A placebo is an “imitation” tablet. It looks like an ASA tablet but it doesn’t contain any active medicine. The ASAC trial uses a placebo so that we can make as clear an assessment as achievable about the effects of ASA. To make sure the results are as reliable as possible, neither the participants, nor the doctors, will know or be able to choose who is taking active medication and who is taking a placebo. This is called a double-blind trial.