The Number Needed to Treat (NNT) is a clinical effect measure that indicates how many patients need to be treated with a particular treatment for one additional patient to experience a beneficial outcome (e.g. cure, prevention of an event). The NNT is the reciprocal of the Absolute Risk Reduction (ARR): NNT = 1 / ARR.
Example: if an event (e.g. myocardial infarction) occurs in 10% of patients in the treatment group and in 15% of patients in the control group, the ARR is 5% (0.05) and the NNT = 1 / 0.05 = 20. This means that 20 patients need to be treated to prevent one myocardial infarction. An NNT of 1 corresponds to a perfect treatment (100% success), while higher NNT values indicate lower absolute efficacy.
The NNT is particularly useful for clinical decision-making, for benefit-risk considerations, and for communication with patients, since, unlike the relative risk reduction (RRR), it reflects absolute clinical relevance. The related Number Needed to Harm (NNH) describes the risk of harm analogously. For CROs, calculating and communicating the NNT in Clinical Study Reports (CSR) is an important task. Regulatory reference: ICH E9.