Each of these compounds binds to opioid receptors — specific proteins found in the brain, spinal cord and other organs in the body — just like morphine. But they activate a different signaling pathway, a route through which information flows from one molecule to another, than conventional opioids.

These distinctions mean the new opioids could decrease the risk of addiction and eliminate a leading cause of overdose death: respiratory depression. The fatal side effect drew attention recently after it was deemed the cause of death for popular pop artist Prince.

Brian Shoichet, professor of pharmaceutical chemistry at University of California, San Francisco, and co-author of a paper on PZM21 published last month in Nature, says eliminating this side effect is one of the most important goals in creating new opioids.

“The whole field is sort of opening up, and there’s hope that there’s molecules that at least don’t cause respiratory depression,” he said. “That’s what kills people on opioids.”