Background: Morphine is commonly used to relieve pain, anxiety and dyspnea in STEMI but it lowers blood pressure and delays the activity of oral antiplatelet agents. The impact of morphine on clinical outcomes remains unknown. This analysis was performed to determine if morphine use was associated with increased risk of adverse clinical events among STEMI patients treated with fibrinolytic therapy and clopidogrel or ticagrelor. Methods: In the Ticagrelor in Patients with ST Elevation Myocardial Infarction Treated with Pharmacological Thrombolysis (TREAT) study, 3799 STEMI patients treated with fibrinolysis were randomized to receive clopidogrel or ticagrelor. Morphine use was left to the discretion of the treating physicians. In this pre-specified analysis, we evaluated clinical outcomes based on the use and timing of morphine administration. Outcomes were stratified by randomized treatment group. Multivariable analysis was performed using Inverse Probability Treatment Weighting (IPTW) weighting. Results: Morphine was used in 53% of patients. After adjustment using IPTW weighting, morphine use was associated with higher hazard of reinfarction at 7 days (HR 4.9, P = .0006) and 30 days (HR 1.7, P = .04), and lower hazard of major bleeding (HR 0.37, P = .006). There was no significant difference in mortality at any time point. Conclusions: Among patients with STEMI treated with fibrinolytic therapy, morphine use was associated with a higher risk of early reinfarction and a lower risk of major bleeding but no difference in mortality. Clinical trial registration: clinicaltrials.gov Identifier: NCT02298088. © 2022 Elsevier Inc.