Number needed to harm

It is defined as the inverse of the absolute risk increase, and computed as

NNH is similar to number needed to treat (NNT), where NNT usually refers to a positive therapeutic result and NNH to a detrimental effect or risk factor.

[2] The NNH is an important measure in evidence-based medicine and helps physicians decide whether it is prudent to proceed with a particular treatment which may expose the patient to harms while providing therapeutic benefits.

If a clinical endpoint is devastating enough without the drug (e.g. death, heart attack), drugs with a low NNH may still be indicated in particular situations if the NNT is smaller than the NNH.

[dubious – discuss][citation needed] However, there are several important problems with the NNH, involving bias and lack of reliable confidence intervals, as well as difficulties in excluding the possibility of no difference between two treatments or groups.

Illustration of two groups: one exposed to a risk factor, and one unexposed. Exposed group has larger risk of adverse outcome (NNH = 4).
Group exposed to a risk factor (left) has increased risk of an adverse outcome (black) compared to the unexposed group (right). 4 individuals need to be exposed for 1 adverse outcome to occur (NNH = 4).