By NATASHA SINGER


First it was big lips and frozen foreheads. Now it’s long eyelashes.


Allergan, the company that turned an obscure muscle paralyzer for eyelid spasms, Botox, into a blockbuster wrinkle smoother, hopes to perform cosmetic alchemy yet again. At the end of the month, the company plans to introduce Latisse, the first federally approved prescription drug for growing longer, lusher lashes.


The product has the same formula as Allergan’s eye drops for glaucoma, called Lumigan. It is one of several drugs in a category known as prostaglandin analogs, which are meant to reduce dangerous pressure in the eyeball. But as a side effect, the treatment tends to make the eyelashes of many patients longer and fuller.


Some medical experts say they worry that cosmetic customers may occasionally experience some of the glaucoma drug’s other side effects, which can include red, itchy eyes and changes in eyelid pigmentation. Some financial analysts, meanwhile, wonder how many people will want to spend $120 for a monthly dose of lashlengthening Latisse.


But other analysts predict that in a world where people spend about $5 billion a year on mascara, Latisse could be the biggest thing to hit cosmetic medicine since, well, Botox. Sales for the cosmetic use of Botox were $600 million in 2007.


“I think this is a fairly big deal,”said Ronny Gal, a senior research analyst in specialty pharmaceuticals at Sanford C.Bernstein, an equity research company.“It is a new idea in a market that loves new ideas and, frankly, it works.”


David E.I.Pyott, Allergan’s chief executive, predicted that in the long term, worldwide sales of Latisse would exceed $500 million. And he suggested that many women would not hesitate at spending $120 for a one-month, three-milliliter supply of the drug. He compared the cost of longer lashes to a daily cup of coffee.


“If you think about it in terms of luxury, it’s four dollars a day,”he said.


But one analyst, Gary Nachman, director of specialty pharmaceuticals at the investment bank Leerink Swann, said the expense of Latisse and the inconvenience of obtaining a doctor’s prescription might deter many women from trying it. He predicted Latisse would have sales of only $80 million by 2012.“I don’t think this is going to be a huge game changer,”Mr. Nachman said.


Some women who have already tried Latisse are fans.“People would say to me‘Are you wearing false eyelashes?’- even my own mother asked,”said Cindy Ross, vice president for sales at Young Pharmaceuticals in Wethersfield, Connecticut, who participated in a Latisse clinical trial.


Ms.Ross said she liked the effect so much that she had a doctor prescribe the glaucoma drug to use on her lashes until Latisse becomes commercially available.“I wouldn’t stop,”Ms. Ross said. “I found a way to get it.”