BACKGROUND: Studies have shown disparate results on the consequences of morphine use in ST-segment elevation myocardial infarction (STEMI). No study has evaluated alternative treatments that could be at least non-inferior to morphine without its potentially damaging consequences for myocardial function and platelet reactivity. The aim of this study was to evaluate whether nitrous oxide/oxygen plus intravenous acetaminophen (NOO-A) is non-inferior to morphine to control chest pain in STEMI patients. METHODS: This multicenter, open-label, cluster-randomized, controlled, non-inferiority study compared NOO-A with morphine in 684 prehospital patients with ongoing suspected STEMI of < 12 h duration and a pain rating score ≥ 4. The primary endpoint was the proportion of patients achieving pain relief (numeric rating score ≤ 3) after 30 min. Secondary safety endpoints included serious adverse events and death at 30 days. RESULTS: The median baseline pain score was 7.0 in both groups. The primary endpoint occurred in 51.7% of the NOO-A group and 73.6% of the morphine group (absolute risk difference - 21.7%; 95% confidence interval - 29.6 to - 13.8). At 30 days, the rate of serious adverse events was 16.0 and 18.8% in the NOO-A and morphine groups respectively (p = NS). The rate of death was 1.8% (NOO-A group) and 3.8% (morphine group) (p = NS). CONCLUSION: Analgesia provided by NOO-A was inferior to morphine at 30 min in patients with acute STEMI in the prehospital setting. Rates of serious adverse events did not differ between groups. TRIAL REGISTRATION: ClinicalTrials.gov: NCT02198378.