What is the recommended intramuscular morphine dose for an opioid‑naïve adult with moderate to severe pain, and how should it be adjusted for opioid‑tolerant patients, elderly individuals, those with hepatic or renal impairment, and pediatric patients?

Intramuscular Morphine Dosing

For opioid-naïve adults with moderate to severe pain, start with 2-5 mg IM morphine, reassess every 15 minutes, and administer additional 2 mg boluses as needed—this is the safest and most effective initial approach. 1

Initial Dosing for Opioid-Naïve Adults

• Start with 2-5 mg IM morphine for patients experiencing moderate to severe pain 1
• The lower end of this range (2 mg) should be used for elderly patients (>70 years), small body size, or frail individuals 1
• Reduce the initial dose by 30-50% in elderly patients to account for decreased clearance and increased opioid sensitivity 1
• Pain should be reassessed every 15 minutes after IM administration 1

Important Context on Route Selection

• IM morphine should not be used as first-line if IV access is available, since IV administration allows for more precise titration with faster onset (5 minutes vs 20 minutes) and easier reversal if needed 1, 2
• IV morphine provides significantly better initial analgesia than IM morphine at equivalent doses 2
• The equivalent IV dose is approximately one-third of the IM dose (so 2-5 mg IM equals roughly 1-2 mg IV) 3

Dose Titration Protocol

• If pain persists or is unchanged after initial assessment, increase the dose by 50-100% of the previous dose 3, 1
• If pain decreases to moderate levels (4-6 on a 0-10 scale), repeat the same dose and reassess in 15 minutes 3
• There is no dose ceiling when titrating to symptoms during acute pain management 1
• Continue this cycle until pain is adequately controlled (typically 0-3 on pain scale) 3

Critical Safety Consideration

• Never start with doses ≥20 mg in opioid-naïve patients, as this significantly increases adverse effects without proportional analgesic benefit 1

Special Population Adjustments

Renal Impairment

• Start with one-fourth to one-half the usual dose in patients with renal impairment due to accumulation of morphine-6-glucuronide, an active metabolite that can cause neurotoxicity 1, 3
• Consider alternative opioids like fentanyl in severe renal failure (CKD stages 4-5) 4, 5

Hepatic Impairment

• Use morphine with caution in hepatic impairment 3, 5
• Fentanyl is the safest option in this population 5

Elderly Patients

• Reduce initial dose to approximately 2 mg IM for patients >70 years 1
• This accounts for age-related pharmacokinetic changes and increased opioid sensitivity 1

Opioid-Tolerant Patients

• For patients already taking chronic opioids (defined as ≥60 mg oral morphine/day, 25 mcg transdermal fentanyl/hour, 30 mg oral oxycodone/day, 8 mg oral hydromorphone/day, or equivalent for ≥1 week) 3
• Calculate the previous 24-hour total opioid requirement and increase the rescue dose by 10-20% 3
• Reassess every 15 minutes for IM administration 3
• If pain is unchanged after 2-3 cycles, consider changing to IV route or alternative management strategies 3

Pediatric Dosing

• For pediatric patients, start with 5-10 mg oral morphine every 4 hours (not IM as first-line) 4
• If IV/IM morphine is necessary for urgent pain control, use 2-5 mg IV (parenteral morphine is 2-3 times more potent than oral) 4
• Avoid starting with standard adult doses, as this causes excessive adverse effects in pediatric patients 4

Mandatory Adjunctive Management

• Initiate a bowel regimen simultaneously with stimulant laxatives and stool softeners from the first dose, as opioid-induced constipation occurs in nearly all patients 1, 4
• Order antiemetics pro re nata to manage opioid-induced nausea, which is common particularly during the first few days 1, 4
• Ensure naloxone is immediately available to reverse accidental overdose 4

Monitoring Requirements

• Monitor respiratory rate, sedation level, and vital signs every 15-30 minutes during initial titration 1
• Document the specific rationale for each dose administered (e.g., "for severe pain rated 8/10") 1
• A 10 mg IV bolus dose does not cause severe respiratory depression if there is a certain level of pain at administration 2

Common Pitfalls to Avoid

• Do not assume the same dose for all patients—individualization based on age, renal function, and body habitus is essential 1
• Do not use fixed doses at fixed intervals—this approach has been largely ineffective; flexibility in dose size and interval with titration for effect is crucial 6
• Do not use transdermal fentanyl for initial opioid therapy or rapid titration—it should only be used after pain is controlled with other opioids in opioid-tolerant patients 3, 4
• Do not omit breakthrough doses from the initial prescription, as transient pain exacerbations require immediate treatment options 4

Conversion to Long-Acting Formulations

• Once pain is adequately controlled and the 24-hour opioid requirement is stable, convert to extended-release oral formulations or transdermal fentanyl 3
• Provide rescue doses of short-acting formulation (10% of total daily dose) for breakthrough pain 3, 7
• If patients require >4 breakthrough doses per day, increase the scheduled around-the-clock dose 4, 7