Number needed to harm
- Measure
How many people must be treated, over a given time, for one to suffer an adverse effect caused by the treatment.
What it is
The number needed to harm (NNH) is the inverse of the absolute risk increase for an adverse effect: the risk of the event in the treated minus the risk in controls. It is the mirror image of the NNT. A high NNH means harm is uncommon. Putting NNT and NNH side by side, over the same period, shows how many people benefit for each one harmed.
Formula and reading
- ARI (absolute risk increase) = EER − CER for the adverse effect; NNH = 1 / ARI.
- Events per 1,000 treated: benefits = 1,000 / NNT; harms = 1,000 / NNH.
- Reading: an NNH of 40 over one year = one extra harm for every 40 people treated for a year.
Worked example
- Data: an anticoagulant is tested for one year. Major bleeding: 4% in the treated and 1.5% in controls.
- Absolute risk increase = 0.04 − 0.015 = 0.025.
- NNH = 1 / 0.025 = 40 over one year.
- The same trial lowers embolism from 9% to 4%: ARR = 0.05 and NNT = 1 / 0.05 = 20.
- Per 1,000 treated for a year: 1,000 / 20 = 50 embolisms prevented and 1,000 / 40 = 25 bleeds caused.
- Reading: for each extra major bleed, 2 embolisms are prevented; the decision depends on how serious each is.
NNT and NNH from the example, per 1,000 treated for one year
| Measure | Value | Events per 1,000 treated |
|---|---|---|
| NNT (embolism prevented) | 20 | 50 prevented |
| NNH (major bleed caused) | 40 | 25 caused |
Markers
- NNH
- ARI
How to read it
- Clinical practice guidelines weigh the NNT against the NNH together with the severity of each outcome and patient preferences.
- A mild adverse effect with a low NNH may be acceptable; a serious one with a high NNH may not.
Common pitfalls
- Comparing a 5-year NNT with a 6-month NNH.
- Reading a low NNH as good news.
- Subtracting the risks in the wrong order and getting a negative number.
Test yourself
Think of the answer, then tap the question to see it.
An adverse effect occurs in 7% of the treated and 2% of controls. What is the NNH?
Absolute increase = 0.05; NNH = 1 / 0.05 = 20.
Is an NNH of 500 better or worse than one of 50?
Better: the harm is less frequent; 500 people must be treated to cause one extra adverse effect.
What must be true to compare the NNT with the NNH?
They must come from the same population and the same follow-up period.
Connections
Educational content on epidemiological and statistical methods; the examples use made-up numbers to teach the calculation. Not a clinical or legal guide: current Colombian rules are in the official sources.
Sources: Chapter 15: Interpreting results and drawing conclusions (Cochrane, 2024); Chapter 6: Choosing effect measures and computing estimates of effect (Cochrane, 2024).
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Representative model, for educational purposes only.