An approving nod from the Food and Drug Administration gave Amylin Pharmaceuticals (NASDAQ:AMLN) stock a big boost -- more than 17% yesterday, and an additional 7% so far today.

The FDA will accept Amylin's alternate plan to prove that the once-weekly version of diabetes drug Byetta, which is made at its new commercial-scale plant, is the same as the drug used in the clinical trials. Plan A, to use laboratory data, was rejected by the FDA last month.

Plan B involves using patients in the maintenance phase of the pivotal phase 3 trial, who have already been switched to the commercial-scale drug. Assuming the data from the maintenance phase looks like the data from earlier in the trial, Amylin, marketing partner Eli Lilly (NYSE:LLY), and development partner Alkermes (NASDAQ:ALKS) should be able to gain approval for the drug.

Plan C, the backup to the backup plan, was to run a new trial with the drug from the new plant. It's a good thing this won't be needed, as it likely would have pushed the filing for the marketing application beyond the first half of next year. Investors are clearly excited about avoiding this delay.

With an FDA decision on Novo Nordisk's (NYSE:NVO) liraglutide expected in the spring, and sales of Byetta running into tough competition from oral diabetes medications like Merck's (NYSE:MRK) Januvia and GlaxoSmithKline's (NYSE:GSK) Avandia, Amylin really needs the once-weekly version approved. Yesterday's announcement got it one giant step closer.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool has a disclosure policy.