Some people taking an injcted asthma medication marketed as Xolair have experienced a delayed and potentially life-threatening allergic reaction known as anaphylaxis. Anaphylaxis typcially causes hives, swelling of the mouth and throat, dizziness, and other symptoms. If not treated quickly, anaphylaxis can be fatal.

The FDA recently ordered the drug manufacturer Genentech to strengthen label warnings after reports that some patients experienced delayed anaphylaxis or reacted to a subsequent dose even if they had no problems with the first dose.

The new warning for Xolair states:

• Doctors should observe patients for at least two hours after the drug is injected and be ready to manage a potentially life-threatening reaction.
• Anaphylaxis can occur even if a patient did not react to a previous Xolair injection.
• Anaphylaxis can occur up to 24 hours after the injection is given.
• Patients using Xolair should be prepared to treat themselves for anaphylaxis in emergency situations.