All 18 people biotin helped already had a diagnosed condition
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.
What the study found
Every published success was in someone with a diagnosed condition
A review of biotin for hair and nails found 18 reported cases, and all 18 improved. All 18 also had something causing the problem in the first place: an inherited or acquired biotin deficiency, brittle nail syndrome, or uncombable hair syndrome. Not one was a healthy person.
The caveat · Case reports skew to successes
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.