# Numbers needed to harm

This way of presenting information about drugs can also be used to express the measure of potential harmful effects of an intervention. In this case the control event rate and the experimental event rate are calculated as the percentage of individuals in respective treatment groups that show a harmful effect of the intervention.

In this case subtracting the control event rate from the experimental event rate gives us a measure known as absolute risk increase (ARI). From this, the value known as the number needed to harm (NNH) can be calculated by dividing 1 by the absolute risk increase, and again multiplying by 100 when the absolute risk increase is expressed as a percentage.

The NNH shows how many individuals would need to be treated with the drug in order for 1 to show the harmful effect.