A B vitamin that helps hair only if you are deficient, which is rare. In shampoo it rinses away before it can do anything useful.
Biotin is a B vitamin the body needs to make keratin. Genuine deficiency (from rare genetic conditions, some medicines, or eating lots of raw egg white) causes hair loss, and correcting it helps.
Most people are not deficient. Added to shampoo or conditioner, biotin sits on the scalp for a minute or two and is rinsed off; there is no evidence it is absorbed in useful amounts or changes hair.
A systematic review found that every reported case of biotin improving hair or nails involved a deficiency or an underlying disorder, with insufficient evidence for healthy people. There are no controlled trials of topical biotin in shampoo.
All 18 reported cases where biotin improved hair or nails involved an underlying disorder or deficiency; there was no sufficient evidence for biotin in healthy people.