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To gain a rapid picture of the benefit (B) or harm of a given treatment, it is sometimes useful to calculate the number of people that would need to be treated to save or lose one life. That number is simply the reciprocal of the difference in absolute (not relative) risk (R). Considering again the example of thrombolytic therapy for an acute MI, the absolute risk difference is 12% – 9%, or 3% (B), so the number needed to treat (NNT) is 1/0.03 or 33. In the setting of an acute MI, one would need to treat 33 patients with thrombolytic therapy to save one life. On the other hand, the absolute risk difference for an intracranial hemorrhage is 1% (R), so the number needed to harm (NNH) is 1/0.01 or 100. One would need to give 100 patients thrombolytic therapy to cause one fatal intracranial hemorrhage. These simple calculations do not weigh the relative value of the benefit and the harm, but the NNT and the NNH can provide a useful snapshot of benefits and risks.
Last full review/revision November 2005
Content last modified November 2005
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