Morphine

What it is (and why we use it)

• Morphine is a strong opioid analgesic used for moderate–severe pain (e.g., post-op pain, trauma, cancer pain).
• Works mainly at μ (mu) opioid receptors in the brain and spinal cord to reduce pain perception and distress.
• Can be given IV (titrated boluses), oral (immediate or modified release), SC/IM (less preferred perioperatively), neuraxial (specialist use).
• Key principle: balance analgesia with safety—opioids can cause respiratory depression and sedation.

Common perioperative uses

• Intra/post-op analgesia: IV boluses in PACU/theatre, often alongside paracetamol/NSAID and regional techniques.
• Patient-controlled analgesia (PCA): allows patient to self-administer small IV doses with lockout to reduce peaks/troughs.
• Bridging to oral opioids once eating/drinking and pain is stable.
• Avoid routine IM morphine for post-op pain (unpredictable absorption, delayed respiratory depression).

Dosing basics (adult—typical starting points; always individualise)

• IV titration for acute pain: give small boluses (e.g., 1–2 mg IV every few minutes) until comfortable and alert; reassess frequently.
• Oral morphine: less potent than IV; use local guidance for conversion and starting doses (especially in opioid-naïve patients).
• Elderly/frail: start lower and go slower (increased sensitivity and higher risk of delirium/respiratory depression).
• Renal impairment: avoid large/repeated doses; consider alternatives (see below) because active metabolites can accumulate.
• Always prescribe/prepare a naloxone plan and monitoring strategy when giving significant opioid doses.

Pharmacology you need on day 1

• Onset: IV within minutes; oral slower (tens of minutes). Duration: IV a few hours; oral longer depending on formulation.
• Metabolism: liver glucuronidation to morphine-3-glucuronide (M3G) and morphine-6-glucuronide (M6G).
• Excretion: renal—metabolites accumulate in renal failure (M6G contributes to prolonged opioid effect/respiratory depression).
• Histamine release can occur (flushing, itch, hypotension), especially with rapid IV administration.

Side effects (what to look for and what to do)

• Respiratory depression: reduced respiratory rate and/or rising CO2, increasing sedation—treat as an emergency.
• Sedation: often precedes respiratory depression; use sedation scoring and respond early.
• Nausea/vomiting: common—use antiemetics (e.g., ondansetron, cyclizine) and reduce opioid dose if possible.
• Pruritus (itch): common; consider antihistamine (may worsen sedation) or low-dose naloxone infusion if severe (specialist/local policy).
• Constipation: very common—start laxatives early for ongoing opioid use.
• Urinary retention: especially post-op and with neuraxial opioids; assess bladder and manage appropriately.
• Hypotension: can occur (vasodilation, histamine, reduced sympathetic tone), especially if hypovolaemic.

Monitoring and safety checks

• Before giving: check allergies, current opioids/sedatives, renal function, respiratory status, and level of consciousness.
• After giving: monitor pain score, sedation score, respiratory rate, SpO2 (and consider capnography if available/high risk).
• High-risk groups need closer monitoring: OSA, obesity, elderly, opioid-naïve, renal impairment, concurrent benzodiazepines/gabapentinoids, head injury.
• Document: dose, time, effect, and any adverse effects; communicate clearly at handover.

When to avoid or use extra caution

• Severe respiratory disease or acute respiratory compromise: avoid large doses; titrate carefully with senior input.
• Renal failure (especially eGFR <30): avoid repeated dosing; consider fentanyl/oxycodone/hydromorphone depending on local policy and patient factors.
• Reduced consciousness, head injury, raised intracranial pressure: opioids can mask neurological deterioration and cause CO2 retention.
• Concurrent sedatives (benzodiazepines, alcohol, gabapentinoids): markedly increased risk of respiratory depression.

Practical first-time scenarios

• PACU patient in pain: use IV morphine in small increments; reassess pain, sedation, and respiratory rate after each dose.
• Patient too sleepy after morphine: stop further opioid, stimulate, give oxygen, check airway/ventilation, call for help early; consider naloxone if significant respiratory depression.
• Renal impairment with ongoing pain: avoid “top-up” cycles of morphine; discuss alternatives and multimodal analgesia with senior.
• Itch and nausea limiting analgesia: treat side effects and reduce opioid requirement (paracetamol/NSAID if appropriate, regional techniques).