Number needed to treat

The number needed to treat (NNT) is a measure of the effectiveness of a healthcare intervention. It represents the average number of patients who need to receive a specific treatment (instead of a control or alternative treatment) for one additional patient to experience a positive outcome, or for one adverse outcome to be prevented. For example, an NNT of 50 to prevent a myocardial infarction (MI) means that 50 patients need to be treated for 1 MI to be prevented – or, for every 1,000 patients treated, 20 MIs are expected to be prevented. The NNT is equivalent to the reciprocal of the absolute risk reduction. NNTs have been popularised as a central concept in evidence-based medicine (clinical epidemiology) as a simple way of communicating the results of clinical trials. They require a defined endpoint (which may make them less useful for chronic, progressive conditions) and are time-specific: in the example provided above, the duration of treatment and the time period over which MIs may be prevented need to be specified. The degree of patient benefit associated with different endpoints may vary. As similar NNTs may be generated with different combinations of underlying risk and risk reductions, they should not be used directly in meta-analysis.