Statistics and risk
Number needed to harm
The number of people exposed to an intervention for one additional person to be harmed.
What it means
Definition
Number needed to harm. Number needed to harm describes adverse effects in the same practical language as number needed to treat. It is useful when comparing likely benefits and harms.
What it can tell you
It can help clarify what sort of evidence or claim is being discussed.
What it cannot tell you
It cannot, by itself, prove that the headline is accurate or clinically important.
Headline trap
Why this matters in headlines
A drug with impressive benefit may still need caution if the NNH for serious side effects is low.