opioid antagonists

o·pi·oid an·tag·o·nists

agents such as naloxone and naltrexone that have high affinity for opiate receptors but do not activate these receptors. These drugs block the effects of exogenously administered opioids such as morphine, heroin, meperidine, and methadone, or of endogenously released endorphins and enkephalins.

o·pi·oid an·tag·o·nists

(ōpē-oyd an-tagŏ-nists) Agents with a high affinity for opiate receptors but do not activate them.