Mention Merck and what comes to mind? Vioxx? Gardasil? Vytorin? Layoffs? But probably not Singulair, the asthma med that rang up $4.3 billion in sales last year, which makes the pill its biggest seller. The patent doesn’t expire until 2012, so one would think much-needed sales are safe for awhile, right?

Well, maybe. Merck faces a looming patent challenge from Teva Pharmaceuticals and, while the outcome is never assured, the litigation raises the prospect of a so-called ‘at risk’ launch of a generic Singulair as early as this August, which is when a 30-month stay, or waiting period, expires for generic versions, according to an investor note from Credit Suisse analyst Catherine Arnold.

The drugmakers square off in federal court in February to resolve Singulair patent litigation and she writes that Merck is likely to prevail, which means nothing much would change. Then again, a settlement giving Teva some exclusivity could yet occur, because Merck would probably like to avoid the experience Wyeth encountered with Teva.

Last year, you may recall, Teva surprised Wyeth by launching a generic Protonix in the midst of patent litigation, grabbing sales and forcing Wyeth to sell its own generic version (back story), although Teva soon halted shipments. The episode continues to spook investors, some of whom may fear a repeat with Singulair. And that fear, Arnold writes, just “can’t be eliminated.”