This number is similar to the number needed to treat (NNT) in the opposite direction: It is the number of people who need to receive an intervention (a medication, a surgery, a treatment, etc.) before one of them is harmed. Whereas a good NNT is a very low number – such as only two people taking a drug for one to benefit – a good NNH is a very high number, such as giving a medication to 1,000 people for one to experience an adverse event. The smaller the NNH, the more common adverse events are, and the riskier the intervention is.
NNH can also refer to removing a therapy. For example, anti-epileptic drugs for epilepsy can have negative long-term effects. In a systematic review, researchers looked for the appropriate timing for discontinuing medication without having patients suffer a relapse. The NNH for discontinuing medication was 8. For every 8 individuals who discontinued anti-epileptics, one would relapse and experience a seizure.