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Number needed to treat

The number needed to treat (NNT) is an epidemiological measure that indicates how many patients would require treatment with a form of medication to reduce the expected number of cases of a defined endpoint by one. It is defined as the inverse of the absolute risk reduction. It was described in 1988.[1]

For example, consider a hypothetical drug which reduces the risk of colon cancer by 50%. Even without the drug, colon cancer is fairly rare, maybe 1 in 3,000 in every 5 year period. The NNT for a 5-year treatment with the drug is therefore 6,000: by treating 6,000 people with the drug, one can expect to reduce the number of colon cancer cases from 2 to 1.

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 NNT is an important measure in pharmacoeconomics. If a clinical endpoint is devastating enough (e.g. death, heart attack), drugs with a high NNT may still be indicated in particular situations. If the endpoint is minor, health insurers may decline to reimburse drugs with a high NNT.


Worked example

Abbreviation Variable Equation Value
- subjects in control group - 250
- subjects in experimental group - 150
- events in control group - 100
- events in experimental group - 15
CER control event rate = events / subjects in control group 0.4, or 40%
EER experimental event rate = events / subjects in experimental group 0.1, or 10%
ARR absolute risk reduction (or increase) = CER - EER 0.3, or 30%
RRR relative risk reduction (or increase) = (CER - EER) / CER 0.75
NNT number needed to treat/number needed to harm = 1 / ARR 3.33
OR, RR odds ratio, relative risk (not really identical, but similar -- see articles for details) = CER / EER 4

See also

Online Calculators

  • EBEM's Calculator for NNT


  1. ^ Laupacis A, Sackett DL, Roberts RS. An assessment of clinically useful measures of the consequences of treatment. N Engl J Med 1988;318:1728-33. PMID 3374545.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Number_needed_to_treat". A list of authors is available in Wikipedia.
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