Biotin
Supplements · Vitamins · Supplement, Typed Supps
The hair and nails vitamin — with a genuine lab test problem.
Stage by stage
- Intake (Tablet): Absorbed efficiently in the small intestine.
- Small intestine (Absorbed): Absorbed efficiently in the small intestine.
- Cells (Fatty acid metabolism): Used as a cofactor in fatty acid synthesis, relevant to skin and hair cell growth.
- Excess (Excreted in urine): Water-soluble, so intake beyond what’s needed is simply excreted.
What the evidence supports
- Correcting a documented deficiency — strong evidence. Clear. Deficiency is rare but real — seen in biotinidase deficiency, long-term raw egg white consumption, and some anti-seizure medication.
- Improving brittle nails in people with that specific problem — limited evidence. A few small, old, uncontrolled studies. It is the least weak of the cosmetic claims, which is not saying much.
- Growing hair or strengthening nails in people who are not deficient — insufficient evidence. The claim on which an entire market rests, and there is no good evidence for it. Hair loss has many causes and biotin deficiency is very rarely one of them.
Typical dose
30 mcg daily is the adequate intake for adults. The small brittle-nail studies used 2.5 mg (2,500 mcg) daily for several months, and products sold for hair and nails routinely supply milligram doses — hundreds of times what the body needs.
Upper limit: No UL. The Food and Nutrition Board could not set one because there are no reports of toxicity from high biotin intakes; research has used doses far above the adequate intake without adverse effects being reported
Timing
Any time, with food.
Time to effect
- Months: Any visible hair/nail change, only if you were actually deficient.
Medication interactions
- Laboratory blood tests (troponin, thyroid, hormone assays): High-dose biotin interferes with many immunoassays, producing falsely high or falsely low results. This has caused missed heart attacks and misdiagnosed thyroid disease. Stop it several days before testing and tell whoever takes the sample. (high)
- Anti-seizure medication (carbamazepine, phenytoin, primidone): These deplete biotin over time — a reason it is sometimes needed rather than avoided. (moderate)
Common myth
“Biotin supplements fix hair/nails for everyone.” In reality: True deficiency is rare. It can also skew certain thyroid and heart lab tests — mention it before bloodwork.
Food sources
More in Vitamins
In the glossary
Sources