Number needed to harm
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The number needed to harm (NNH) is an epidemiological measure 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, the drug warfarin, which reduces the risk of ischemic stroke, increases the risk of gastrointestinal 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 probabilities pA and pB of this endpoint under treatments A and B, respectively, are known, then the NNT is computed as 1/(pB-pA).
The NNH is an important measure in evidence-based medicine and helps physicians decide whether it is prudent to proceed with a particular treatment. 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 number needed to treat, or NNT, (the converse for side-effects, or the drug's benefit) is less than the NNH.
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