What is the onset and duration of action of intravenous (IV) morphine?

Onset and Duration of Action of Intravenous Morphine

Intravenous morphine has an onset of action of 1-2 minutes and a duration of effect of approximately 4 hours. 1, 2

Pharmacokinetic Profile

• After IV administration, morphine reaches peak plasma concentrations within 15-30 minutes 1
• The onset of analgesic effect is rapid, occurring within 1-2 minutes after administration 2
• The duration of analgesic effect typically lasts 4 hours, which is why standard dosing intervals for IV morphine are every 4 hours 1, 2
• IV morphine has a plasma elimination half-life of 2-4 hours, with steady state achieved within 24 hours after the start of treatment 2

Clinical Efficacy

• IV morphine provides more rapid pain relief compared to oral morphine, with studies showing significantly better pain control within the first hour of administration 3
• When administered as a bolus, IV morphine shows significantly faster onset of analgesic effect (5 minutes) compared to intramuscular morphine (20 minutes) 4
• The rapid onset of IV morphine makes it particularly useful for managing severe acute pain that requires immediate relief 3

Pharmacodynamic Considerations

• Faster infusion rates of IV morphine produce greater positive subjective effects and higher peak plasma levels than slower infusion rates 5
• The relative potency of IV morphine compared to oral morphine is between 1:2 and 1:3 (i.e., 10 mg IV morphine is approximately equivalent to 20-30 mg oral morphine) 2
• When converting from oral to IV morphine, the oral dose should be divided by three to achieve roughly equianalgesic effect 2

Clinical Applications

• IV morphine is preferred in patients who:
  • Already have an in-dwelling intravenous line
  • Have generalized edema
  • Develop erythema or sterile abscesses with subcutaneous administration
  • Have coagulation disorders
  • Have poor peripheral circulation 2
• For severe cancer pain requiring rapid control, IV morphine boluses can provide total or satisfactory pain relief within one hour in most patients 3

Adverse Effects

• The primary adverse effect of IV morphine is respiratory depression, which may persist longer than the analgesic effect 2
• Other common side effects include nausea, vomiting, sedation, and potential cardiovascular effects including slight reductions in blood pressure and heart rate 2, 1
• Naloxone can be used to reverse opioid-induced respiratory depression, with an onset of action of 1-2 minutes and a half-life of 30-45 minutes 2

Important Clinical Considerations

• Dose reduction of at least 50% is indicated in elderly patients due to altered pharmacokinetics 2
• Continuous IV morphine infusions may be appropriate for terminal pain control, with dosages ranging from 0.5 to 300 mg/hour based on individual patient needs 6
• When administering IV morphine with benzodiazepines, dose reduction may be necessary due to synergistic respiratory depressant effects 2