Makindo Medical Notes

Related Subjects: | Analgesia | Sedation and Analgesia on ITU | Codeine | Dihydrocodeine | Diamorphine | Morphine | Opiates

💉 Key point: Give doses slowly and titrate to effect. Always be ready to manage respiratory depression with Naloxone and airway support. 👵 Frail or opioid-naïve patients are particularly sensitive to morphine.

🧠 About

• Morphine is the reference opioid against which all others are compared. It provides powerful analgesia by acting on central opioid receptors.
• Available in immediate-release (e.g. Oramorph) and modified-release (e.g. MST Continus) preparations.
• Always check the BNF for up-to-date guidance and dosing in renal impairment, palliative care, and conversions.

⚙️ Mode of Action

• Agonist at the μ-opioid receptor in the brain and spinal cord → inhibits pain transmission and alters emotional response to pain.
• Also acts on κ- and δ-receptors but to a lesser extent.
• Reduces neurotransmitter release (substance P, glutamate) in the dorsal horn and enhances descending inhibitory pain pathways.
• Causes sedation and respiratory depression by direct action on the medullary respiratory centre.

💊 Indications & Dosing

• Start at 2.5 mg if frail, elderly, or opioid-naïve.
• Acute severe pain: 5–10 mg SC/IM every 4 h; use half-dose if IV (2.5–5 mg slow IV).
• Acute MI: 2.5–5 mg slow IV bolus; monitor for hypotension and respiratory depression.
• Acute LVF: 2.5–5 mg slow IV over 5 min (may relieve anxiety and preload but use cautiously).
• Chronic pain: Start with 10 mg modified-release morphine twice daily (MST Continus), titrating every 24–48 h based on need.
• Breakthrough pain: Give 1/6 of the total daily dose as immediate-release morphine (e.g. Oramorph) PRN.
• Palliative care: 10–20 mg SC over 24 h in a syringe driver, titrate to comfort.
• Usually co-prescribe a laxative and an antiemetic.

📊 Typical Dose Ranges

⚠️ Interactions

• Avoid with MAOIs (current or recent) – risk of severe CNS/respiratory depression.
• Additive CNS depression with benzodiazepines, alcohol, antihistamines, or sedating antidepressants.

⚕️ Cautions

• May worsen hypotension or respiratory depression.
• Reduce dose in hepatic or renal impairment (active metabolites accumulate in renal failure).
• Use lower doses in the elderly, frail, or hypovolaemic.
• Monitor for opioid-induced hyperalgesia with prolonged use.

🚫 Contraindications

• Significant respiratory depression, acute asthma, or severe COPD.
• Raised intracranial pressure or head injury.
• Paralytic ileus or bowel obstruction.
• Myasthenia gravis, hypotension, or concurrent MAOI therapy.

💥 Side Effects

• Nausea, vomiting, constipation, pruritus, sedation, hypotension.
• Dry mouth, urinary retention, miosis, reduced cough reflex.
• Delirium, restlessness, hallucinations, or myoclonus (high doses).
• Biliary tract spasm, bronchospasm, rhabdomyolysis (rare).
• Endocrine effects with chronic use: ↓ libido, hypogonadism, adrenal suppression.

📖 Educational Summary

Morphine remains the benchmark opioid in UK practice. Its efficacy, predictability, and broad familiarity make it first-line for most severe pain states. Always assess renal function and frailty before prescribing; accumulation of active metabolites (morphine-6-glucuronide) can cause prolonged sedation. In palliative care, diamorphine may be substituted for morphine if volume in syringe driver is limiting or nausea is troublesome. Safe practice involves anticipating respiratory depression and co-prescribing laxatives and antiemetics. Conversion between oral and parenteral routes should use the approximate ratio: Oral : SC/IV = 2–3 : 1.

📚 References

• Morphine – British National Formulary
• Faculty of Pain Medicine, Opioids Aware (RCoA 2023)
• Palliative Care Formulary, 7th Edition
• NICE NG140: End-of-Life Care for Adults (2019)