Rationale: Once subjects have a heart attack, they remain at much higher than average risk of another heart attack and stroke, despite the best current therapies to lower their cholesterol and blood pressure and thin their blood. Many subjects with heart disease also have problems metabolising (i.e. processing) sugar even if they do not have diabetes. There is some evidence that metformin, a drug which improves sugar metabolism, decreases the risk of future heart attacks in diabetic patients. However, whether metformin further reduces the risk of heart disease beyond established treatments in people without diabetes is unknown.