Number Needed to Harm Calculator

Number needed to harm (NNH) refers to the average number of patients who need to be exposed to some risk factor to cause harm in an average of one person who would not have been harmed otherwise.
The number needed to harm (NNH) is calculated as:
NNH = 1 / (IT – IC)
where:
  • IT: Incidence rate in treatment (or “exposed”) group
  • IC: Incidence rate in control group
To calculate NNH, simply fill in the boxes below and then click the “Calculate” button.

Number Needed to Harm (NNH): 50.00

Interpretation: 50.00 patients need to be exposed in order for one to experience a harmful effect, on average.

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