“What is the best vitamin for hair loss?”

Unfortunately, there is really no one vitamin that directly helps hair loss. Biotin, which was recommended for years, has been shown to not being particularly effective, as was once thought. Instead, its use, especially in the high doses that were recommended in the past, has been shown to interfere with certain critical blood tests and can lead to misleading results. Your best best is a consultation with a dermatologist, a specialist in hair loss conditions, who can diagnose your specific type of hair loss and prescribe certain topical and/or medications that have proven safety and efficacy for retaining and even growing hair. It is best not to wait, since the early one intervenes, the more likely one is to be successful. Best of luck.

See my answer above. If you are losing hair you should see a board certified dermatologist for evaluation and treatment. Taking a full a to z multivitamin is helpful. Using Rogaine may also be heloful.