See also separate article Is This New Treatment Any Good?

The number needed to treat (NNT) is the number of patients who need a specific treatment to prevent one additional bad outcome (e.g. myocardial infarction, stroke).¹ The NNT specifies the treatment, its duration, and the adverse outcome being prevented. Therefore, if a drug has an NNT of 10, it means you have to treat 10 people with the drug to prevent one additional bad outcome.

The number needed to harm (NNH) is similar to NNT and indicates how many patients need to be exposed to a risk factor to cause harm in one patient who would not otherwise have been harmed. The lower the NNH, the worse the risk factor.

Calculation

• The NNT is the inverse of the absolute risk reduction (ARR).²
• The ARR is the absolute difference in the rates of events between a given activity or treatment relative to a control activity or treatment; i.e. control event rate (CER) minus the experimental event rate (EER), or ARR = CER - EER.
• NNTs are always rounded up to the nearest whole number and accompanied by the 95% confidence interval.
• Example: if a drug reduces the risk of a bad outcome from 50% to 40%, the ARR = 0.5 - 0.4 = 0.1. Therefore, the NNT = 1/ARR = 10.
• A nomogram can be used to find the NNT by using the proportion of events in the control group and the relative risk reduction.³

It has been argued that the NNT and NNH are not the best way to report and assess the results of randomised clinical trials and that the ARR is preferable.⁴

Benefits of numbers needed to treat

• NNTs can be used either for summarising the results of a therapeutic trial or for medical decision-making about an individual patient.²
• The NNT provides a more clinically useful measure of the relative benefit of an active treatment over a control than the use of the relative risk, the relative risk reduction (RRR) or the odds ratio (the ratio of the odds of an event occurring in one group to the odds of it occurring in another group).⁵
• NNTs are sensitive to factors that change the baseline risk, such as the outcome considered, patients' characteristics, secular trends in incidence and case fatality, and the clinical setting.⁶

Disadvantages

• Although NNTs are easy to interpret, they cannot be used for performing a meta-analysis. Pooled NNTs derived from meta-analyses can be seriously misleading because the baseline risk often varies appreciably between the trials.⁶
• Applying the pooled RRRs from meta-analyses or individual trials to the baseline risk relevant to a specific patient group produces a useful NNT.⁶ Multiplying the RRR by the CER gives the ARR.

Document references

1. Number Needed to Treat (NNT), Centre for Evidence Based Medicine
2. Chatellier G, Zapletal E, Lemaitre D, et al; The number needed to treat: a clinically useful nomogram in its proper context. BMJ. 1996 Feb 17;312(7028):426-9. [abstract]
3. Cook RJ, Sackett DL; The number needed to treat: a clinically useful measure of treatment effect. BMJ. 1995 Feb 18;310(6977):452-4. [abstract]
4. Hutton JL; Number needed to treat and number needed to harm are not the best way to report Br J Haematol. 2009 Jun;146(1):27-30. Epub 2009 Apr 27. [abstract]
5. Smeeth L, Haines A, Ebrahim S; Numbers needed to treat derived from meta-analyses--sometimes informative, usually misleading. BMJ. 1999 Jun 5;318(7197):1548-51.
6. Number needed to treat (NNT), Bandolier

Internet and further reading

• Odds and odds ratio, Bandolier