Number needed to harm

The number needed to harm (NNH) is an epidemiologicalmeasure that indicates how many patients would require a specific treatment to cause harm in one patient. It is defined as the inverse of the absolute risk increase.

For example, consider the drug warfarinwhich reduces the risk of ischemic stroke. The drug increase the risk of GI bleeds.

In general, NNT is always computed with respect to two treatments A and B, with A typically a drug and B a placebo(in our example above, A is a 5-year treatment with the hypothetical drug, and B is no treatment). A defined endpoint has to be specified (in our example: the appearance of colon cancer in the 5 year period). If the probabilitiesp_A and p_B of this endpoint under treatments A and B, respectively, are known, then the NNT is computed as 1/(p_B-p_A).

The NNH is an important measure in evidence-based medicineand helps physicians decide whether it is prudent to proceed with a particular treatment. If a clinical endpoint is devastating enough (e.g. death, heart attack), drugs with a low NNH may still be indicated in particular situations if the NNT(the benefit) is less than the NNH.

See also

• Number needed to treat- the converse for side-effects

External link

• Bandolier article on NNT

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It uses material from the http://en.wikipedia.org/wiki/Number+needed+to+harm Wikipedia article Number needed to harm.