U.S. health regulators declined to approve a nasal spray that would have been the first needle-free emergency treatment for allergic reactions and asked for further tests, a move that left its developer ARS Pharmaceuticals "very surprised".

ARS Pharma's Neffy is to be an alternative to EpiPen and other autoinjectors like Sanofi's Auvi-Q filled with epinephrine, a life-saving drug used by people at risk of anaphylaxis and other allergic reactions.

The U.S. Food and Drug Administration (FDA), in its so-called "Complete Response Letter", asked the company to complete a repeat-dose study on the treatment before a potential approval, rather than after as was mutually agreed in August, ARS Pharma said late on Tuesday.

"We are very surprised by this action and the late requirement at this time ... particularly given the positive Advisory Committee vote," CEO Richard Lowenthal said in a statement.

Lowenthal was referring to a vote in May when a panel of advisers to the FDA voted 16 to 6 in favor of adults using Neffy and 17 to 5 in favor of children using the nasal spray.

ARS Pharma said it expects to re-submit its application in the first half of 2024, with an FDA "action date" likely in the second half, adding that it will appeal the decision.

The company's application was based on trials in healthy patients and in those having a rhinitis attack, with the nasal spray showing a comparable response to injectable products.

It did not test the spray in people with anaphylaxis due to ethical concerns. That lack of tests in a real-world setting with people with anaphylaxis was the biggest concern, said James Tarbox, an allergist at Texas Tech University Health Sciences Center.

ARS Pharma's rival Viatris in June petitioned the FDA to require that ARS conduct more trials that closely mimic real-world conditions. Viatris has an apprved/under test needle-free treatment for rhinitis. (Reporting by Christy Santhosh, Sriparna Roy and Jahnavi Nidumolu in Bengaluru; Editing by Maju Samuel, Dhanya Ann Thoppil and Varun H K)