The US Food and Drug Administration says that Genentech’s asthma treatment Xolair should carry a stronger label warning after the agency received new reports of serious and life-threatening allergic reactions, anaphylaxis, in patients after treatment with the medicine.

The FDA has notified asthmatic patients and healthcare professionals of these new reports and notes that these reactions occur within two hours of receiving a Xolair subcutaneous injection. However, these cases also include patients who had delayed anaphylaxis, with onset two to 24 hours or even longer, after receiving the drug, which is co-marketed with Novartis.

The symptoms anaphylaxis in these patients have included bronchospasm, hypotension, syncope, urticaria and angioedema of the throat or tongue and the FDA added that health care professionals who administer Xolair should observe their patients for at least two hours after the treatment is given. No deaths have been reported but the agency has asked Genentech to add a boxed warning, the strongest possible, to a revised product label and provide a medication guide for patients.

Genentech said it was still discussing the FDA’s request but noted that “the updated wording and its placement in the label is not yet final." The label on Xolair already includes warnings about the chance of anaphylaxis after treatment with Xolair and one in 1,000 patients suffered such a reaction in clinical trials before the drug was approved in 2003.

Analysts feel that the FDA’s request is unlikely to hurt revenues from Xolair, US sales of which reached $425 million last year, as most doctors are already aware of the risks.