opioid /ō″pē·oid/ [Gk, opion, poppy juice, eidos, form] , strictly speaking, pertaining to natural and synthetic chemicals that have opium-like effects similar to morphine, though they are not derived from opium. Examples include endorphins or enkephalins produced by body tissues or synthetic methadone. Morphine and related drugs are often included in this category because the term narcotic has lost its original meaning.
