[Dictionary] Number Needed to Harm (NNH)

Number needed to harm (NNH) is the average number of patients who must receive a specific treatment, or be exposed to a specific risk factor, over a defined time period, for one additional patient to experience a specified harmful outcome compared with a control condition. It is the reciprocal of the absolute risk increase (ARI): NNH = 1 / ARI, where ARI is the harmful-event rate in the exposed or treated group minus the harmful-event rate in the unexposed or control group. NNH is the arithmetic mirror of number needed to treat (NNT) applied to an adverse rather than a beneficial outcome, and the two are only meaningfully compared for the same intervention, the same outcome definitions, and the same follow-up duration — an NNH computed for one adverse event at one time horizon cannot be validly compared against an NNT for a different outcome measured over a different period. A calculated NNH is only interpretable as evidence of harm when the underlying absolute risk increase is statistically significant; when a trial's confidence interval for the risk difference crosses zero, the resulting NNH is not a meaningful single number (its reciprocal passes through infinity at zero difference) and should be reported as a non-significant risk difference instead.