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[[File:Number needed to harm plain.svg|alt=Illustration of two groups: one exposed to a risk factor, and one unexposed. Exposed group has larger risk of adverse outcome (NNH = 4).|thumb|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:Medicine|medicine]], the '''number needed to harm''' ('''NNH''') is an [[Social:Epidemiology|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  <math>1/(I_e - I_u)</math>, where <math>I_e</math> is the [[Medicine:Incidence (epidemiology)|incidence]] in the treated (exposed) group, and <math>I_u</math> is the incidence in the control (unexposed) group.<ref name=":02">{{Cite book|title=Dictionary of Epidemiology - Oxford Reference|language=en|doi=10.1093/acref/9780199976720.001.0001|year=2014|isbn=9780199976720|last1=Porta|first1=Miquel|last2=Greenland|first2=Sander|last3=Hernán|first3=Miguel|last4=Silva|first4=Isabel dos Santos|last5=Last|first5=John M.}}</ref> 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 [[Medicine:Number needed to treat|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.<ref name=>{{cite web 
  |url=https://training.cochrane.org/handbook/current/chapter-15
  |accessdate=3 October 2023
  |title=Cochrane Handbook for Systematic Reviews of Interventions
  |last1=Schünemann 
  |first1=Holger J 
  |date=2023<!-- version 6.4 -->  
  |website=cochrane.org 
  |publisher=Cochrane Training
  |quote=<!-- section 15.4.2 -->[NNH] can easily<sup>''[citation needed]''</sup> 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.
}}</ref>

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== Relevance ==
The NNH is an important measure in [[Medicine:Evidence-based medicine|evidence-based medicine]] and helps physicians decide whether it is prudent to proceed with a particular treatment which may expose the patient to harms while providing therapeutic benefits.  If a clinical endpoint is devastating enough without the drug (e.g. [[Biology:Death|death]], [[Medicine:Myocardial infarction|heart attack]]), drugs with a low NNH may still be indicated in particular situations if the NNT is smaller than the NNH.{{dubious|reason=risk is exposure times effect; comparing number exposed while neglecting size of effect is statistically clueless|date=October 2023}}{{Citation needed|date=September 2018}} However, there are several important problems with the NNH, involving bias and lack of reliable confidence intervals, as well as difficulties in excluding the possibility of no difference between two treatments or groups.<ref>{{cite journal |author=Hutton JL | title= Misleading Statistics: The Problems Surrounding Number Needed to Treat and Number Needed to Harm | journal=Pharm Med |volume=24 |issue=3 |pages=145–9 |year=2010 |doi=10.1007/BF03256810| s2cid = 39801240 }}</ref>

==Numerical example==
{{RCT risk increase example}}

==See also==
* [[Medicine:Pharmacoeconomics|Pharmacoeconomics]]

==References==
{{reflist}}{{Clinical research studies}}
{{DEFAULTSORT:Number Needed To Harm}}
[[Category:Epidemiology]]
[[Category:Evidence-based medicine]]
[[Category:Medical statistics]]

{{Sourceattribution|Number needed to harm}}