The clinical effects of calcium supplements on adverse events reporting have not been well described. This study reviews randomized controlled trial (RCT) evidence of adverse events to clarify the epidemiology of these events. The hypothesis that patient self-report of myocardial infarction (MI) is increased in individuals receiving calcium supplementation is because of an increase in non-MI events incorrectly perceived by the patient as being because of MI, is examined. In seven RCTs summary self-reported gastrointestinal (GI) adverse event rates were more common in participants receiving calcium. These were described as constipation, excessive abdominal cramping, bloating, upper GI events, GI disease, GI symptoms, and severe diarrhoea or abdominal pain (calcium 14.1%, placebo 10.0%), relative risk (RR) 1.43 95% confidence interval (CI) 1.28 to 1.59, p?