FDA warnings about potential side effects of a company's drugs can range from mildly annoying to crippling, as has been the case for Johnson & Johnson's (NYSE:JNJ) and Amgen's (NASDAQ:AMGN) anemia drugs. I'd put the agency's warning yesterday about Amylin Pharmaceuticals' (NASDAQ:AMLN) diabetes drug Byetta closer to the annoying end of the spectrum.

The FDA issued a warning regarding 30 post-marketing reports of acute pancreatitis in patients taking the drug. Most of them had other risk factors for developing pancreatitis -- gallstones, severe hypertriglyceridemia, and alcohol use -- but the FDA recommended that doctors advise all patients taking the drug that they should seek medical care if they experience severe abdominal pain, and discontinue use of Byetta unless another explanation for the pancreatitis is found.

Amylin and marketing partner Eli Lilly (NYSE:LLY) have agreed to send out a "dear doctor letter" informing physicians of the new warnings and to change the label to make the precautions known.

While the number of patients who will go off the drug after developing pancreatitis is extremely low -- two incidences per 10,000 patients -- the big question is how many patients who have risk factors for developing pancreatitis will be removed from the drug by cautious doctors.

Investors will need to wait a quarter or two to get a handle on any decrease in sales, but after GlaxoSmithKline's (NYSE:GSK) black-box warning on diabetes drug Avandia, my guess is that diabetes patients probably won't be that spooked by the warning. The 3%-4% drops in the stock prices of Amylin and Alkermes (NASDAQ:ALKS), Amylin's partner in the development of a sustained-release version of Byetta, could be a good buying opportunity for investors.

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