Calculating Numbers Needed to Treat (NNT) may not be as straight forward as you might think! A good example of how the calculation can be quite problematic concerns the treatment of Chronic Obstructive Pulmonary Disease (COPD) with inhaled corticosteroids (ICS). The evidence from randomised trials shows that treatment with ICS increases the risk of pneumonia, whilst reducing the risk of exacerbations of COPD. The occurrence of exacerbations in the trials was more common than episodes of pneumonia, and this difference in the rate of the two conditions means that the NNT early in the trials favours treatment with ICS, but the tables appear to turn the other way after a couple of years.
To find out more have a look at the online-ahead-of-print version of a Thorax Editorial that I have written about this (due out in print in early 2013).