WASHINGTON (AP) -- The government is taking steps to curb use of some long-acting asthma drugs used by millions, issuing safety restrictions Thursday to lower a life-threatening risk that asthma could worsen suddenly.

The Food and Drug Administration's warnings cover the drugs Advair, Symbicort, Foradil and Serevent. The FDA said they should be used only by asthmatics who can't control their lung disease with other medications -- and even then only for the shortest time possible.

Nor should LABA-containing drugs ever be used without simultaneous use of a different asthma-controlling medication, such as an inhaled corticosteroid -- a move that specifically targets two of the drugs, Foradil and Serevent, the FDA said.

Why? These four drugs contain an ingredient that relaxes muscles around stressed airways, called a long-acting beta agonist or LABA. While they're very helpful for some patients, the way LABA-containing drugs work also sometimes masks that inflammation is building in the airways. That means patients may not realize a serious asthma attack is brewing until they're gasping for air.

The FDA cited studies that showed an increased risk of hospitalization and even some deaths, particularly among children. One study found three extra adverse events -- mostly hospitalizations -- for every thousand patients who took a LABA-containing drug compared to another asthma medication, said FDA's Dr. Gerald Dal Pan.

Medical guidelines already urge people to use a LABA together with an inhaled corticosteroid to relieve inflammation. Advair and Symbicort combine both kinds of medicine in one inhaler. Over a year ago, the FDA's scientific advisers had urged that LABA-only medications, Foradil and Serevent, no longer be used to treat asthma -- and that none of the four drugs be used in children.

The FDA said Thursday it was taking a somewhat stronger step. It can't pull those medications off the market because people with other lung diseases, such as chronic obstructive pulmonary disease or COPD, use them without the asthma risk. Just saying they shouldn't be used for asthma would have little practical effect, said Dr. John Jenkins, FDA's director of new drugs. So FDA labeled LABA-containing medications as contraindicated without simultaneous use of a different asthma-controlling medication -- a legal term with more enforcement muscle to limit prescription. FDA will monitor that, to see if doctors follow the rules.

"Our goal is to overall reduce the use of LABAs, to manage the risk while at the same time keeping them available for those patients who really need them," said Jenkins, a pulmonologist.

"The reality is the available options to treat asthma are not that great," he added. For patients not well-controlled by inhaled corticosteroids alone, "their options for additional therapy are, in and of themselves, drugs with a lot of risk."

Other warnings:

-Quit LABA-containing drugs as soon as asthma is brought under control, in favor of a corticosteroid or other asthma-maintenance medicine.

-Children and teens should be prescribed only the combination LABA drugs to ensure compliance with both medications. That mostly happens today, as pediatric use of the single-agent drugs has plummeted with publicity about the risk.

-Manufacturers also will be required to study whether combination LABA use indeed lowers the risk.

Advair and Serevent are marketed by GlaxoSmithKline, Foradil by Novartis AG and Symbicort by AstraZeneca.

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