Prevention. How much harm? How much benefit? 1. Influence of reporting methods on perception of benefits

Before a physician or a patient can decide whether a preventive program is worth while, each must understand the nature and degree of its benefits and the frequency and magnitude of its adverse effects. Preventive interventions can be divided into two major categories: those with infrequent or minor adverse effects and those with adverse effects that are frequent or serious. Accident prevention, avoidance of high-risk behaviour and healthy lifestyle choices such as breast-feeding and moderate exercise are associated with few adverse consequences. By contrast, screening populations for disease, risk classification for the purpose of selective preventive interventions, dietary intervention and prophylactic drug treatment may lead to more frequent and serious adverse effects. When assessing whether the benefits of a preventive intervention outweigh the harm, one must be aware that the methods used to report benefits of clinical trials may distort the reader's perception of their magnitude. The relative reduction of morbidity or mortality rate often grossly exaggerates benefits and should never be used as a basis for clinical decision making. More realistic ways of recording benefits are the absolute reduction of morbidity or mortality rate, the number of patients that need to be treated to avoid one adverse event, and the total cohort mortality rate.