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Cause: Genetics (the most likely culprit)

Treatment: Minoxidil, the only topical ingredient that's FDA approved to regrow hair in women. Rogaine has just launched a 5 percent minoxidil foam for women ($30; drugstores). (Previously, the only option for women was 2 percent.) Sadick also suggests in-office red-light laser treatments, which can increase the number of hairs as well as the diameter of the hair shaft.