Supplement Fact Check: Which Popular Claims Hold Up?

Researched by DoseRoutine Research TeamReviewed for accuracy by Nicholas Alexander, RSE, SO, PMPLast updated Educational reference only — not medical advice. Always confirm dosing and safety decisions with a licensed clinician.

Most supplement claims you meet online are repeated far more often than they are checked. This page takes the headline claim behind each of our dosing guides, reads the health authority document or the original trial, and gives a plain verdict — including where a claim we would like to be true simply is not supported.

An open printed medical journal article with a highlighter and reading glasses beside three plain supplement bottles
Short answer

Of the 24 headline supplement claims checked here, 10 are supported by a named health authority document or a published trial, 12 are not supported, and 2 hold only under specific conditions. The pattern is consistent: claims about meeting a nutrient requirement, about upper limits and about drug interactions usually survive scrutiny, while claims that a supplement prevents a disease — dementia, cancer, fractures, colds — usually do not, because the large randomised trials that tested those questions were negative.

All 24 claims, checked against their source

All 24 claims, checked against their source
Use caseAmount reported in sourcesWhat to know
Adults should not exceed 40 mg of zinc a day from food and supplements.SupportedThe NIH Office of Dietary Supplements sets the adult tolerable upper intake level for zinc at 40 mg a day from food and supplements combined. Many single capsules supply 50 mg. (NIH Office of Dietary Supplements — Zinc fact sheet.) Source
Zinc definitively shortens the common cold.Partly supportedMeta-analyses of lozenge trials delivering high elemental doses within about a day of symptom onset reported shorter illness. A 2024 Cochrane review was more cautious. The evidence does not support a swallowed high-dose capsule taken on day four. (Hemilä H. Zinc lozenges and the common cold: a meta-analysis. JRSM Open. 2017.) Source
Taking 1,000 mg of vitamin C in one dose delivers roughly ten times as much as 100 mg.Not supportedVitamin C absorption uses saturable transporters. Pharmacokinetic work shows the fraction absorbed falling and plasma levels plateauing as single doses rise, with the excess excreted in urine. (Levine M, et al. Vitamin C pharmacokinetics in healthy volunteers. PNAS. 1996.) Source
Vitamin C prevents colds in the general population.Not supportedThe Cochrane review found regular supplementation did not reduce cold incidence in ordinary adults, though it modestly shortened illness. A clear reduction in incidence appeared only in people under extreme physical stress. (Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane. 2013.) Source
A three-year trial tested 180 mcg a day of MK-7 in postmenopausal women.SupportedKnapen and colleagues ran a three-year randomised placebo-controlled trial at 180 mcg a day of MK-7, reporting reduced age-related decline in bone mineral density at the spine and femoral neck. (Knapen MHJ, et al. Osteoporosis International. 2013.) Source
Vitamin K2 is proven to keep calcium out of arteries and prevent heart events.Not supportedMatrix Gla protein requires vitamin K to inhibit soft-tissue calcification, which is real biochemistry. Trials measuring arterial outcomes have been mixed, and no health authority concludes that supplementation prevents cardiovascular events. (NIH Office of Dietary Supplements — Vitamin K fact sheet.) Source
People on warfarin must keep vitamin K intake consistent.SupportedWarfarin works by blocking vitamin K recycling, so changes in vitamin K intake shift the INR. Supplements should be discussed with the anticoagulation clinic before starting. (NIH Office of Dietary Supplements — Vitamin K fact sheet.) Source
Selenium supplements prevent prostate cancer.Not supportedSELECT randomised more than 35,000 men to 200 mcg a day of L-selenomethionine and found no reduction in prostate cancer risk. The trial was stopped early for futility. (Lippman SM, et al. SELECT. JAMA. 2009.) Source
A single brazil nut can supply more selenium than an adult needs in a day.SupportedThe NIH ODS puts one brazil nut at roughly 68–91 mcg of selenium, against an adult requirement of 55 mcg and an upper limit of 400 mcg. Content varies with soil selenium. (NIH Office of Dietary Supplements — Selenium fact sheet.) Source
Folic acid before conception reduces the risk of neural tube defects.SupportedThe MRC Vitamin Study demonstrated a substantial reduction in neural tube defect recurrence with periconceptional folic acid. The CDC advises 400 mcg daily for anyone who could become pregnant, starting at least a month before conception. (MRC Vitamin Study Research Group. Lancet. 1991.) Source
