Morphine and Opioid Analgesics

Introduction & terminology

• Analgesic — a drug that selectively relieves pain by acting on the CNS or peripheral pain mechanisms without significantly altering consciousness; relieves pain as a symptom without affecting its cause.
• Opium — dried resinous exudate of the unripe capsule of the poppy Papaver somniferum. Two alkaloid classes: phenanthrenes — morphine (~10%), codeine (~0.5%), thebaine (non-analgesic precursor); and benzylisoquinolines — papaverine (smooth-muscle relaxant) and noscapine (antitussive), both non-analgesic.
• Opiate vs opioid vs narcotic — opiate = natural opium alkaloids + semisynthetic morphine derivatives; opioid = any natural, synthetic or endogenous agent acting at an opioid receptor (so pethidine and endorphins are opioids but not opiates); narcotic = originally "sleep-inducing", now chiefly a legal term.
• Prototype — Sertürner isolated morphine (after Morpheus, god of dreams) in 1803–06; it remains the standard against which all strong analgesics are compared. The µ (mu) receptor is the major analgesic receptor.