An unnamed drug maker has applied for U.S. regulatory approval to sell a generic version of Allergan Inc.’s eyelash-enhancing drug Latisse before its patent protection expires.

Latisse, approved in late 2008 with $73.3 million in sales last year, has market exclusivity until late 2011. But the  Food and Drug Administration disclosed last week that a company filed an application in May to market a copycat version by challenging Allergan’s patent for the drug. The FDA didn’t identify the applicant of the generic drug. An Allergan spokeswoman acknowledged the FDA filing but couldn’t comment further because the Irvine, Calif., company hasn’t officially been notified by the generic challenger.

Allergan makes a range of cosmetic medical products and eye drugs, and Latisse is  a small but high-profile contributor. The company posted total sales of $4.5 billion in 2009.

Latisse represents a case of a drugmaker conjuring something new from the side effect of another product. In this case it’s Lumigan, an Allergan glaucoma drug that has the same underlying medication — called bimatoprost — and side effects that include a change in eye color and thicker lashes.

J.P. Morgan projects that sales of Latisse will hit $140 million this year and $250 million in 2013.

The patent covering  bimatoprost expires in September 2012 for both drugs. Latisse has other patents covering the method of the drug’s use that stretch to 2024, according to an FDA listing.

Even if the challenger is successful, the launch of a generic is unlikely for years. The application was filed with a “Paragraph IV” certification, meaning that the applicant believes the relevant patent is invalid or will not be infringed by the generic.

Under the law, if a patent owner sues a generic applicant for infringement within 45 days of receiving notice, FDA approval of the generic is stayed for 30 months.

The automatic 30-month stay allows time for resolution of the litigation while protecting the patent holder from harm that could ensue from the FDA granting marketing approval to a potentially infringing product. If the suit isn’t resolved by then, the generic company can launch the product “at risk” and be liable for triple damages if it loses the patent case.

Latisse has a heavy marketing budget for the drug, which actresses Brooke Shields and Claire Danes promote. Latisse sales were soft in the first quarter, when Allergan said it learned users of the relatively new product were stretching out their supply. The company in April said it aimed to combat this with more advertising and offering Latisse buyers discounts on Botox and the facial filler Juvederm, both Allergan products.