Number needed to harm (NNH)
This is calculated in the same way as for NNT, but used to describe adverse events. For NNH, large numbers are good, because they mean that adverse events are rare. Small values for NNH are bad, because they mean adverse events are common.
An example of how NNH values can be calculated along with NNT is that of inhaled corticosteroids used for asthma, where increasing dose made small improvement in efficacy, but large worsening for dysphonia and oral candidiasis.
Other material on NNT is useful for NNH as well: