The number needed to treat (NNT) represents the number of patients a doctor would have to treat to benefit one patient over a specified length of time. It is calculated as one divided by the absolute risk reduction.

Ideally, every treatment would have an NNT of one; that would mean that every patient treated benefits from treatment. However, most treatments are not that effective.

A reasonable NNT depends on the risks and costs of the treatment and the degree of benefit. For example, we might accept a higher NNT if the benefit is saving a life than if the benefit is allowing someone to return to work 0.7 days sooner. This is especially true if the treatment has real risks, such as allergic reactions or side effects. Even if the treatment is shown to be effective, the adverse effects might not justify its use.