The number needed to harm (NNH) is the number of people that must be treated with a specific intervention for one extra person to experience a specific adverse effect (or harm) over a defined time.^[1] The NNH is in contrast to the number needed to treat (NNT), which indicates the number of people who must be treated for one extra person to benefit. A large NNH means the harm is rare, and a small NNH means the harm is more common. However, NNH is only meaningful in the context of the relevant NNT.

For example, in the SECURE trial, adults within 6 months after a heart attack were randomized to receive a daily “polypill” (aspirin, a statin, and ramipril) or standard treatment.^[2] For nonfatal serious adverse events, the trial reported an absolute risk increase of 0.9% (or 0.009), so the NNH was approximately 111 (1 ÷ 0.009) over 3 years.^[2] In other words, 111 people would need to receive the polypill over three years for one extra person to experience harm from it. On the other hand, for the primary outcome — a composite of cardiovascular death, nonfatal heart attack, nonfatal ischemic stroke, or urgent revascularization — the trial reported that the NNT was approximately 31 over three years. That means 31 people would need to receive the polypill over three years for one extra person to benefit. Since the number of people needed to receive the therapy for one extra person to benefit — 31 — was far lower than the number needed for one extra person to experience harm — 111 — the polypill looks like a good choice of intervention; the benefits outweigh the harm.