27 findings · 8 substances · 2 well-evidenced
Controlled evidence that a hair supplement grows hair barely exists, and the best-studied one is better documented for skewing blood tests, heart-attack tests included. Skin gains that do show up are modest.
Regulators handle hydrolyzed collagen peptides as an ordinary food protein. The largest dose in the trials, 35g, went to ten men on one occasion, as an absorption test.
That 35g went to ten men on one occasion, with no long-term follow-up. All ten were men, so nothing here speaks to anyone else. And no formal ceiling means nobody has set one, which is not the same as evidence that any dose is safe indefinitely.
Six trials in 388 women compared metformin alone with metformin plus inositol. Cycles came more regularly and hair-growth scores fell about a point. Weight, glucose and insulin did not move.
The cycle-regularity interval's lower bound is 1.01, so one more small trial could take it to nothing. A one-point change on a hair-growth scale may not be visible to the person looking in the mirror. Six trials, 388 women, three to six months, and no fertility or pregnancy outcomes.
Many lab tests capture what they measure using biotin. Extra biotin competes for that capture: in one test format the result reads too high, in another too low.
The direction rule is settled; the amount of biotin needed to trigger it is not. That threshold differs by manufacturer, by test and by how recent the reagent is, and cannot be predicted from one analyser to another. The panel study behind it added biotin to pooled serum in a laboratory rather than measuring people who had swallowed any.
An inherited fault in the enzyme that recycles biotin leads to seizures and lost sight or hearing untreated. Consistent high doses halt much of it, and US newborns are screened.
This is a reference chapter, and it reports no incidence figure. The high doses used here treat a specific inherited fault in recycling biotin, and say nothing about biotin in people who do not have it. Reading across from this condition to general supplementation is precisely the error its existence invites. This reflects the chapter as it stood in December 2025.
Biotin is in eggs, nuts and legumes, and gut bacteria make more. Where people do run short there is usually a cause: long-term antibiotics, anticonvulsants, or intravenous feeding.
A clinical reference chapter rather than primary research: it synthesises rather than measures, and puts no number on how rare rare is. The at-risk list is qualitative, with no estimate of how much any of those exposures raises the risk. These chapters are revised continuously, and this reflects the text as it stood in September 2025.
The US Food and Drug Administration says many hair, skin and nail supplements hold 20mg of biotin, and some pills 100mg. The recommended daily intake is 0.03mg.
The dose figures are what the agency was aware of in 2019, not a survey of what is on shelves now. The 0.03mg intake figure is itself extrapolated from the biotin content of human milk rather than measured against a requirement, and no Recommended Dietary Allowance was ever set, for lack of evidence.
The US Food and Drug Administration lists, by manufacturer, cleared troponin devices whose makers had not addressed biotin interference. Fourteen are on it.
The table names devices the agency judged had not addressed the risk, and puts no number on the size of the error on any of them. Several entries are point-of-care strip tests rather than the laboratory analysers most hospitals run, and some clearances date to the 1990s. The page carries no version marker, so the count of fourteen is a snapshot of one day in September 2026.
A review of biotin for hair and nails found 18 reported cases, and all improved. Every one of them had a cause: a biotin deficiency, brittle nail syndrome or uncombable hair syndrome.
These are case reports, the weakest design there is: uncontrolled, unblinded and uniformly positive, which is the pattern publication bias produces. No randomised trial appeared in the search at all. The conditions involved are individually rare, so the group in which biotin did help is very small.
A search found three controlled studies of biotin for hair. The strongest, double-blind and placebo-controlled, found no difference from placebo; the other two carried risks of bias.
Three studies is a very thin base, and the review is limited by that rather than by its method: PubMed only, no registration, no pooled estimate. What it found is an absence of demonstrated benefit in people who are not deficient, which is not the same as demonstrated absence of benefit. The authors are practising dermatologists writing about a supplement their patients ask for.
In 23 samples from people on high-dose biotin, most of ten hormones read abnormal. Removing the biotin from the sample returned them to normal, and biotin-free assays read normal.
