The one placebo-controlled biotin hair study found no difference
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.
What the study found
Three controlled studies exist, and the best one found nothing
Biotin's reputation for hair rests on almost no controlled evidence. A search found three studies: two in people on isotretinoin and in women after sleeve gastrectomy, both with multiple risks of bias. The strongest, double-blind and placebo-controlled, found no difference from placebo.
The caveat · No benefit shown, none ruled out
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.