The asthma drug Xolair now has a black-box warning — the strongest alert the FDA imposes — since reports showed the risk of anaphylaxis, an extreme allergic reaction, to be greater than originally thought. The reaction is rare but potentially very dangerous. Xolair, known generically as omalizumab, was approved in 2003 to treat moderate to severe allergic asthma in some patients 12 and older for whom inhaled steroids aren't sufficient. Users and their doctors must balance the benefits and the perils of the drug.