Genetech and FDA informed healthcare professionals and asthmatic patients in the united state that the prescribing information for Xolair was revised to include a new BOXED WARNING, and updated WARNINGS, PRECAUTIONS, and ADVERSE REACTIONS sections that address the risk of anaphylaxis (the onset of action can be delayed for 24 hours or more) when taking this medication. In addition, a new MEDICATION GUIDE was developed and will be provided to patients when a prescription for Xolair is filled or refilled at the pharmacy. Due to the risk of anaphylaxis, Xolair should only be administered to patients in a healthcare setting under direct medical supervision. Patients should be observed for an appropriate period of time following each Xolair injection.