The Food and Drug Administration on Friday issued a public health advisory about potentially fatal side effects from two GlaxoSmithKline ( GSK) asthma drugs. The company vigorously disputed the FDA's analysis.

The agency is asking the maker of the drugs, Advair Diskus and Serevent Diskus, to add warnings to their labels that these medications "may increase the chance of severe asthma episodes, and death when those episodes occur."

Advair is GlaxoSmithKline's biggest product, with $4 billion in sales during the first nine months of 2005. Serevent's sales were $450 million. By late afternoon, the company's stock was down $2.20, or 4.2%, to $49.85 on trading that was more than triple the average daily trade for the past three months.

The FDA made the same request of Schering-Plough ( SGP) and Novartis ( NVS), which make and sell a similar asthma product called Foradil Aerolizer. They had no immediate comment.

Schering-Plough's stock was up 23 cents, or 1.2%, to $19.73, and Novartis was adding 29 cents to $54.15.

Each of the drugs cited by the FDA is a long-acting beta 2-adrenergic agonist, or LABA, which provides asthma relief by relaxing muscles around the airways to the lungs. Although they can reduce the number of asthma episodes, the FDA says they "may increase the chances of a severe asthma episode when they do occur."

In one study, the agency says, "an increased number of people taking a LABA in addition to their usual asthma care died from their asthma compared to people taking a placebo in addition to their usual asthma care, although the number of asthma deaths in the study was small."

The FDA proposal is "inconsistent" with treatment guidelines established by the National Institutes of Health, as well as "the standard of care for asthma treatment, which could put many patients at risk of uncontrolled asthma," GlaxoSmithKline said.

LABA drugs shouldn't be the first line of defense vs. asthma, the FDA says. Instead, they should be "added to the asthma treatment plan only if other medicines do not control asthma, including the use of low-or-medium dose corticosteroids."

But GlaxoSmithKline says the combination of LABA and inhaled corticosteroid drugs is an effective first-line treatment. The FDA's proposal would limit an asthma sufferer's choices "until after a patient has failed on other treatment options and therefore may be at risk for severe outcomes, such as exacerbations and potentially death," said Dr. Kathy Rickard, a respiratory medicine executive for GlaxoSmithKline, in a prepared statement.

"Sufficient safety information about these medicines is already in the labels to help guide physicians about their appropriate use," Rickard said. "However, we will work with FDA to address the differences of opinion about how best to communicate the benefit-risk profile of these medicines for optimal patient care."

Serevent's safety has been questioned by well-known drug industry critics -- Public Citizen and Dr. David J. Graham , a veteran FDA researcher. Graham told senators during a hearing last year on Merck's ( MRK) now-wthidrawn arthritis drug Vioxx that several other marketed drugs needed more scrutiny or tougher labels. Serevent was one of those drugs.