The number needed to treat is the number of patients who are required for a specific treatment to prevent a bad outcome. The numbers need to treat (NNT) cannot be used for the meta-analysis. It cannot be calculated for different treatments unless the risk of that disease is similar. The absolute risk reduction can be found by the difference between two treatments. The number needed to treat formula can be used to determine the absolute risk reduction.
The NNT is applied to radiotherapy, respiratory medicine, etc. The number needed to treat formula in estimating the number of patients that need to be treated in order to have an impact on one person.