Anaes · Anaesthetic adjuncts

Morphine

Also known as Prototype mu-opioid agonist · Phenanthrene opioid analgesic · Natural opium alkaloid analgesic

Morphine is the prototype phenanthrene opioid, a full agonist at the mu-opioid receptor and the natural alkaloid of opium against which every other opioid is compared. It produces the canonical mu-opioid constellation: analgesia, sedation, miosis, euphoria, constipation and nausea, and its principal hazard is respiratory depression from a reduced medullary response to carbon dioxide, reversed by the competitive antagonist naloxone. Two features make morphine pharmacologically distinctive and exam-critical. First, it UNIQUELY releases histamine from mast cells, producing flushing, urticaria, pruritus and vasodilation with hypotension, a feature it does not share with fentanyl and a caution in asthma. Second, it is metabolised by hepatic glucuronidation to the active metabolite morphine-6-glucuronide (M6G), which has higher mu-receptor affinity than the parent and ACCUMULATES in renal failure, producing prolonged respiratory depression, so morphine is avoided in renal impairment and teaching prefers fentanyl. Tolerance, dependence and opioid-induced hyperalgesia develop with repeated use, and opioid stewardship means using opioids only when necessary. Built on the histamine-release comparison (Hermens 1985), the M6G quantitative review (Klimas 2014), the dialysis toxicity case (Slater 2020), the respiratory-depression review (Dahan 2010), the intrathecal-morphine hypoxia cohort (Ladha 2017), the conversion-ratios review (Davis 2024), the full-agonist pharmacology review (Edinoff 2021), the naloxegol sphincter-of-Oddi case (Davies 2019), the clonidine-versus-fentanyl hernia trial (de Souza 2026), the M3G hyperalgesia case (Juba 2013), and the intrathecal dose-response trial (Palmer 1999).

Checked against its sources on 11 Sept 2026