Concepts to be considered when appraising applicability of evidence to older people - number needed to harm

Definition

Number needed to harm (NNH) - the number of patients who would need to receive a treatment/exposure for one additional patient to experience a specified adverse outcome, compared with a control - the harm-side counterpart to number needed to treat
NNH is reported far less consistently than NNT in both trial publications and clinical guidelines, despite being equally important for a genuinely balanced risk-benefit judgement, particularly in older patients where treatment burden and adverse effect risk are often higher

NNH is calculated the same way as NNT (1 divided by the absolute risk increase in the harm outcome between treatment and control groups) - a smaller NNH means the treatment causes harm more readily, the opposite direction from NNT, where a smaller number indicates greater benefit

A treatment can have a favourable NNT for its intended benefit but an unfavourable (low) NNH for a significant harm - the clinically meaningful judgement requires comparing NNT and NNH together, not looking at benefit in isolation
In frail older patients, both baseline absolute risk of the harm (e.g. falls, bleeding, delirium) and the treatment's added risk are often higher than in trial populations, meaning the true NNH in this population may be considerably lower (more harm) than the published figure suggests
Explicitly weigh NNT against NNH (and the relative severity/reversibility of the benefit versus the harm) when making a treatment decision, rather than focusing on benefit alone - this comparison is especially important in older patients given their generally higher harm susceptibility and often smaller absolute benefit given shorter life expectancy

Anticoagulation risk-benefit balance, statin therapy in the very elderly, any treatment with a narrow therapeutic margin in frailty

A treatment that appears clearly beneficial when only NNT is considered can appear far more marginal, or net harmful, once NNH is weighed alongside it - both figures are needed for an honest risk-benefit conversation with the patient

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