on the label. In the vitamin D study, pills from the USP-verified maker were generally more accurate than the rest [12]; in the Amazon immune-support study, none of the 30 products carried a seal [9].
• Take extra care with products sold for sexual performance, weight loss or muscle building, the categories where the FDA has found the most hidden drugs [3].
• Search the FDA's Health Fraud Product Database before buying anything that promises fast, dramatic results, and remember that it lists only a fraction of problem products [6].
• Skip proprietary blends for the ingredients you care most about. You should be able to see the dose of each one [20].
• Check the form, not just the milligrams, for example magnesium oxide versus better-absorbed forms [22].
• Buy from the brand or its authorized channel, in sealed packaging that shows a lot number and an expiration date.
• Be wary of words like "clinical grade" and "pharmaceutical grade", which are marketing terms. NIH notes that label words such as "standardized," "verified" or "certified" do not necessarily guarantee quality or consistency [2].

How we solve this

Our answer is to take the guesswork out of each step where testing keeps finding problems. We call the result practitioner-grade, and we mean something specific by it: made under GMP rules in facilities with third-party cGMP certification, tested by outside labs, sold to you through a professional platform rather than an open marketplace, and bought directly from the brand.

We do not use "clinical grade" or "pharmaceutical grade", because neither term has a legal definition for supplements.

• Approved brands only. Our picks, and our protocols when they open, use only brands on our approved list, chosen for their testing, manufacturing standards and use of the forms and doses that studies used.
• Bought direct from the brands. Our store runs on Fullscript, which says it sources its products directly from the brands, without exception, so there is no reseller in the chain [19].
• Vetted by the platform. Fullscript requires each brand and its contract manufacturer to hold third-party cGMP certification and to prove it every year [25]. It reviews labels, stability data, certificates of analysis and FDA citations [26].
• Pulled off the shelf and tested. Each quarter, Fullscript pulls products straight off its own warehouse shelves and sends them to ISO 17025-accredited labs to check for heavy metals, microbes and label accuracy, removing products with serious safety or potency results [26]. When a top-selling vitamin B12 failed potency testing, Fullscript says it was delisted immediately [25].
• Studied doses and honest grades, with our full site. When our full site opens, each protocol will show the dose the research used and an evidence grade for each ingredient, so a sprinkle of something cannot hide behind a long ingredient list.
• Certifications in plain text, with our full site. Where a product carries an independent certification, such as USP Verified or NSF Certified for Sport, our full site will name it in plain words so you can check it yourself.
• Stored and shipped with care. Products are stored and shipped from Fullscript's NSF-certified facilities [19]. Our article on heat, light and shipping explains why that matters.

What we can and cannot promise

No store can make a supplement work for everyone, and we do not promise results. What we can do is choose a supply chain in which the bottle you open comes straight from the brand, made to a documented standard, and is stored and shipped by certified facilities. Our store ships to US addresses only.