Ferris State University | College of Pharmacy | Professor of Pharmacy Practice

One Minute Genius

Number Needed to Treat / Harm

John and Albert skydiving Albert and I developed an acute interest in risk reduction at about 3500 feet.

Scroll down for what we discovered

Number Needed to Treat

Definition:The Number Needed to treat is the number of patients that you would need to treat to prevent one primary outcome (heart attack, death, stroke, whatever)
This applies to patients:
  • with the same problem studied
  • treated for the same duration as the study

  • First calculate the absolute risk reduction (ARR)
  • Then take the ARR in decimal form (e.g. .05 for 5%) and divide it INTO 1. (1/ ARR = NNT)
    • 8% stroke rate with A. Fib decreased to 3% with Coumadin
    • Absolute risk reduction of 5% .
    • NNT = 1 / ARR or 1/.05 = 20

    • Therefore you need to treat 20 A. Fib patients for one year with warfarin to prevent one stroke.

    Number Needed to Harm: this is the same concept as the Number Needed to Treat except that you use the Incidence of an adverse effect in the treatement group minus the incidence in the placebo group (or alternative treatment groug) as the Absolute Risk Increase (instead of Absolute Risk Reduction).

    The calculation is then the same using Risk Increase instead of ARR.

    Example: You need to treat 10 patients with spironolactone to cause one case of gynecomastia.