A 2 by 2 factorial, multicenter, prospective, randomized, open-label, blinded endpoint trial. Patients undergoing primary PCI for STEMI will be eligible. Enrolled patients will be randomly assigned to the ticagrelor group or the clopidogrel group in a 1:1 ratio. After emergent coronary angiography, patients who have thrombolysis in myocardial infarction (TIMI) flow grade <2 in coronary angiogram will be randomized again, to either bolus intracoronary injection of morphine sulfate or saline in a 1:1 ratio. Randomization will be stratified by infarct location (anterior vs. non-anterior), and morphine use for pain control before study enroll (for only intracoronary morphine).