Flirtatious looks may be getting a boost, now that a drug that makes eyelashes grow longer and darker has been approved for that use by the Food and Drug Administration.

Latisse, manufactured by Allergan Inc., will be marketed as a lash-enhancement product beginning in February. Originally a glaucoma treatment with the brand name Lumigan, it turned out to have an unexpected side effect.

Doctors and patients found the seven-year-old drug not only reduced eye pressure, but also made for lusher lashes, leading to increasingly popular off-label use of the eye drops. Both products are based on a substance called bimatoprost, and contain it in the same strength.

"It's a very strong drug that lowers eye pressure, and it's been noted one of the side effects is increased eyelash growth," said Dr. Robert Noecker, an ophthalmologist at the University of Pittsburgh Medical Center who has treated thousands of patients with the glaucoma drug since 2001.

"We didn't anticipate the eyelash growth but it was noted in clinical trials. It's been used off-label ever since the drug came out.

"There are no systemic safety issues," Dr. Noecker said, "but it can make your eyes bloodshot and cause discoloration of the eyelid skin. If your eyes are light brown or hazel they can get darker brown."

The company's Web site says the change in iris color may be permanent.

With the federal approval, Latisse will be sold as what the company calls "the first and only science-based treatment approved by the FDA to enhance eyelash prominence as measured by increases in length, thickness and darkness of eyelashes."

Other companies, including Jan Marini Skincare, have used bimatoprost for lash thickening in some of their products, but this new application renews Allergan's patent and the company is moving to force the other products off the market. The patent also will keep any generic forms of the drug at bay for years.

"It's the same thing that happened when Allergan got approval to market Botox as an anti-wrinkle treatment," said dermatologist Suzan Obagi, director of the UPMC Cosmetic Surgery and Skin Health Center.

"This will be a huge money-making drug," she continued. "I haven't jumped yet because I wanted to make sure we heard from the FDA about what other things to look out for."

Even with federal approval, she said, "I'm going to dispense it judiciously. If the darkening of the eye color can be permanent, that raises red flags. I need to see what the studies found about the effect on the pigment-making cells in the skin, because those can produce not just color but melanoma."

Melanoma is the most serious form of skin cancer and can be fatal.

Latisse will be sold by prescription ($120 a month, and insurance may not cover it) to be applied once a day with a special applicator to the base of the upper eyelashes. The company warns against use on the lower lashes because it may cause dark circles under the eyes. It says users can expect lusher lashes in eight to 16 weeks. But to maintain the effect, continued treatment is required.

Hair grows in three stages, Dr. Noecker explained. Bimatoprost works by increasing the time a follicle is in the growth stage.

"It pretty much grows lashes in 100 percent of cases," he said. "You don't need mascara when you're on this drug."

Some patients have had to trim their lashes because they were brushing up against their glasses, he said, but eyelashes fall out regularly anyway.

"Lots of bald ophthalmologists have looked at using it to grow hair on men's heads," he added with a laugh, "but it doesn't seem too effective."

Eyelid skin is the thinnest skin on the body and easiest to penetrate, he said, and eyelash follicles are thin as well. Skin and hair on the head are much thicker.

Dr. Paul Leong, facial plastic surgeon at Western Pennsylvania and Sewickley Valley hospitals, said he reviewed the FDA trial studies closely and found them convincing.

"It's a well designed, double-blind, placebo-controlled study," he said. "Neither the patients nor the observers knew who got the real stuff and who got the placebo. What they found was that for those who got the drug, their lashes grew up to 25 percent longer, the thickness more than doubled, and the darkness improved by 20 percent.

"It's a cosmetic treatment, so do we need it? No. But it's definitely a complaint that many patients have, and this addresses it."