Number needed to treat (revision 3)

The number needed to treat (NNT) is the number of people who must receive a treatment, rather than the comparator, for one additional person to experience the benefit over a defined period. It is the reciprocal of the absolute risk reduction.^[1]

Its value is that it expresses effect on an absolute scale. A relative risk reduction of 20% describes a large benefit where the baseline risk is high and a negligible one where it is low; the number needed to treat carries the baseline risk within it.^[1]

\text{NNT} = 1 / (\text{risk}_{\text{control}} - \text{risk}_{\text{treated}})

For SELECT, the primary composite occurred in 8.0% of the placebo group and 6.5% of the semaglutide group over a mean 39.8 months. The absolute risk reduction is 1.5 percentage points, so the number needed to treat is 1/0.015 ≈ 67 over roughly 3.3 years.^[2]