The number needed to harm: is it too optimistic?

The number needed to treat (NNT) to prevent one adverse outcome event is a useful measure of clinical trial results and represents the therapeutic effort required to realize a specific clinical benefit. With many therapies, there are both positive and negative consequences. Analogous to the NNT, there is the number needed to harm (NNH), which is the reciprocal of the absolute risk increase in situations where the experimental treatment harms more patients then the control treatment. The NNH is the number of patients who, if they received the experimental treatment, would lead to one additional person being harmed compared with patients who receive the control treatment. The NNH may, however, provide an optimistic measure of the true risks caused by therapy.