Breathe Deep. It Might Not Be That Bad.

The Food and Drug Administration has asthma-drug makers on their heels again. The agency issued a statement yesterday warning about the potential problems from use of long-acting beta agonists (LABAs) drugs, including Merck (NYSE: MRK  ) and Novartis' (NYSE: NVS  ) Foradil and GlaxoSmithKline's (NYSE: GSK  ) Serevent. It also includes AstraZeneca's (NYSE: AZN  ) Symbicort and Glaxo's Advair, which combine LABAs with a steroid.

This isn't the first time the FDA has brought up the issue of LABAs increasing hospitalizations and even deaths, but the grumblings seem to be intensifying. The agency doesn't want patients using LABAs alone -- Foradil and Serevent should be taken with a separate steroid -- and it wants doctors to reduce the use of LABAs alltogether. The agency isn't questioning the effectiveness of LABAs, but the FDA would like to see patients discontinue their use as soon as the asthma is under control and then continue on other asthma controllers, like Merck's Singulair or Teva Pharmaceutical's (Nasdaq: TEVA  ) Qvar.

Any changes to prescribing habits could mean losses for one of Glaxo's most important products. Sales of Advair topped $7.7 billion last year and made up more than 17% of Glaxo's revenue. Symbicort isn't quite as important to AstraZeneca, but it's still a blockbuster making up 7% of revenue for the U.K.'s other major pharmaceutical company. The non-combination products -- Foradil and Serevent -- aren't that important because they only make up 5% of the total prescriptions for products containing LABA.

But there's a reason sales of Advair and Symbicort are so high: They work really well. It's possible that despite the FDA's grumblings, doctors won't discontinue prescribing them for fear that symptoms will return. For its part, Glaxo claims that the evidence of Advair increasing hospitalizations or deaths is nonexistent, and FDA's own advisory panel seemed to agree that any risks outweighed the benefits.

Fools, this one could take a while to shake out.

What is going on with this $376 billion bubble?