Morphine
First line strong opioid
Mechanism of action: mu-opioid agonist
Presentation:
- Oral:
- Immediate release (IR) available as oral solution (e.g. Oramorph) or tablets (e.g., Sevredol)
- Modified release (MR) formulations as tablet (e.g., Morphgesic) or capsule (e.g., Zomorph) or granules
- Solution for injection for subcutaneous, intramuscular, or slow intravenous bolus, and patient controlled analgesia.
- Rectal suppositories
Suggested initial dose: Immediate release morphine 10-20mg orally 2 hourly prn for adults over 50kg with normal hepatic and renal function; reduce in old age and frailty. Increase dose as necessary if pain is poorly controlled.
Modified release opioid preparations should not be initiated for patients with acute pain.
Monitor all patients for opioid toxicity using sedation score and respiratory rate. The risk of opioid-induced ventilatory impairment is increased if other sedatives are co-prescribed, including benzodiazepines and gabapentinoids, and by the use of modified release preparations in opioid naive patients. Monitor renal impairment as any deterioration will increase the risk of opioid accumulation. Reduce the morphine dose if the patient is sedated. Consider a titrated Naloxone reversal if patient is not responsive and has a low respiratory rate (less than 8 breaths per minute).
Consider increasing the morphine dose if pain is poorly controlled and functional activity is compromised (e.g., unable to take a deep breath or cough, or unable to comply with physio). Do not continue dose escalation if higher doses of morphine are not providing adequate pain management: call the pain team for advice.
If unreliable enteral absorption, consider morphine PCA or 5-10mg sc 4 hourly prn for adults over 50kg with normal hepatic and renal function, reduce in frailty.
Co-prescribe prn Naloxone
Oral absorption: Bioavailability of 30%, peak plasma concentration reached at 30-90 min after immediate release oral pre>
Protein binding: 30-40%, primarily to albumin
Metabolism: Mainly hepatic. Glucuronidation produces morphine-6-glucuronide (M6G) and morphine-3-glucuronide (M3G) in a ratio of 6:1, while approximately 5% of the drug is demethylated to normorphine. M6G has analgesic activity, while M3G may produce hyperalgesia. Hydromorphone and codeine are minor metabolites.
Extrahepatic metabolism can account for up to 30% of its metabolism and may play an important role in morphine metabolism in severe liver failure.
Elimination: mainly renal excretion, with less than 10% of the dose excreted as unchanged drug.
Half-life: approx. 2 hours
Further prescribing information (side effects, contraindications, interactions):