Once we have established the size (and meaning) of the benefit associated with a treatment, we need to consider the size (and meaning) of the harms also. The kinds of side effects associated with cancer treatments would clearly be unacceptable in a treatment for headache. We cannot decide whether a treatment is worthwhile without considering the harms as well as the benefits.
The number needed to harm (NNH) is a very convenient means of doing this, and is precisely analogous to the NNT.
Click on the "exercise" link below for a short quiz on NNH...