The number needed to treat (NNT) is a valuable information in treatment decisions. This is the result of a calculation based on published data, collected from an intervention study. NNT is the inverse of the absolute risk reduction (1/ARR) between two treatment options. It is always expressed by a value rounded up to whole numbers, between 1 and infinity, depending on the effectiveness of the new treatment being studied. The NNT is the average number of patients needed to be treated for a duration equal to the study period to achieve one additional positive response under the conditions of the study. The more the treatment will be effective, the more the NNT will be low (tending towards 1). The NNT should be calculated based on the results of a clinical study of good methodological quality, involving a well-defined and homogeneous group of patients, whose baseline risk is known and using a dichotomous endpoint (the event occurs or not). The study must have shown a statistically significant difference between the two groups. In the process of medical decision making, the NNT must be balanced with the safety profile of the treatment (represented by the number needed to harm, or NNH), the costs and stress associated with it. It is possible to calculate a NNT and its confidence interval, from the published data of a clinical study. Some authors report the NNT in the text of the publication.