Aspirin reduces the risk of heart attacks and strokes but also increases the risk of bleeding. New Zealand guidelines recommend aspirin for people at high risk of a heart attack or a stroke and who have not already had such an event, taking into account both the benefits and harms of aspirin. But determining whether the benefits of aspirin outweigh its harms is difficult because these vary according to individual characteristics (such as age and sex). Further, there is no accepted method to objectively determine the balance of benefits and harms of aspirin. This proposal is to develop an evidence-based tool that will quantify the likely harms (additional bleeds) and benefits (reduction in heart attacks and strokes) of aspirin for an individual patient. The aspirin harm benefit calculator will facilitate evidence-based decision-making by clinicians and support discussions with their patients regarding the optimal use of aspirin.