Amphastar Pharmaceuticals Inc. AMPH, +0.61% shares plummeted 15.8% in morning trade Tuesday after the company said the Food and Drug Administration failed to approve its nasal spray for opioid overdoses. The FDA said the company needed to resolve various issues, including studying users' ability to understand and administer the product correctly and evaluation of the device itself, according to Amphastar, which currently sells the opioid overdose treatment naloxone in pre-filled syringes. Chief Executive Officer Dr. Jack Zhang said the company will work with the FDA to address the regulator's concerns. Naloxone nasal sprays are seen as the easiest to use in an emergency situation, and amid an opioid crisis across the U.S., prices have spiked. Amphastar shares have dropped 22.0% over the last three months, compared with a 7.6% rise in the S&P 500 SPX, -1.28% .