There are lots of reasons why we lose hair; fortunately, some of them are partly or completely reversible. Here's a list of causes and what you can or can't do about them.

• Age: Well, everyone ages -- there's just no getting around it. We produce less hair, and the individual hairs we do grow are thinner. Plus, older hair is more prone to dryness and breakage, so be sure to take that into account in your daily hair-care regimen.

• Male-pattern baldness (androgenetic, alopecia): Yes, this can happen to women, too. But while men tend to lose hair at the top, women lose it equally from all parts of the scalp. Male-pattern baldness is caused by a reduction in estrogen, so it's most common in postmenopausal women. If this sounds like you, you should see a dermatologist. Minoxidil (Rogaine and other brands, used on the scalp) and finasteride (Propecia, a pill) are the only two FDA-approved treatments for this condition. However, finasteride must be avoided by women who might become pregnant. Some birth control pills are formulated to increase hair growth; estrogen-replacement therapy usually helps, too. There are a handful of other topical products that haven't been as well researched, but they may help some people; these include tretinoin (used in conjunction with minoxidil), azelaic acid and copper peptides. Cimetidine (an antacid) can sometimes reduce hair loss -- and you'll never have heartburn again. As a last resort, consider hair transplants, which are looking better than in the past.