Expressing the magnitude of adverse effects in case-control studies: "the number of patients needed to be treated for one additional patient to be harmed"

Results of epidemiological studies need to be expressed in understandable terms if they are to be of practical use to clinicians and policy makers Case-control studies are often used to stud) adverse effects of treatment; odds ratios from these are used to express the magnitude of adverse effects, but ar-e not intuitively understandable estimates of risk A more understandable and informative means of expressing the risk of adverse events in case-control studies is "the number of patients needed to be treated for one additional patient to be harmed" This is calculated from the odds ratio and the unexposed event rate-that is, the rate of occurrence of the adverse event of interest in people not exposed to the treatment.