High-dose folic acid can hide a vitamin B12 deficiency.SupportedFolic acid can correct the anemia of B12 deficiency while neurological damage continues. The adult upper limit of 1,000 mcg of synthetic folic acid exists largely to reduce that masking risk. (NIH Office of Dietary Supplements — Folate fact sheet.) Source
Anyone with an MTHFR gene variant needs methylfolate instead of folic acid.Not supportedMTHFR variants reduce enzyme activity, but most people reach adequate folate status on ordinary folic acid, and public health advice for pregnancy still specifies folic acid because that is the form the prevention trials used. (NIH Office of Dietary Supplements — Folate fact sheet.) Source
Calcium is best absorbed in single doses of 500 mg or less.SupportedAbsorption efficiency is highest at single doses of 500 mg or less, so two smaller doses deliver more calcium than one large tablet. (NIH Office of Dietary Supplements — Calcium fact sheet.) Source
Calcium plus vitamin D supplements reliably prevent hip fractures.Not supportedThe Women's Health Initiative trial of 1,000 mg calcium plus 400 IU vitamin D in over 36,000 postmenopausal women found a small bone density gain, no significant hip fracture reduction overall, and more kidney stones. (Jackson RD, et al. N Engl J Med. 2006.) Source
Calcium can stop levothyroxine and some antibiotics from working properly.SupportedCalcium reduces absorption of levothyroxine, quinolone and tetracycline antibiotics and bisphosphonates. Spacing the doses apart is standard advice. (NIH Office of Dietary Supplements — Calcium fact sheet.) Source
Rhodiola rosea is a proven treatment for fatigue.Not supportedTwo small placebo-controlled trials of the standardized SHR-5 extract reported reduced fatigue. NCCIH states the studies are small and of variable quality and the evidence is insufficient to conclude rhodiola is effective for any condition. (National Center for Complementary and Integrative Health — Rhodiola.) Source
The rhodiola night-duty trial used a low daily amount of a standardized extract.SupportedDarbinyan and colleagues used a repeated low-dose regimen of the standardized SHR-5 extract over two weeks in physicians on night duty, reporting improvement on a mental fatigue index. (Darbinyan V, et al. Phytomedicine. 2000.) Source
Lion's mane improves memory in healthy adults.Not supportedThe most cited trial studied older adults who already had mild cognitive impairment, over 16 weeks, and scores fell back after supplementation stopped. There is no good evidence of memory benefit in healthy people. (Mori K, et al. Phytotherapy Research. 2009.) Source
Lion's mane regenerates nerves in people.Not supportedThe nerve growth factor results come from cell culture and rodent studies. They justify further research; they are not evidence of nerve regeneration in humans. (National Center for Complementary and Integrative Health — Mushrooms.) Source
Ginkgo biloba prevents dementia.Not supportedThe Ginkgo Evaluation of Memory study followed over 3,000 adults aged 75+ for a median of about six years on 240 mg a day and found no reduction in dementia incidence. The five-year GuidAge trial also found no reduction in progression to Alzheimer's disease. (DeKosky ST, et al. JAMA. 2008.) Source
Ginkgo should usually be stopped before surgery.SupportedGinkgo affects platelet function and bleeding events have been reported, so surgical teams commonly ask for it to be stopped one to two weeks beforehand. (National Center for Complementary and Integrative Health — Ginkgo.) Source
Raw ginkgo seeds are safe to eat.Not supportedFresh and roasted ginkgo seeds contain a toxin; ingestion has caused seizures and deaths. Supplements use a standardized leaf extract, which is a different product. (National Center for Complementary and Integrative Health — Ginkgo.) Source
Vitamin D needs a blood test to decide the right amount.Partly supportedHealth authorities publish a daily requirement and a 4,000 IU adult upper limit that apply generally. Correcting a suspected deficiency, rather than meeting the requirement, is what a 25-hydroxyvitamin D test informs. (NIH Office of Dietary Supplements — Vitamin D fact sheet.) Source
Creatine damages the kidneys in healthy people.Not supportedCreatine raises serum creatinine, a marker used to estimate kidney function, without evidence of kidney damage in healthy people. Anyone with existing kidney disease should ask their doctor first. (NIH Office of Dietary Supplements — Dietary supplements for exercise and athletic performance.) Source

Figures are amounts reported in the cited sources, not a personal recommendation. Your own amount depends on your health, medicines and blood work.

