Biotin is the most sold supplement for nails, and the most confidently marketed. The science behind it is a handful of small trials from more than thirty years ago, all without a placebo group, all in people whose nails were already brittle. That is not nothing. It is also far less than the labels suggest.
Since 2017 there has been a second reason to read carefully before buying. The United States Food and Drug Administration has warned that biotin at supplement doses can produce falsely high or falsely low results in laboratory tests, including tests used to diagnose heart attacks. The nail question and the safety question belong together.
What is biotin and what does it do?
Biotin is a water-soluble B vitamin (vitamin B7) that acts as a helper for enzymes that build fatty acids and process glucose and amino acids. The NHS describes it as needed in very small amounts to help the body make fatty acids, and notes that bacteria in the gut make biotin, so it is not clear that most people need any extra from the diet (NHS).
The United States National Institutes of Health sets the adequate intake for adults at 30 micrograms a day (NIH Office of Dietary Supplements). Cooked beef liver provides about 30.8 micrograms per 85 g serving, a whole egg about 10 micrograms, canned salmon about 5 micrograms per 85 g. Deficiency is rare in people who eat a mixed diet.
The link to nails comes from keratin. Biotin-dependent enzymes are involved in the lipid metabolism of keratinised tissue, and true biotin deficiency causes hair loss and skin rashes. Whether extra biotin improves nails in people who are not deficient is the open question.
What did the biotin and nail studies find?
The NIH fact sheet summarises three studies that are still the basis of every biotin-for-nails claim. All three used the same dose, 2.5 mg a day, in people with brittle nails, and none compared biotin with a placebo (NIH Office of Dietary Supplements).
| Study | Participants | Dose and duration | Reported result |
|---|---|---|---|
| Swiss study, 1989 | 45 patients with brittle nails | 2.5 mg a day for about 5.5 months | Firmer and harder fingernails in 41 of 45 (91 percent) |
| Nail thickness study, 1990 | 22 women, 10 controls | 2.5 mg a day for 6 to 15 months | Nail thickness up 25 percent in 8 patients measured at once; up 7 percent, not significant, in 14 measured later |
| Retrospective review, 1993 | 35 patients | 2.5 mg a day for 6 to 15 months | Clinical improvement in 22 of 35 (63 percent) |
Read together, the studies say that a group of people with brittle nails who took 2.5 mg of biotin for around six months mostly reported better nails. They do not say how many would have improved anyway, because nobody in the trials took a dummy pill for comparison.
How strong is that evidence?
Weak, by the usual standards of medicine. The NIH conclusion is that claims for biotin and nails are supported, at best, by only a few case reports and small studies, and that future studies are needed (NIH Office of Dietary Supplements). The three trials date from 1989 to 1993, together cover about 100 people, and were not blinded.
Brittle nails also improve on their own. A plate takes about six months to grow out, so anyone who starts a supplement and also starts wearing gloves, using cuticle oil or taking a break from gel will see better nails by month six regardless. Without a placebo group, a study cannot separate the biotin from the rest.
There is one more gap. All three studies recruited people whose nails were already brittle. There is no trial showing that biotin makes normal nails stronger, longer or faster-growing. For someone with healthy nails, the realistic expectation is no visible change.
Why does the FDA warn about biotin and blood tests?
Many laboratory tests use a biotin-based chemistry to capture the molecule being measured. Extra biotin in the blood competes with that chemistry and can push the result up or down. The NIH fact sheet reports the FDA's warning that high biotin intake can cause clinically significant falsely high or falsely low laboratory test results, which could lead to inappropriate patient management or misdiagnosis (NIH Office of Dietary Supplements).
The FDA first published its safety communication on biotin in November 2017 and updated it in November 2019, with particular attention to troponin, the blood marker used to diagnose a heart attack. A falsely low troponin result can hide a heart attack (FDA safety communication). Thyroid function tests are also affected. According to the NIH, even a single 10 mg dose of biotin has interfered with thyroid function tests taken within 24 hours.
The practical rule is disclosure. Anyone taking biotin should tell every doctor, nurse and midwife who orders a blood test, and should ask whether to stop the supplement for a few days beforehand. In an emergency, tell the hospital staff at once.
How much biotin did the trials use, and how does that compare with supplements?
