First it was frozen foreheads. Now it’s Betty Boop 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 lash-lengthening 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.