In this trial, the investigators will evaluate the effect of metformin therapy following ST-elevation myocardial infarction (STEMI) in a total of 380 non-diabetic patients. This trial is a randomized, double blind, controlled trial. The intervention, which consist of metformin 500mg twice daily or placebo twice daily, will commence within three hours after the percutaneous coronary intervention, and will be continued for 4 months. The primary endpoint is the difference between the two intervention groups (metformin vs placebo) in left ventricular ejection fraction, as measured with magnetic resonance imaging after 4 months. The investigators hypothesize that metformin therapy results in a significantly higher ejection fraction in this population.