# Number needed to treat

**Editor-In-Chief:** C. Michael Gibson, M.S., M.D. [1]

## Overview

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.

## Historical Perspective

The number needed to treat was described in 1988.^{[1]}

## Calculation of NNT

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 *p _{A}* and

*p*of this endpoint under treatments

_{B}*A*and

*B*, respectively, are known, then the NNT is computed as 1/(

*p*-

_{B}*p*).

_{A}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

## Variations

### Number Needed to Screen

In screening, the number needed to screen (NNS) is the number of patients who must be screened for one patient to benefit.^{[2]}^{[3]} Number need to screen is smaller than number needed to invite.

### Number Needed to Invite

In screening, the number needed to invite (NNI) is the number of patients who must be invited for screening for one patient to benefit. NNI is the number needed to screen divided by the rate of acceptance of screening.^{[2]}

## References

- ↑ 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. - ↑
^{2.0}^{2.1}Richardson A (2001). "Screening and the number needed to treat".*J Med Screen*.**8**(3): 125–7. PMID 11678550. - ↑ Rembold CM (1998). "Number needed to screen: development of a statistic for disease screening".
*BMJ*.**317**(7154): 307–12. PMC 28622. PMID 9685274.