Examining IV Acetaminophen as Adjunct to Titrated IV Morphine

A multicenter trial evaluated whether IV morphine plus placebo was noninferior to IV morphine plus IV acetaminophen for early analgesic response in adults presenting to emergency departments with acute severe pain.

The double-blind randomized study was conducted across 11 French EDs with stratification by traumatic versus nontraumatic pain. Eligible patients were adults (≥18 years) with acute pain of less than 24 hours’ duration and an NRS score of 5 or higher who were able to communicate and rate pain. Key exclusions included unstable vital signs, Glasgow Coma Scale score less than 15, pregnancy, body weight less than 50 kg, need for immediate fracture treatment or joint reduction, acute cardiopulmonary emergencies, acute intoxication, analgesic use within 8 hours, inability to obtain venous access, hypersensitivity to study drugs, kidney or hepatic insufficiency, chronic pain treatment, and use of buprenorphine, nalbuphine, or pentazocine.

Randomization kits contained either acetaminophen 1 g in a 100-mL ready-to-use IV bag or 100 mL of 0.9% sodium chloride placebo, administered concurrently with morphine titration: an initial 0.10 mg/kg bolus (maximum 10 mg) over 2 minutes followed by 0.05 mg/kg boluses every 10 minutes to protocolized stopping criteria. Outcomes were tracked at early ED time points through 60 minutes, with the primary assessment at 30 minutes.

Noninferiority was prespecified with a 1-point margin on the 0-to-10 NRS scale, using 95% confidence intervals (CIs) for the mean between-group difference in pain-score reduction (placebo group compared with acetaminophen group) adjusted for baseline NRS.

In the per-protocol (PP) analysis, the between-group difference in mean NRS reduction at 30 minutes was 0.32 points (95% CI, −0.29 to 0.94) in traumatic pain and 0.80 points (95% CI, 0.19 to 1.41) in nontraumatic pain; in the modified intention-to-treat (mITT) analysis, the corresponding differences were 0.36 points (95% CI, −0.28 to 1.01) and 0.76 points (95% CI, 0.11 to 1.41).

Because noninferiority required support in both the PP and mITT analyses and the upper bound of the 95% CI exceeded the 1-point margin in at least one analysis/stratum, the authors concluded that morphine plus placebo did not demonstrate noninferiority to morphine plus acetaminophen for initial pain relief.

Across prespecified time points (10, 20, 30, 45, and 60 minutes), pain scores decreased in both groups, and the reported trajectories showed reductions over time regardless of pain stratum. For opioid use at 30 minutes, the authors reported similar morphine consumption overall, with stratum-specific estimates including nontraumatic pain totals of 11.0 mg (SD 4.4) with acetaminophen vs 12.1 mg (SD 4.3) with placebo and traumatic pain totals of 12.0 mg (SD 4.6) vs 11.3 mg (SD 4.7), alongside corresponding mg/kg summaries. Successful analgesia at 30 minutes (defined as NRS ≤3) was reported at 81/117 (69%) vs 71/115 (62%) in nontraumatic pain and 57/85 (67%) vs 59/90 (66%) in traumatic pain (acetaminophen vs placebo). Rescue analgesia at 30 minutes was more frequent with placebo in the nontraumatic stratum (15/117 vs 3/120, placebo vs acetaminophen), while the traumatic stratum showed low use in both groups. These secondary measures were presented as supportive endpoints alongside the primary analysis.

During 60-minute active monitoring, reported adverse effects included nausea, vomiting, respiratory depression, hypotension (systolic blood pressure <90 mm Hg), dizziness, decreased consciousness (GCS ≤13), and rash or pruritus, with generally similar patterns between groups in each pain stratum. In the subset with extended 24-hour follow-up data, no statistically significant difference in adverse events was observed beyond 30 minutes (9/49 with placebo vs 5/40 with acetaminophen; P = .56), and no serious adverse event requiring withdrawal or medical intervention was recorded.

Key Takeaways:

• The trial reported that, under a prespecified 1-point noninferiority margin and a requirement for concordant PP and mITT support, confidence bounds around the 30-minute pain outcome did not consistently remain within the margin across traumatic and nontraumatic strata.
• Investigators observed broadly similar 30-minute morphine consumption and successful-analgesia proportions between groups, while reporting more rescue analgesia with placebo than with acetaminophen in the nontraumatic stratum.
• The authors noted low short-term adverse-event rates during 60-minute monitoring and described planned 24-hour follow-up for delayed events, with subset data not showing an increase in adverse events beyond 30 minutes.