Washington ? Patients with serious asthma caused by allergies are getting a new weapon to block the attacks, with approval of the drug Xolair by the Food and Drug Administration.

The drug, approved late Friday, is expensive, requiring monthly shots that will cost between $5,000 and $10,000 a year, depending on the dose. It is intended only for sufferers from serious allergic asthma who get no relief from standard medication.

Doctors have been eagerly awaiting it, however, because it works differently from any other asthma medicine, blocking an allergic reaction well before it triggers the hallmark gasping for air.

That same reaction is behind a range of allergic ailments, from hay fever to peanut allergy, that scientists hope Xolair one day will prove key to treating, too.

“This is radically new, it’s not another antihistamine or another inhaler,” said Dr. Bob Lanier of the University of North Texas Health Science Center, who led research on the drug for its three developers: Genentech Inc., Tanox Inc. and Novartis Pharmaceuticals.

Xolair will be available by prescription in late July.