Number Needed to Treat
How many people need the treatment for one of them to benefit.
What it means
How many people have to take a treatment, for a given time, for one of them to get the benefit.
- NNT of 1 — everyone benefits. Almost nothing achieves this
- NNT of 10 — ten people treated, one benefits, nine get nothing
- NNT of 100 — common for preventive medicines taken by people at low risk
It is arguably the most honest single number in medicine, and it is rarely offered unprompted — partly because it is uncomfortable, and partly because people find it discouraging.
The mirror figure is number needed to harm — how many people take it before one has a significant side effect. The two together are the real picture.
In practice
In guidelines and research papers, and occasionally in a good decision-aid leaflet.
Almost never in a consultation, unless you ask.
Why it matters
Ask for it when deciding about a long-term preventive medicine — statins, blood pressure tablets, bone protection. "How many people like me take this for one to benefit?"
Ask for the number needed to harm alongside it.
A high NNT is not automatically a reason to decline. If the benefit is avoiding a stroke and the treatment is cheap and safe, an NNT of 100 can still be entirely worth it — but that should be your judgement made with the numbers, not without them.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026