NNT as a reciprocal
Number needed to treat is the inverse of absolute risk reduction. If a treatment prevents 10 events per 100 people (10% ARR), you must treat 10 people to prevent one event (NNT = 1/0.10 = 10). If it prevents only 2 events per 100 (2% ARR), NNT jumps to 50. This transformation makes the same relative reduction look vastly different depending on baseline risk.
A 50% relative risk reduction sounds impressive, but the NNT clarifies whether it matters clinically. If you must treat 1,000 people to prevent one event, the practical benefit may not justify cost or side effects.
Comparing treatments across conditions
NNT lets you compare preventive treatments on equal ground. An antihypertensive drug might have NNT=50 for preventing stroke in the general population, while a specific intervention in high-risk patients might have NNT=5 for the same outcome. The lower NNT signals greater clinical efficiency, making resource allocation decisions more transparent.