(This March 5 story has been corrected to fix the nasal spray estimate to 23% from 15%, in paragrapgh 7)

By Christy Santhosh

(Reuters) - The U.S. Food and Drug Administration approved the expanded use of ARS Pharmaceuticals' ( SPRY ) nasal spray for severe allergic reactions in patients who weigh between 15 and 30 kilograms, the company said on Wednesday.

Shares of the company were up 5.2% in extended trading.

The spray, sold under the brand name neffy, will be dosed at 1 milligram (mg) for the new patient population, compared to the previously approved dose of 2 mg for people who weigh above 30 kilograms.

Neffy, which was first approved by the FDA in August, is seen as an alternative to EpiPen and other autoinjectors that are filled with epinephrine.

It is designed to be given at the first sign of a severe allergic reaction to prevent life-threatening conditions such as anaphylaxis.

Anaphylaxis is a life-threatening allergic reaction that can occur within seconds of being exposed to an allergen.

Raymond James analyst Ryan Deschner said, "Despite being a relatively small portion of the overall neffy market (about 23% according to the company), 15-30kg patients are a big portion of patients currently asking for neffy (via their parents)."

The nasal spray can easily slip into a pocket and gets rid of the burden of having to carry bulky autoinjectors.

Deschner said the expanded approval will accelerate prescriptions for neffy going into August, as it is traditionally the big "back to school" peak for epinephrine sales.

ARS said neffy 1 mg is expected to be available by the end of May in the U.S.

CEO Richard Lowenthal told Reuters ahead of the FDA decision that the 1 mg version of neffy will be priced similarly to the 2 mg version, which is available at $199 for two doses.

The company is working on a lower dose for smaller children along with a modified sprayer, Lowenthal said.