The US FDA has rejected an application from Amphastar Pharmaceuticals Inc to market an intranasal version of the emergency opioid-overdose treatment, naloxone.

The FDA’s complete response letter identifies issues with user human factors study – which is designed to evaluate the user interface of a product – among others, Amphastar said.

Amphastar already sells naloxone in pre-filled syringes, as does privately held Kaleo Pharmaceuticals, which came under fire earlier this month for raising the price of its naloxone device Evzio by 550 percent to $4,500.