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

Duration of Action of Intravenous Morphine

Intravenous morphine has a duration of action of approximately 4 hours, with peak effects occurring within 15-30 minutes after administration. 1

Pharmacokinetic Profile

• The terminal half-life of intravenous morphine ranges from 1.5 to 4.5 hours, with the accepted elimination half-life in normal subjects being 1.5 to 2 hours 2
• Morphine has an apparent volume of distribution ranging from 1.0 to 4.7 L/kg after parenteral administration 2
• Total plasma clearance ranges from 0.9 to 1.2 L/kg/h in postoperative patients, but shows considerable interindividual variation 2
• The major pathway of clearance is hepatic glucuronidation to morphine-3-glucuronide, which is pharmacologically inactive 2

Clinical Duration of Effect

• In clinical practice, normal release morphine (including intravenous) does not need to be given more often than every 4 hours for pain management 1
• When converting from continuous intravenous fentanyl to intermittent intravenous morphine, the standard dosing interval is every 4 hours, reflecting morphine's duration of action 1
• The onset of analgesic effect after intravenous administration is rapid, with effects beginning within 5 minutes compared to 20 minutes for intramuscular administration 3

Factors Affecting Duration

• There is large interindividual variation in morphine pharmacokinetics and dynamics in cancer patients 1
• Morphine concentrations are not predictive of analgesic response, especially in patients previously treated with opioids 2
• The minimum effective concentration varies widely and is influenced by factors including:
  • Extent of previous opioid use
  • Age
  • General medical condition 2

Clinical Applications

• For breakthrough pain in patients on regular oral morphine, intravenous morphine can provide rapid relief with effects lasting approximately 4 hours 4
• When titrating intravenous morphine for severe pain, bolus doses can be given every 5-10 minutes until pain relief is achieved, with the total effect lasting about 4 hours 5, 6
• In pain management protocols, the standard interval between regular intravenous morphine doses is 4 hours, reflecting its duration of action 1

Common Side Effects and Monitoring

• Common adverse effects during intravenous morphine administration include:
  • Nausea and vomiting
  • Sedation
  • Respiratory depression (dose-dependent)
  • Hypotension 2, 3
• Respiratory depression risk is highest within the first hour after administration but can persist throughout the duration of action 3
• Careful monitoring is required, particularly in opioid-naïve patients, for the full 4-hour duration of effect 2

Remember that while the pharmacologic half-life of morphine is 1.5-2 hours, the clinical duration of analgesic effect is approximately 4 hours, which is why dosing intervals are typically set at 4 hours for intermittent intravenous administration 1, 2.