What is a good intramuscular (IM) dose of morphine for an opioid-naive patient weighing 73 kilograms (kg) with severe abdominal pain due to appendicitis?

IM Morphine Dosing for Severe Appendicitis Pain in Opioid-Naive Patient

For this 73 kg opioid-naive patient with severe appendicitis pain unresponsive to NSAIDs and acetaminophen, administer 5-10 mg of morphine IM initially, with the option to repeat every 15-30 minutes as needed for adequate pain control.

Rationale for Initial Dosing

The recommended starting approach is conservative but effective:

• Start with 5-10 mg IM morphine for opioid-naive patients with severe pain requiring parenteral administration 1, 2
• The CDC guidelines specify that the lowest starting dose for opioid-naive patients is often equivalent to approximately 5-10 MME, which translates to this dose range 1
• For parenteral administration, the equivalent dose is approximately one-third of the oral dose, meaning 5-10 mg IM is roughly equivalent to 15-30 mg oral morphine 1

Route Considerations

While guidelines generally favor subcutaneous over IM administration for chronic pain, IM morphine is appropriate in this acute emergency setting 1:

• IM administration provides faster onset than oral routes, which is critical for severe acute pain 3
• The oral-to-parenteral conversion ratio is 2:1 to 3:1, so 5-10 mg IM provides substantial analgesia 1

Titration Strategy

Reassess pain every 15-30 minutes and administer additional 5 mg boluses as needed 1:

• Parenteral morphine bolus doses can be offered as frequently as every 15-30 minutes for acute severe pain 1
• If two boluses are required within an hour, consider doubling subsequent doses or transitioning to continuous infusion 1
• There is no specified dose limit when titrating to symptoms in acute severe pain 1

Evidence Supporting Morphine Use in Appendicitis

Morphine does not mask physical examination findings in acute appendicitis 4:

• A prospective double-blind trial using 0.15 mg/kg morphine (approximately 11 mg for this 73 kg patient) demonstrated significant pain relief without affecting physical signs 4
• Morphine improved patient cooperation during examination without compromising diagnostic accuracy 4

Weight-Based Dosing Consideration

For this 73 kg patient, a weight-based approach of 0.1 mg/kg would yield approximately 7 mg, which falls within the recommended 5-10 mg range 4:

• The research evidence used 0.15 mg/kg (maximum 20 mg) successfully in appendicitis patients 4
• Starting at the lower end (5-7 mg) allows for safer titration while still providing meaningful analgesia

Common Pitfalls to Avoid

• Do not withhold opioid analgesia due to outdated concerns about masking examination findings—this practice is not evidence-based and causes unnecessary suffering 4, 5
• Avoid starting with excessively high doses (>10 mg) in opioid-naive patients, as this increases risk of respiratory depression and sedation without proportional benefit 1, 2
• Do not use fixed dosing intervals—reassess frequently and titrate to effect rather than waiting predetermined time periods 1
• Monitor respiratory status closely after IV/IM administration, particularly in the first 15-30 minutes when peak effects occur 3

Monitoring Parameters

After administration, assess:

• Pain intensity at 15-minute intervals 3
• Respiratory rate and oxygen saturation (watch for rates <10/min) 3
• Level of sedation 3
• Need for additional doses based on inadequate pain relief 1