The scientific community has yet to make its' mind up about using Retin-A as a hair loss product. The medicine has been approved by the FDA for treating acne and sun-damaged skin, which at least means it's reasonably safe. However this doesn't mean that it has any effect on hair loss.
The anecdotal word on the grapevine is that it does in fact promote hair re-growth, by unblocking follicles, and works best in combination with minoxidil. Retin-A is the brand name for tretinoin (a 'cousin' of vitamin A) and must be used with caution; too much and it will actually cause hair loss.
So far clinical trials studying the combined effects of tretinoin and minoxidil have been limited to such a point that no real statistical significance can be drawn from the results (due to the size of the trial group), nevertheless they look encouraging; with more than 60 per cent of the group showing some degree of re-growth after a year. A number of dermatologists have noted that this combination of medicines appears to work all over the scalp (rather than predominantly on the crown), making it ideal for the treatment of female pattern baldness (which is marked by overall thinning, rather than a receding hairline). Tretinoin is also though to work on its own, although not as well.
Retin-A comes as a topical solution (which means that it's applied directly to the scalp as a liquid, cream or a gel) and is available in a variety of strengths. Research indicates that there isn't all that much difference in efficacy between the weakest (0.01%) and strongest (0.1%) solutions, and because of the risk of side effects most people opt for something in the middle (typically 0.025%). Common side effects include; itchy scalp (Aloe Vera or Lamin gel may help) and photosensitivity. Often initial tenderness will disappear after a week or so, but if you have any concerns contact your dermatologist. Retin-A should be used with caution if you're already taking vitamin supplements.