The return address on the envelope was exciting, but the letter inside failed to deliver the goods.

Amylin Pharmaceuticals (NASDAQ:AMLN) and marketing partner Eli Lilly (NYSE:LLY) got word yesterday that the FDA is finishing up its review of the companies' supplemental marketing application to use diabetes drug Byetta as a monotherapy. (The companies don't expect to hear about an approval until next year.)

Unfortunately, that wasn't the version of the drug that Amylin investors care about.

Shareholders really want to hear about how much longer it'll be before the duo can submit the marketing application for the once-weekly version of Byetta. Last month, Amylin and co-developer Alkermes (NASDAQ:ALKS) said that the easiest path to approval was blocked, but there's still no word on how long the delay will be.

I wouldn't expect twice-daily injectable Byetta to get much of a bump in sales from monotherapy approval. After all, there are so many effective oral drugs available -- like Merck's (NYSE:MRK) Januvia, Takeda's Actos, and metformin, the long-genericized version of Bristol-Myers Squibb's (NYSE:BMY) Glucophage.

Byetta does promote weight loss, but is that really worth it for patients who would have to poke themselves with a needle twice a day? Once a week -- sure. And that's why everyone's waiting.

Getting the drug approved as a monotherapy isn't a complete waste of time, though, because that label will carry over if the once-weekly version is eventually approved. Doctors should have a much easier time persuading patients to take one shot a week, and thus that version may be able to cut into the initial-therapy territory, currently dominated by the oral meds.

Amylin isn't a doomed stock in the long term, but the uncertainty over when once-weekly Byetta will be approved is killing its near-term prospects. It's unlikely that it'll break out until this issue is resolved, and a belated Christmas present from the FDA in the form of a monotherapy indication won't help much.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Eli Lilly is a Motley Fool Income Investor pick. The Fool's disclosure policy has a soft spot for the poky little puppy.