How a claim gets a verdict

A claim is marked supported only when a named health authority document or a published trial states it directly. Where a claim rests on laboratory or animal work, or on trials too small or too short to answer the question, it is marked not supported — even when the supplement itself is popular, cheap and well tolerated.

Partly supported is reserved for claims that are true under conditions most people do not meet: a specific format, a specific dose, a specific starting point, or a population under unusual stress. Zinc lozenges started within a day of a cold are the clearest example; the same zinc swallowed on day four is a different claim.

Nothing here is a recommendation about what you should take. Every amount quoted anywhere in the library is a figure reported in a cited source, which is not the same thing as a dose that suits your body, your bloods or your prescriptions.

  • Supported: a health authority or trial states the claim directly.
  • Partly supported: true only for a specific format, dose, timing or population.
  • Not supported: the human outcome data is absent, negative, or comes from cells and animals only.
A hand holding a supplement bottle turned to show its facts panel
A claim on the front of a bottle is marketing. The verdicts below come from the source documents behind the panel on the back.

The pattern behind the failures

The claims that fail share a shape. A supplement corrects a deficiency, or moves a marker in a laboratory, and the leap is made to preventing a disease. Vitamin K activates matrix Gla protein, which really does inhibit soft-tissue calcification — and trials of arterial outcomes have still been mixed. Lion's mane raises nerve growth factor in cell culture, and there is still no evidence of nerve regeneration in people.

The other recurring failure is population. The most cited lion's mane trial studied older adults who already had mild cognitive impairment, and the scores fell back once supplementation stopped. That is not a memory claim for a healthy 30-year-old. Vitamin C reduces cold incidence in people under extreme physical stress, not in ordinary adults.

Where large, long, randomised trials exist, they tend to settle the argument in the unglamorous direction. SELECT randomised more than 35,000 men to selenium and found no reduction in prostate cancer. The Ginkgo Evaluation of Memory study followed over 3,000 adults aged 75 and older for about six years and found no reduction in dementia. The Women's Health Initiative gave calcium plus vitamin D to over 36,000 postmenopausal women and found no significant reduction in hip fracture, with more kidney stones.

The claims that do hold up

The verdicts are not uniformly negative, and the supported claims are worth more attention than they usually get, because most of them are about safety rather than benefit. Zinc has a 40 mg adult upper limit that a single capsule can breach. Synthetic folic acid has a 1,000 mcg limit that exists largely because folic acid can correct the anemia of B12 deficiency while nerve damage continues.

Periconceptional folic acid reducing neural tube defects is one of the strongest findings in nutrition, and it is a claim about prevention that did survive a randomised trial. Calcium absorbing better in doses of 500 mg or less is a practical finding that changes how a supplement is taken rather than whether it is taken.

Interaction claims survive scrutiny more often than benefit claims. Vitamin K and warfarin, calcium and levothyroxine, ginkgo and surgical bleeding — these are established in clinical practice and are the claims most worth acting on.

What the published studies found

Each entry below links straight to the paper or the health authority page so you can read the original rather than take our word for it.

Side effects reported in the literature

Commonly reported

  • Zinc: nausea and stomach upset, and copper depletion with sustained intakes above the 40 mg adult upper limit
  • Vitamin C: diarrhea and abdominal cramps at large single doses, largely because absorption saturates
  • Calcium: constipation and bloating, and a higher risk of kidney stones in the Women's Health Initiative trial
  • Folic acid: usually well tolerated, with masking of B12 deficiency the reason for the 1,000 mcg limit

Serious, seek medical advice

  • Selenium: hair and nail loss and neurological symptoms from chronic intakes near or above the 400 mcg upper limit
  • Ginkgo: bleeding events have been reported, and raw ginkgo seeds are toxic
  • Zinc: prolonged high intake can blunt immune function through copper deficiency
  • Any supplement: a new rash, breathlessness or swelling after starting needs urgent medical attention

Summarized from NIH Office of Dietary Supplements fact sheets for health professionals. Report anything unexpected to your own doctor or pharmacist.

