Biotin and hair growth: the myth and the real risk
Biotin is the headline ingredient in most hair, skin and nail supplements, usually at doses hundreds of times what anyone needs. The evidence for the hair claim is much thinner than the shelf space suggests, and there's a safety issue attached that almost nobody mentions.
What the evidence actually supports
Start with a labelling correction, because it matters. Biotin doesn't have an RDA. The Food and Nutrition Board found the data insufficient to set one and established an Adequate Intake instead — 30 mcg/day for adults. Anyone quoting 30 mcg as an RDA is quoting the right number with the wrong label.
On hair and nails, NIH is blunt: the claims are supported, at best, by a few case reports and small studies. The brittle-nail studies had no placebo control and didn't report baseline biotin status. For hair specifically, the evidence is largely case reports in people with rare deficiency conditions. If you are genuinely biotin-deficient, repleting helps. Most people taking these supplements are not.
The safety issue nobody mentions
This is the part worth knowing regardless of whether you care about the hair claim. High-dose biotin interferes with lab tests that use biotin-streptavidin assay technology. A single 10 mg dose was enough to interfere with thyroid tests within 24 hours, and NIH documents falsely diagnosed Graves' disease and a patient death from falsely low troponin results.
The FDA continues to receive adverse event reports of biotin causing falsely low troponin results — the test used to diagnose a heart attack — and lists assays that still haven't addressed the risk.
What to actually do
If you take a hair supplement, look at the biotin number. Doses of 5,000 to 10,000 mcg are common and are hundreds of times the AI.
More importantly: tell any provider ordering bloodwork that you take biotin, and ask whether to stop it beforehand. That single sentence is worth more than the supplement is. If your hair is shedding, the more useful workup is ferritin, thyroid function and protein intake — not a bigger bottle of biotin. Those three explain far more shedding than biotin status does, and unlike biotin they're worth measuring before you treat.
