The U. S. Food and Drug Administration announced Thursday it is conducting a safety review of the asthma drug Xolair after receiving reports of increased risk of heart attack, abnormal heart rhythm, heart failure, and stroke among patients who use it.

The drug Xolair (omalizumab) made by Genentech and co-marketed by Novartis, given by injection, is approved for use by adults and adolescents (12 and older) with moderate to severe persistent asthma whose symptoms are not well controlled with inhaled corticosteroids.

The drug was approved in 2003 and the study, started in 2004 and is slated to end in 2012. The study "Evaluating the Clinical Effectiveness and Long-Term Safety in Patients with Moderate to Severe Asthma" EXCELS was mandated by the agency as part of its post-approval process.

It counted outcomes among some 5,000 Xolair-treated patients and compared them with a control group of about 2,500 patients who were not taking Xolair. In early results the trial indicated that the risk of heart problems among users of the drug was heightened compared with nonusers.

The agency said the data "suggest a disproportionate increase in ischemic heart disease, arrhythmias ... cardiac failure" and other conditions "in patients treated with Xolair compared to the control group of patients not given the drug."

Although the FDA is not recommending stopping Xolair, however, the agency believes patients and doctors should be aware of potential risks associated with the drug.

The agency said, "Posting this information does not mean that FDA has concluded there is a causal relationship between the drug products and the emerging safety issue. Nor does it mean that FDA is advising health care professionals to discontinue prescribing these products. FDA is considering, but has not reached a conclusion about, whether this information warrants any regulatory action."

The observational study is not a randomized trial, the kind that researchers consider to be the gold standard in determining cause and effect. In an observational trial, there can be differences in underlying risk factors for heart attack and stroke between the two study groups.

In an e-mail, Novartis said that the study was being "conducted in a real-world setting" so the two groups "differed in their baseline characteristics."

Genentech spokeswoman Tara Cooper said, "Limitations of the existing data preclude a definitive association with Xolair use. At this time, there is insufficient information for the companies to change their assessment of the benefit-risk profile for Xolair. Physicians should continue to assess whether Xolair is appropriate for their individual patients."

However Dr. Clifford Bassett, a fellow at the American Academy of Allergy, Asthma and Immunology, and medical director of Allergy and Asthma Care of New York said, "In my experience, in properly selected patients, [Xolair] has been a great benefit and clinical improvement can be dramatic, on a case-by-case basis. As far as the possible events associated as indicated by the FDA release, I am surprised as I have not observed or seen those type of problems to date," he said.