What is NNH and how is it interpreted?

• A. NNH is the number of patients to treat to prevent one adverse outcome.
• B. NNH refers to the proportion of patients harmed.
• C. NNH refers to the time to harm rather than count.
• D. NNH is the number of patients treated for one additional harm.

Answer: (D)

NNH is the number of patients who must be treated for one additional harmful outcome to occur due to the treatment. It comes from the absolute risk difference for the harm outcome between the treatment and control groups. Specifically, NNH = 1 / (risk of harm with treatment − risk of harm with control). If the treatment increases the risk of harm, this difference is positive and NNH is a finite number; a larger NNH means fewer people experience harm per treated population, indicating a safer option. If the treatment lowers the risk of harm, the difference is negative, and NNH isn’t interpreted the same way—in that case you’d discuss harm reduction or use the related NNT concept to prevent harm.

For example, if 8% of patients taking the treatment experience the adverse event versus 5% with control, the absolute risk increase is 3 percentage points, and the NNH is 1 / 0.03 ≈ 33. So about 33 patients would need to be treated for one additional adverse event to occur because of the treatment.