The trial dose of 2.5 mg is about 80 times the adult adequate intake of 30 micrograms. Supplements sold for hair, skin and nails often contain more than the trial dose, and the NIH reports interference with thyroid tests from a single 10 mg dose (NIH Office of Dietary Supplements). The NHS states that taking 0.9 mg or less a day of biotin in supplements is unlikely to cause harm, and that there is not enough evidence to know the effects of high daily doses (NHS).
| Amount per day | What it represents | Source |
|---|---|---|
| 30 micrograms (0.03 mg) | Adult adequate intake | NIH |
| 0.9 mg (900 micrograms) | Supplement level the NHS considers unlikely to cause harm | NHS |
| 2.5 mg | Dose used in all three brittle-nail studies | NIH |
| 10 mg | Single dose reported to interfere with thyroid tests within 24 hours | NIH |
Two things follow. The nail dose is above the NHS comfort threshold, so it is a decision to make with a clinician, not a default. And there is no evidence that more than 2.5 mg does more for nails; higher doses add test interference without adding benefit.
Who may benefit from biotin, and who should leave it?
The people in the trials had brittle nails with no other explanation. That remains the group most likely to see a change. The Cleveland Clinic lists biotin among treatments for nail ridges and brittleness, with the condition that it is taken with a clinician's approval (Cleveland Clinic).
Brittleness has causes that biotin will not touch. Repeated wet and dry cycles, solvents in removers, iron deficiency, thyroid disease and ageing all produce splitting or ridged nails. A GP can check iron and thyroid with a simple blood test, and that test should be done before any biotin is started, because biotin distorts the thyroid result.
Leave biotin, or ask first, if you are pregnant or breastfeeding (ask a midwife or GP), if you have a thyroid condition monitored by blood tests, if you have heart symptoms or a planned cardiac work-up, or if you take any supplement that already contains biotin, as many multivitamins and hair formulas do.
What should you do before trying biotin for your nails?
You will need:
- A GP or pharmacist appointment
- A note of every supplement you already take
- A phone photo of your nails in daylight, dated
- Gloves and a cuticle oil, so the supplement is not the only change
- Rule out other causes first. Ask a GP to check iron and thyroid function before you start, because biotin can distort those results afterwards.
- Fix the mechanical causes. Gloves for wet work, oil on the cuticles daily, a break from gel and acetone soaks. Many brittle nails recover on this alone.
- Agree a dose. The trials used 2.5 mg a day; there is no evidence for more.
- Set a time limit. Nails take about six months to grow out, so judge the result at six months, not six weeks.
- Tell every clinician who orders a blood test that you take biotin, and ask whether to pause it beforehand.
- Compare the dated photos at six months and stop if there is no change.
- Three small studies from 1989 to 1993 gave 2.5 mg of biotin a day to people with brittle nails and reported improvement in 63 to 91 percent, with no placebo group.
- There is no trial showing biotin improves nails that are already healthy.
- The FDA warns that supplement-level biotin can produce falsely high or low blood-test results, including thyroid and troponin tests.
- The trial dose is about 80 times the adult adequate intake of 30 micrograms and above the 0.9 mg a day the NHS calls unlikely to cause harm.
- Check iron and thyroid with a GP before starting, tell every clinician you take it, and judge the result at six months.
Questions people ask
How long does biotin take to work on nails?
The studies ran for about 5.5 to 15 months and a fingernail takes about six months to grow out fully. Any change from biotin appears in new nail growth, so six months is the realistic point to judge it.
How much biotin should I take for nails?
The three published studies all used 2.5 mg a day. There is no evidence that higher doses work better, and the NHS regards 0.9 mg a day or less as unlikely to cause harm. Agree a dose with a GP or pharmacist.
Can biotin affect blood tests?
Yes. The FDA warns that biotin at supplement doses can cause falsely high or falsely low results, including thyroid tests and the troponin test used to diagnose heart attacks. Tell any clinician ordering a blood test that you take biotin.
Does biotin make nails grow faster?
No study has shown a faster growth rate. The trials measured firmness and thickness in people with brittle nails. Growth speed is set by the nail matrix, at around 3 to 4 mm a month.
Is it better to get biotin from food?
For general health, yes. Eggs, liver, salmon, sunflower seeds and sweet potato all contain biotin, and gut bacteria make some too. The 2.5 mg dose used in the nail studies is far above what food provides, so the nail effect, if real, is a supplement effect.