I originally started using prenatal vitamins for iron deficiency, but it wasn't bad enough for iron pills. But they're good for every woman, pregnant or not, it's a multivitamin... originally used to increase your vitamin intake during pregnancy. But there's no harm in taking them when not pregnant. My doctor suggested I start taking them when I was 14, and when I remember to take them regularly my hair grows faster, and nails grow thicker and faster as well.
This is relative to loss of head hair? My wife has thyroid problems and is losing haid hair...que sera...and least she has a full bush and was never so stupid that she got rid of her pubes permanently...(:
Ultimately, the benefits of vitamins are negligible if you have a balanced diet.
I take prenatal vitamins and I have noticied that it works wonders for my nails especially and I am sure my hair benefits as well. The downsides are limited enough unless you take too much, I think it's not a bad idea.