Number needed to harm (NNH)

The number needed to harm, or NNH, is how many people would need to receive a treatment for one extra person to experience a particular harm, such as a side effect.

This term is specific to research interpretation and is defined on AnxietyResearch. Shrinktionary, the network's language layer, doesn't currently carry a canonical entry for it.

NNH is the mirror image of the number needed to treat. NNT counts benefit. NNH counts harm. A high NNH is good, because it means harm is rare. A low NNH is concerning, because it means harm is common. Reading the two together gives a balanced picture of a treatment.

Every anxiety treatment has trade-offs. Medications can cause side effects. NNH puts a number on how often. Comparing NNT and NNH helps a clinician and patient weigh whether a treatment's likely benefit is worth its likely harm.

A medication has an NNT of 6 for a meaningful response and an NNH of 30 for a bothersome side effect. Benefit is more common than this particular harm, which supports its use for many patients, though not all.

NNH depends entirely on which harm is being counted. A drug can have a high NNH for a rare serious harm and a low NNH for a common mild one. Check which harm the number describes. A single NNH doesn't capture every risk.