23 samples, and the exposure is multiple sclerosis treatment at doses far above any hair supplement, so the blood levels here are an upper bound rather than what a shopper reaches. Two authors worked for the company developing that high-dose product, which had a commercial interest in characterising this interference. Roche assays only, and the removal step is not perfectly neutral: it shifts parathyroid hormone by about 8%.
No upper limit for biotin has been set, for lack of data, and no toxicity has been seen up to 200mg by mouth. A 10mg daily dose was already enough to disturb a troponin test.
Absence of reported harm is not demonstrated safety; it is what an unset upper limit actually means, and the panel that could not set one said so. That panel was writing in 2012 about intravenous nutrition products, before the interference literature existed, and is silent on blood tests.
The results were markedly abnormal and did not match how the patient looked. After biotin was stopped they normalised far faster than thyroxine's half-life allows.
A single case, paired with a review of other single cases: this shows that something can happen, never how often. The interference was inferred from how fast the results normalised rather than by measuring biotin in the sample. Assay formulations from 2016 have since been revised for biotin tolerance, and this patient's own dose is not a market-typical exposure.
Healthy volunteers took 10mg of biotin a day, about half what hair supplements commonly hold. Two of three troponin assays were untouched; on the newest, a missed diagnosis is possible.
Nobody in this study was misdiagnosed; what was measured was the assay, not a patient outcome. Only three assays were tested, so it cannot say what any particular hospital runs. Whether interference of this size changes a decision depends on where a person's true troponin sits relative to the cut-off, which the design does not test.
Adding biotin to plasma suppressed a Roche troponin result by 24% at 50 micrograms per litre, 56% at 100 and 97% at 500. A genuinely high result fell as far as a low one.
Nobody in this experiment had swallowed any biotin; it was added to plasma in a laboratory, and added biotin may not behave quite like biotin absorbed alongside its own breakdown products. The two highest concentrations tested sit far above what ordinary supplement users have been shown to carry. What was measured was the size of the error, not a missed diagnosis.
The trials ran 4 to 16 weeks at daily amounts spanning 0.4g to 12g, and no recommended figure falls out of them. One 160-person trial used 10g a day.
The joint trial tested one branded product, and two of its authors work for the manufacturer and for the contract firm that ran it, so an independent repeat is still missing. Several outcomes in the largest synthesis, skin roughness among them, came out flat.
It is broken into amino acids and short peptides before absorption. Across 113 trials the gains in skin elasticity, hydration and joint pain are real anyway.
Finding the fragment in blood shows it survives digestion; it does not show it did anything afterwards. The signalling step rests on animal and cell-dish work rather than on the human trials the reviews cover. That headline pools 113 trials through 16 earlier reviews, so its precision is theirs rather than its own, and several outcomes within it, including skin roughness and two joint sub-scores, came out flat. The peptide work came from a research institute owned by a collagen manufacturer.
Collagen is digested into free amino acids and short peptides. The marker fragments peak in blood 1 to 2 hours after a dose and stay detectable for up to 24.
These are absorption curves for a few marker fragments, not a clearance time for the ingredient as a whole. Collagen from different animal sources may behave differently. And finding the tough fragment in blood shows it lasts, not that it does anything while it is there.
In the one placebo-controlled trial of these injections, eight of sixteen had disturbed liver tests, one went into anaphylaxis, and the lightening never reached significance.
Thirty-two people finished that trial, and the gap between glutathione and placebo ran close to the significance line, so the honest reading is no proven benefit rather than proven uselessness. The August recall reports came from the pharmacies themselves rather than independent checking, and neither notice names a death or a hospital admission.
Single doses at 900 and 1,800 mg changed nothing. Two weeks at 1,800 mg a day did raise ultraviolet sensitivity measurably, and that sits at the top of what products give.
Sensitivity was measured on small patches of back skin under standardised lamps, which is a laboratory endpoint rather than sunburn in ordinary life. The two-week arm had no placebo group and compared each person against himself. Blood levels of hypericin, the constituent products are standardised on, did not track photosensitivity at all.
The evidence disagrees with itself, or the finding carries a caution. Either way the marker shows one star, however strong the individual sources are.
Over 12 weeks, 5g of creatine a day left resistance-trained men no different from placebo in hair density, follicle count, thickness or DHT.
