Number Needed to Harm

The number of patients who would need exposure to a treatment or risk factor for one additional patient to experience an adverse outcome.

Source: Cook & Sackett 1995

The number needed to harm expresses how many patients would have to be exposed to a treatment or risk factor for one extra person to suffer a particular adverse outcome. A small number means harm is common, arising after only a few are treated, while a large one means it is rare. Set beside the number needed to treat for benefit, it lets the good a treatment does be weighed against the harm, in the same intuitive units of people affected. Framing risk as people harmed is its purpose. Sackett and colleagues (1991) describe this measure.

Source: Sackett et al. 1991

The number needed to harm is calculated as the reciprocal of the absolute risk increase, that is one divided by the difference in the risk of the adverse outcome between the exposed and unexposed groups. If exposure raises the risk of harm by a certain absolute amount, the number needed to harm is one divided by that amount. So the number needed to harm is calculated from the absolute risk increase in the adverse outcome, taking its reciprocal, which converts the difference in harm between groups into the number who must be exposed for one additional harmful event.

Source: Cook & Sackett 1995

The number needed to harm is interpreted as the number of patients who must be exposed for one extra adverse outcome to occur: a small number means harm arises after relatively few exposures and is therefore more frequent, while a large number means harm is rare. So the number needed to harm is interpreted as how many exposures produce one additional harm, with smaller values indicating greater harm, and it is weighed against the number needed to treat for benefit, since comparing how many must be treated to help one patient with how many to harm one patient informs the balance of benefit and risk.

Source: Cook & Sackett 1995

The number needed to harm relates to the number needed to treat as its counterpart for adverse outcomes: the number needed to treat is the number who must be treated for one to benefit, and the number needed to harm the number who must be exposed for one to be harmed, both being reciprocals of an absolute risk difference. So the number needed to harm and the number needed to treat together express a treatment's benefits and harms on a common, interpretable scale, allowing clinicians and patients to weigh how many must be treated to help one against how many would be harmed.

Source: Cook & Sackett 1995

The limitations of the number needed to harm include that it depends on the baseline risk and the time period, so it varies between populations and durations and is not a fixed property of a treatment; that it requires a reliable estimate of the absolute risk increase; and that it summarises one harm at a time. So the number needed to harm is interpreted with attention to the baseline risk, time frame, and population to which it applies, since a single number needed to harm does not capture all harms or their severity, and comparing across settings requires comparable baseline risks.

Source: Cook & Sackett 1995