Inmedicine, 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 
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:
are also used.[2]
The NNH is an important measure in 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. death, heart attack), drugs with a low NNH may still be indicated in particular situations if the NNT is smaller than the NNH.[dubious – discuss][citation needed] 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.[3]
| Quantity | Experimental group (E) | Control group (C) | Total |
|---|---|---|---|
| Events (E) | EE = 75 | CE = 100 | 175 |
| Non-events (N) | EN = 75 | CN = 150 | 225 |
| Total subjects (S) | ES = EE + EN = 150 | CS = CE + CN = 250 | 400 |
| Event rate (ER) | EER = EE / ES = 0.5, or 50% | CER = CE / CS = 0.4, or 40% | — |
| Variable | Abbr. | Formula | Value |
|---|---|---|---|
| Absolute risk increase | ARI | EER − CER | 0.1, or 10% |
| Number needed to harm | NNH | 1 / (EER − CER) | 10 |
| Relative risk (risk ratio) | RR | EER / CER | 1.25 |
| Relative risk increase | RRI | (EER − CER) / CER, or RR − 1 | 0.25, or 25% |
| Attributable fraction among the exposed | AFe | (EER − CER) / EER | 0.2 |
| Odds ratio | OR | (EE / EN) / (CE / CN) | 1.5 |
[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.
