Latisse is the same formula as Lumigan, Allergan’s eyedrops for glaucoma, which reduces eye pressure but also happens to grow lashes. Both are also known as bimatoprost ophthalmic solution. In early 2009, Allergan introduced the drug as a lash enhancer. According to Ms. Katt, Allergan sold $73.7 million worth of Latisse last year, expects to sell $140 million of it this year and estimates that net sales could ultimately exceed $500 million a year.

Some doctors said it was common knowledge that Latisse was easily available without a prescription.

“They say in the patient packet insert that it is doctor-prescribed, and that’s not true,” said Dr. Seth L. Matarasso, a dermatologist in San Francisco. “Patients in my area say they found it in their health club. It is dispensed by a variety of people. Controls are not that strict.”

Dr. Matarasso injects his fair share of Botox but said his sales of Latisse were “not astronomical,” largely because it is so easy to obtain. He does not sell Latisse online and does require an in-person consultation.

The risks for consumers vary. The Latisse they buy online could be adulterated or fake. But even when the Latisse is the bona fide Allergan product, bypassing a doctor means a patient is not necessarily taught proper application or made aware of the potential problems.

Image Latisse is promoted by Brooke Shields.

“When the F.D.A. approved this product for marketing, they made a determination that the side effects or misuse or inappropriate use could cause harm, and that’s why they restricted it to a prescription drug,” said Carmen A. Catizone, the executive director of the National Association of Boards of Pharmacy, which represents state agencies that regulate pharmacies and pharmacists. “If it was completely safe to use without doctor supervision, they would have deemed it over-the-counter.”

Excluding ophthalmologists, most doctors do not do eye exams before dispensing Latisse, but they do explain how to apply it. Latisse must be dabbed on the upper lash line only, since it can grow unintended hair — on the cheek, for example. People are not supposed to place it on the lower lash line or eyebrows (though some do), and they are meant to use the one-use applicators that come with Latisse to avoid infection and to give the right dose.