** Shares of drugmaker Amphastar Pharmaceuticals AMPH.O rise 2% to $20 premarket

** FDA says it approved Rextovy, a 4mg naloxone nasal spray for opioid overdose, for sale without prescription

** AMPH's naloxone reverses the effects of opioid overdose, as drugs like fentanyl can slow or stop breathing

** FDA says product to be sold via pharmacies, convenience stores and online, increasing access

** Says approval adds competition in OTC naloxone market, helping widen availability and lower costs

** FDA says spray is safe even if opioid involvement unclear and urges users to seek emergency help after use

** As of last close, stock was down ~26% YTD