Number needed to harm

"NNH" redirects here. For other uses, see NNH (disambiguation).
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).

In medicine, the number needed to harm (NNH) is an epidemiological measure that indicates how many persons on average need to be exposed to a risk factor over a specific period to cause harm in an average of one person who would not otherwise have been harmed. It is defined as the inverse of the absolute risk increase, and computed as , where is the incidence in the treated (exposed) group, and is the incidence in the control (unexposed) group.[1] Intuitively, the lower the number needed to harm, the worse the risk factor, with 1 meaning that every exposed person is harmed.

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.

Marginal metrics:

  • NNT for an additional beneficial outcome (NNTB)
  • NNT for an additional harmful outcome (NNTH)

are also used.[2]

  1. ^ Porta, Miquel; Greenland, Sander; Hernán, Miguel; Silva, Isabel dos Santos; Last, John M. (2014). Dictionary of Epidemiology - Oxford Reference. doi:10.1093/acref/9780199976720.001.0001. ISBN 9780199976720.
  2. ^ Schünemann, Holger J (2023). "Cochrane Handbook for Systematic Reviews of Interventions". cochrane.org. Cochrane Training. Retrieved 3 October 2023. [NNH] can easily[citation needed] be read to imply the number of people who will experience a harmful outcome if given the intervention ... The preferred alternative is to use phrases such as 'number needed to treat for an additional beneficial outcome' (NNTB) and 'number needed to treat for an additional harmful outcome' (NNTH) to indicate direction of effect.