Calculate the number needed to harm from adverse event rates.
Number needed to harm mirrors number needed to treat but for adverse effects. A low NNH means harm is common, and it must be weighed directly against the NNT for benefit. Rare but severe harms can outweigh a favourable NNH, so severity must be considered alongside frequency.
Number Needed to Harm
NNH = 1 ÷ absolute risk increase
NNH = 1 ÷ absolute risk increase Number needed to harm mirrors number needed to treat but for adverse effects. A low NNH means harm is common, and it must be weighed directly against the NNT for benefit.
Rare but severe harms can outweigh a favourable NNH, so severity must be considered alongside frequency.
This calculator takes 3 inputs: Adverse event rate with treatment, Adverse event rate without treatment, Population treated. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.