Number needed to treat is incorrect without proper time-related considerations.

The number needed to treat (NNT) is a simple measure of a treatment's impact, increasingly reported in randomized trials and observational studies. Its calculation in studies involving varying follow-up times or recurrent outcomes has been criticized. We discuss the NNT in these contexts, illustrating using several published studies. The computation of the NNT is founded on the cumulative incidence of the outcome. Instead, several published studies use simple proportions that do not account for varying follow-up times, or use incidence rates per person-time. We show that these approaches can lead to erroneous values of the NNT and misleading interpretations. For example, after converting the incidence rate to a cumulative incidence, we show that a trial reporting a NNT of 4 "to prevent one exacerbation in 1 year" should have reported a NNT of 9. A survey of all papers reporting NNT, published in four major medical journals in 2009, found that 6 out of all 10 papers involving varying follow-up times did not correctly estimate the NNT. As the "number needed to treat" becomes increasingly used in complex studies and in the comparative effectiveness of therapies, its accurate estimation and interpretation become crucial to avoid erroneous clinical and public health decisions.