Hair loss is surprisingly common in women, affecting more than 30 million in the U.S., according to the American Academy of Dermatology. It can start as early as puberty or much later in life. In fact, 38 percent of women 70 and older experience hair loss, in part because hair thickness decreases with age, especially after menopause. The condition often leads to low self-esteem, social anxiety, and self-imposed isolation.

Female pattern hair loss (FPHL) is characterized by thinning over the top of the scalp and sometimes the sides. In men, a male hormone known as an androgen is primarily responsible for hair loss, but the main cause of FPHL appears to be less clear-cut. Women normally produce male hormones too, and in some cases the condition is associated with excessive androgen levels. Other types of hair shedding in women may be related to estrogen loss or significant changes in estrogen levels, as in the postpartum period or in menopause. Hair loss can also result from breakage during hair treatments and styling, certain medication—including hormones—and conditions like iron deficiency, severe dieting, thyroid disease, lupus, and even stress.

Finasteride (Propecia) is approved by the Food and Drug Administration to treat male pattern hair loss (also called androgenetic alopecia). At higher doses, it's also approved to reduce the symptoms of enlarged prostate in men. It works by blocking the enzyme that is responsible for the conversion of testosterone to dihydrotestosterone, the androgen that causes the prostate to enlarge. It also reduces dihydrotestosterone levels in the scalp, which slows hair loss and even increases hair growth. Because finasteride has been effective in controlling male pattern hair loss, it has been used to treat female pattern hair loss, although it has not gained FDA approval for that purpose. Medication prescribed to treat a condition that does not have FDA approval for that use is known as "off-label." Doctors can legally prescribe any medication they deem appropriate for treatment.

"If a woman has female pattern hair loss and elevated androgen levels that we can document, then she is likely to respond to treatments that block or decrease androgens, and finasteride is an option," says Elise A. Olsen, M.D
., a professor of dermatology and oncology and director of the Hair Disorders Research and Treatment Center at Duke University Medical Center. "But often we will try it even in women who don't have elevated androgen levels because other treatment options are very limited, and these women may have sensitivity to androgens at the cellular level of the hair follicle that we can't document."
What is the evidence?

The only randomized, double-blind, placebo-controlled trial on this subject involved 137 postmenopausal women with low androgen levels and hair thinning at the front of their scalp. After 12 months, there were no significant differences in hair count between the patients receiving finasteride or a placebo, and both groups continued to lose hair. The authors concluded that finasteride was well-tolerated but did not reduce hair loss in these women.

Another group of researchers studied 48 premenopausal women with FPHL and elevated androgen levels: 36 subjects were randomized to treatment with one of three drugs with anti-androgen effects, including finasteride, and 12 subjects declined treatment but were observed. The study wasn't blinded, meaning that patients and researchers knew which medication was received. After a year, those taking finasteride continued to lose hair.

In contrast, two studies reported some success with finasteride, but neither was randomized, blinded or placebo-controlled. In one, 37 premenopausal women with FPHL but without androgen excess received finasteride along with an oral contraceptive. After 12 months, most of them showed improved hair density, although the contraceptive contained drospirenone, which has anti-androgen actions and may have contributed to that effect. And in a 2010 study, 86 pre- and postmenopausal Asian women with FPHL and normal androgen levels were treated with finasteride for 12 months, resulting in slight increases in hair density for 57 women, moderate increases in 10 women, and more substantial increases for four women. Anecdotal reports and small, uncontrolled trials have also noted beneficial results with hair loss in pre- and postmenopausal women with or without increased androgen levels.