U.S. regulators on Monday finalized a strong new label warning patients and doctors about a potentially severe allergic reaction to Genentech Inc.’s asthma drug Xolair (omalizumab).

The U.S. Food and Drug Administration had proposed the so-called black box warning about anaphylaxis in February but announced final wording of the new label on Monday.

Xolair, given as an injection, is approved to treat adults and adolescents age 12 and above for moderate to severe asthma.

The FDA has warned that anaphylaxis occurs in about 1 in 1000 patients taking the medication.

Novartis collaborates with Genentech to market the drug.