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.