Twelve weeks is short against a process that takes years, so a slow thinning could begin after the trial ended. The 38 men who finished were all resistance-trained and aged 18 to 40, which leaves everyone else untested. And this was maintenance dosing at 5g a day; the older study that started the worry ran a 25g loading week first.
The creatine hair worry and the hair trial measured different things: a hormone shift during a week of 25g a day, and hair itself after twelve weeks of 5g a day, which showed nothing. Both may be right.
Men on these drugs are diagnosed with melanoma somewhat more often. One pooling of 866,049 reads that as sunlight showing through; a later one calls it reason to avoid them.
Every study on both sides is observational, so men who take these drugs differ from men who do not in ways no adjustment removes, sun exposure among them. They also see doctors more often, which alone yields more diagnoses of a cancer found by looking. The newer pooling's studies barely agree.
Men on erectile-dysfunction drugs are diagnosed with melanoma more often. What that means is unsettled: one analysis reads it as shared sun exposure, and a later one urges at-risk men to avoid the drugs without answering that argument.
The US Food and Drug Administration knows of one patient who died after a falsely low troponin result on an affected test. That is one voluntary report, and the agency names no rate.
Voluntary adverse-event reporting shows that something can happen, never how often. The death report carries no published clinical workup, so nothing in it rules out another cause. The agency's figures describe what it was aware of in 2019, in the United States, where assay availability and reagent generations have both moved on since.
In a US emergency department, 7.4% of samples held biotin at the lowest level known to shift a test. In another US cohort it was 1%. In 524 UK samples, none did.
Being above the threshold is not the same as a test actually failing, and none of the three reports a wrong result or a harmed patient. All three read exposure off leftover samples or medical records rather than asking people directly, which undercounts a vitamin that is often an unadvertised ingredient in a beauty product. The UK samples were anonymised, with no record of who those patients were or what they were taking.
There is no single rate of people carrying enough biotin to skew a blood test. Three studies using one threshold disagreed widely, and the gap tracks who was sampled and where, so a clean result may only mean few there take supplements.
Older Roche reagents shifted by 20% once biotin passed 500 nanograms per millilitre; newer ones tolerated 1,000 to 3,000. Troponin was not among the assays tested.
78 patients in one Taiwanese hospital, and the tolerance figures come from spiking reagents rather than from those patients. Comparing two manufacturers in a single laboratory does not establish a general ranking between them. The highest patient level found was about 3 nanograms per millilitre, so this shows the reagents have improved rather than that these patients were ever at risk.
Most of what is known about biotin skewing blood tests was measured on older reagents. Newer ones tolerate far more, but troponin, the heart-attack test, was not checked, so whether the problem has shrunk depends on a lab's kit.
More than 150 published patients had a thyroid result thrown off by one of six known interferences, biotin among them. At least half were misdiagnosed or mismanaged.
One review advises stopping biotin more than 48 hours before a blood test, reasoning from how fast the body clears it. Regulators say there is not enough information to know.
The 48 hour figure is the review authors' inference from biotin's half-life, and no study they cite establishes that it clears interference at any dose. It also cannot help the people most exposed: someone arriving at an emergency department has no notice, and people taking biotin for biotinidase deficiency, a mitochondrial disorder or multiple sclerosis cannot stop.
Stopping biotin more than 48 hours before a blood test is advice reasoned from how fast the body clears it. No study has tested the interval, and the US regulator says there is not enough information to know if any stop works.
At 250 to 500mg a day for four to twelve weeks, forearm and facial skin measured lighter than placebo while covered skin did not change. The lightening fades on stopping.
The trials ran 46 to 124 people over four to twelve weeks. In the twelve-week trial the change concentrated in participants over 40 rather than spreading across the group, and in the four-week trial the count of sun spots never moved even as the melanin reading did. No trial measured glutathione in the blood, so nothing rules out the capsules working through something other than the mechanism the marketing assumes. Participants were mostly Asian adults already seeking lighter skin.
Nobody disputes which way swallowed glutathione pushes skin tone. What is open is whether a handful of small, short trials is enough to call it settled, and reviews split on how much of that evidence to admit.
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