patients and our own clinical efficiency benefit when the number we need to treat to prevent an event is low. We might not hesitate to treat even as many as 400 patients to save one life if the treatment was cheap, easy to apply and comply with, and safe. In reality, however, treatments usually are expensive and they carry risks. When these risks or adverse outcomes are documented in trial reports, users can apply the number needed to treat to judge both the relative benefits and costs of therapy. If, for instance, beta blockers cause clinically important fatigue in 10% of the patients who use them, the NNT to cause fatigue is 1/0.10 or 10. This is shown in Table 4 , where it is seen that a policy of treating low-risk patients after myocardial infarction (NNT = 400 to prevent one death) will result in 40 being fatigued for every life saved. On the other hand, a policy of treating just high-risk patients will result in 4 being fatigued for every life saved. Table 4: Incorporating side-effects into the Number Needed to be Treated If the risk of death at one year without therapy (baseline risk) is: X 1% or 0.01 10% or 0.10 And the risk of death with propranolol is: Y 0.01x0.75= 0.0075 0.10x0.75= 0.075 Then the Absolute risk reduction is: X - Y 0.01-0.0075=0.0025 0.10-0.075=0.025 And the Number Needed to be Treated to prevent one event is: 1/(X - Y) 1/0.0025 = 400 1/0.025 = 40