After a heart attack, patients are prescribed two drugs to 'thin' the blood and to prevent further heart attacks. However, thinning of the blood also means that bleeding is more likely, and with time, this can have potentially life-threatening consequences. We therefore propose a nationwide study that will address this major concern by following what happens to patients if we use two different durations of treatment. The goal of the study is to determine how long we should give blood thinning drugs so that heart attacks can be avoided without undue risk of bleeding. This will address a major international controversy and define how we best treat patients following a heart attack in order to maximise the benefits and minimise the risk. It will also have benefits for New Zealand's health system by potentially avoiding the costs of unnecessary and potentially harmful prolonged treatment.