Measures of effect - number needed to harm

Definition

Number needed to harm is the number of patients who would need to be exposed to a treatment/risk factor for one additional person to experience a specified adverse outcome, compared with a control/unexposed group - calculated analogously to number needed to treat, but for harm rather than benefit, and both measures together inform a complete risk-benefit assessment of a treatment
Used alongside number needed to treat to give a complete picture of a treatment's risk-benefit trade-off, particularly important for treatments with a meaningful adverse effect profile

Calculated as 1 divided by the absolute risk increase in the adverse outcome attributable to the treatment/exposure - a lower number needed to harm indicates a more frequent/serious harm risk (fewer patients need to be exposed for one to be harmed), the inverse relationship direction to number needed to treat, where a lower number indicates a more effective treatment (fewer patients need treatment for one to benefit)

Should be calculated and reported alongside number needed to treat for the same treatment, allowing direct comparison of benefit magnitude against harm magnitude for informed risk-benefit decision-making
Like number needed to treat, number needed to harm varies with baseline risk of the adverse outcome in the specific population being considered, not a fixed property of the treatment alone

Comparing number needed to treat against number needed to harm for the same treatment provides a structured framework for shared decision-making, particularly valuable when discussing a treatment with both meaningful benefit and meaningful risk with a patient
A treatment with a number needed to treat substantially lower (more favourable) than its number needed to harm generally represents a favourable risk-benefit trade-off, though the relative severity/reversibility of the harm versus the benefit also matters beyond the numbers alone
NNT vs NNH direct comparison for shared decision-making, inverse interpretation direction (lower NNH = more harm, lower NNT = more benefit), baseline risk dependence shared with NNT, severity/reversibility considerations beyond the raw numbers
Comparing number needed to treat against number needed to harm provides a structured, quantitative framework for risk-benefit discussion with a patient, but the raw numbers alone don't capture everything relevant - the relative severity and reversibility of the harm versus the benefit also needs separate consideration, since a treatment with an unfavourable NNT-to-NNH ratio might still be reasonable if the harm is minor/reversible and the benefit prevents something severe

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