Interactions to check with a pharmacist

Reported interactions
Medicine or conditionWhat sources report
Warfarin and other vitamin K antagonistsWarfarin works by blocking vitamin K recycling, so any change in vitamin K intake shifts the INR. Discuss K1 or K2 supplements with the anticoagulation clinic before starting. Source
LevothyroxineCalcium reduces levothyroxine absorption. Standard advice is to separate the doses by several hours. Source
Quinolone and tetracycline antibioticsCalcium binds these antibiotics in the gut and reduces absorption; zinc can do the same. Space the doses apart as directed. Source
BisphosphonatesCalcium taken at the same time reduces absorption of the bisphosphonate. Source
Surgery and antiplatelet or anticoagulant medicinesGinkgo affects platelet function and bleeding events have been reported; surgical teams commonly ask for it to be stopped one to two weeks beforehand. Source
Copper statusSustained high-dose zinc competes with copper absorption and can cause copper deficiency, which is the main reason for the 40 mg upper limit. Source

This is not a complete interaction list. Check your own medicines with a pharmacist before combining anything.

Educational information only

This page is reference material, not medical advice. Supplements interact with prescription medicines and with each other. Talk to a doctor or pharmacist before starting anything, especially if you are pregnant, breastfeeding, managing a health condition or taking prescription medicine.

Frequently asked questions

How many supplement claims did you check, and how many held up?

We checked 24 headline claims drawn from our dosing guides. 10 are supported by a named health authority document or a published trial, 12 are not supported, and 2 are supported only under specific conditions such as a particular format, dose or population.

What makes a claim 'not supported' rather than just unproven?

A claim is marked not supported when the human outcome data is absent, negative, or limited to cells and animals. Selenium and prostate cancer, ginkgo and dementia, and calcium plus vitamin D and hip fracture were each tested in large randomised trials that did not find the claimed benefit.

Does 'not supported' mean the supplement is useless or unsafe?

No. It means that one specific claim is not backed by the evidence. A supplement can correct a genuine deficiency, be well tolerated, and still fail to prevent the disease it is marketed against.

Why does zinc for colds get a partly supported verdict?

Meta-analyses of lozenge trials delivering high elemental doses within about a day of symptom onset reported shorter illness, while a 2024 Cochrane review was more cautious. The evidence does not support a swallowed high-dose capsule started on day four of a cold.

Which supplement claims are most worth acting on?

The interaction and upper-limit claims. Keeping vitamin K intake consistent on warfarin, separating calcium from levothyroxine and certain antibiotics, stopping ginkgo before surgery, and staying under the 40 mg zinc and 1,000 mcg folic acid limits are all supported and all consequential.

Do I still need methylfolate if I have an MTHFR variant?

That is not established. MTHFR variants reduce enzyme activity, but most people reach adequate folate status on ordinary folic acid, and pregnancy advice still specifies folic acid because that is the form used in the prevention trials.

Where do the amounts quoted in your guides come from?

Every amount is a figure reported in the cited health authority document or trial, linked from the page it appears on. They are records of what was studied or what a limit is set at, not a recommendation for you.

How often is this fact check updated?

Verdicts are revisited whenever a guide is updated or a major review or trial changes the picture, and each verdict links to its source so you can check the current position yourself.

Keep reading

Related calculators & comparisons

More DoseRoutine calculators and side-by-side comparisons.

Sources

  1. NIH Office of Dietary Supplements — fact sheets for health professionals
  2. National Center for Complementary and Integrative Health — herbs at a glance
  3. Lippman SM, et al. SELECT. JAMA. 2009
  4. DeKosky ST, et al. Ginkgo Evaluation of Memory study. JAMA. 2008
  5. Jackson RD, et al. Calcium plus vitamin D and fractures. N Engl J Med. 2006
  6. MRC Vitamin Study Research Group. Lancet. 1991
  7. Hemilä H, Chalker E. Vitamin C for the common cold. Cochrane. 2013

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