What is the recommended dosage and administration schedule for morphine via Intravenous (IV) or Subcutaneous (SC) injection?

Morphine Dosing Guidelines for IV and SC Administration

For opioid-naïve adults requiring IV morphine, the recommended starting dose is 2-5 mg IV given slowly, titrated to effect, with no specified dose limit when titrating for symptom control. 1, 2

Initial Dosing

Intravenous (IV) Administration

• Opioid-naïve patients:
  • Starting dose: 0.1-0.2 mg/kg (typically 2-5 mg) IV 2
  • Frequency: Every 4 hours as needed, with additional doses for breakthrough pain
  • Administration: Must be injected slowly to avoid chest wall rigidity 2

Subcutaneous (SC) Administration

• Opioid-naïve patients:
  • Starting dose: 2-5 mg SC 1
  • Frequency: Every 4 hours as needed
  • Note: SC route is preferred over IM for chronic cancer pain as it's simpler and less painful 1

Dose Titration

IV Titration

• Bolus doses can be given as frequently as every 15 minutes as required 1
• If a patient receives two bolus doses in an hour, consider doubling the infusion rate 1
• For continuous infusion, start after initial bolus doses have established pain control

SC Titration

• Similar principles to IV titration
• SC infusion is the preferred method for patients requiring continuous parenteral morphine 1

Dose Conversion

Oral to Parenteral Conversion

• Oral to IV morphine potency ratio: 1:2 to 1:3 1
  • Example: 30 mg oral morphine ≈ 10 mg IV/SC morphine

IV to SC Conversion

• IV and SC morphine have similar potency when given as continuous infusion
• For bolus dosing, SC may have slightly lower bioavailability than IV

Special Considerations

Renal Impairment

• Use with caution in patients with renal dysfunction
• Consider dose reduction and increased monitoring due to potential accumulation of morphine-6-glucuronide 1
• Fentanyl or buprenorphine may be safer alternatives in severe renal impairment 1

Elderly Patients

• Start at the lower end of the dosing range (2 mg IV)
• Monitor closely for respiratory depression, sedation, and confusion 2

Continuous Infusion

• For patients requiring ongoing pain control, transition from bolus dosing to continuous infusion
• Initial infusion rate can be calculated based on total 24-hour requirement during titration
• Continue to provide breakthrough doses (typically set at 50-100% of hourly rate)

Monitoring

• Assess for:
  • Pain relief (primary outcome)
  • Respiratory rate (watch for depression <8 breaths/minute)
  • Level of sedation
  • Blood pressure (monitor for hypotension)
  • Common side effects: nausea, constipation, sedation

Side Effect Management

• Constipation: Prophylactic laxatives should be routinely prescribed 1
• Nausea/vomiting: Consider prophylactic antiemetics, especially during initiation 1
• Respiratory depression: Most serious adverse effect; have naloxone available for reversal if needed 2

Common Pitfalls to Avoid

1. Dosing errors: Ensure clear communication between mg and mL to prevent accidental overdose 2
2. Rapid administration: Inject IV morphine slowly to prevent chest wall rigidity 2
3. Inadequate breakthrough dosing: Ensure rescue doses are available for breakthrough pain
4. Failure to prevent constipation: Always prescribe prophylactic laxatives 1
5. Undertreatment due to fear of side effects: Remember that morphine can be safely titrated to effect when properly monitored

Morphine remains the first-choice opioid for moderate to severe cancer pain when administered properly, with appropriate monitoring and management of side effects 1.