How do I calculate the number needed to treat (NNT) for a specific intervention based on available clinical data?

To calculate the Number Needed to Treat (NNT) for a specific intervention, you first need to determine the Absolute Risk Reduction (ARR) from available clinical data. ARR is calculated by subtracting the event rate in the treatment group from the event rate in the control group. Once ARR is known, NNT is simply the reciprocal of ARR (i.e., NNT = 1 / ARR) expressed as a whole number rounded up to the nearest integer to reflect the number of patients who need to be treated to prevent one additional adverse event or achieve one additional beneficial outcome NICE NG238.

For example, if the event rate is 10% in the control group and 5% in the treatment group, ARR = 10% - 5% = 5% (or 0.05), so NNT = 1 / 0.05 = 20, meaning 20 patients need treatment for one to benefit.

This calculation assumes that the clinical data are derived from well-conducted randomised controlled trials or meta-analyses providing clear event rates for both groups. It is important to use absolute risks rather than relative risks to avoid misinterpretation of treatment effect size NICE NG238.

Recent literature emphasises the practical utility of NNT in clinical decision-making by quantifying benefit in a patient-centred manner, as seen in studies analysing NNT for specific conditions such as neurogenic orthostatic hypotension and chronic insomnia disorder François et al. 2016Chalet et al. 2024. These studies demonstrate how NNT can guide clinicians in weighing benefits against harms and support shared decision-making NICE NG238 François et al. 2016Chalet et al. 2024.