U.S. regulators yesterday asked the makers of three popular asthma medications to add warnings to their labels stating that the drugs could increase the chances of severe asthma episodes that could result in death.

The warnings involve long-acting bronchodilator medicines Advair and Serevent, made by GlaxoSmithKline, and Foradil from Novartis AG. Patients use them daily to relax bronchial muscles and prevent asthma attacks.

In a public health advisory issued on its Web site, the Food and Drug Administration said drugs in the class known as long-acting beta 2-adrenergic agonists should only be used after other medicines fail to control asthma.

The FDA said that even though long-acting beta 2-adrenergic agonists decrease the frequency of asthma episodes, these medicines may make asthma episodes more severe when they occur.

GlaxoSmithKline, Europe's biggest drugmaker, disagreed with the proposed labeling, saying it would be inconsistent with the standard of care for asthma treatment and could put many patients at risk of uncontrolled asthma.

Guidelines from the National Institutes of Health recommend steroids and long-acting beta 2-adrenergic agonists as initial therapy for moderate-to-severe persistent asthma.

The new safety alert could have a material impact on profits at Glaxo, which relies on its asthma franchise for about 15 percent of total sales, Prudential Equity analyst Tim Anderson said in a report.

Schering-Plough Corp., which markets Foradil in the United States, said it is still discussing the proposed label changes with the FDA.

"Safety is the most important thing. . . . Patients should be discussing this with their doctors," said Schering spokeswoman Mary-Fran Faraji.

Asthma is a chronic lung disease that afflicts about 15 million Americans, nearly 5 million of them children. Severe asthma attacks can kill.

In July, an advisory panel to the FDA said the three asthma drugs were safe enough to stay on the market.

Glaxo's Serevent and Advair already come with "black box" warnings related to one study showing that Serevent patients had a higher, albeit small, risk of life-threatening asthma attacks and deaths.

The panel urged that Novartis's medicine Foradil, part of the same class of drugs, should carry a similar warning.

Worldwide sales of Advair totaled $4.5 billion last year, making it Glaxo's top-selling product. Serevent sales were $639 million in 2004, and sales of Foradil were $320 million.