Number Needed to Treat (NNT) and Number Needed to Harm (NNH)

Number Needed To Treat (NNT)

Number Needed to Treat (NNT) is a measure used to communicate the clinical impact of an intervention to patients, clinicians, and policymakers.
It specifies the exact number of patients who need to be treated in order to prevent one additional adverse event from occurring.
NNT is not an absolute measure of the effectiveness of a drug.
Instead, it is an estimate of efficacy specifically when compared to a defined comparator, such as a placebo.
It provides a clearer picture of treatment impact than relative risk, which can often over-inflate the perceived benefits of an intervention.

NNT is mathematically calculated as the inverse of the Absolute Risk Reduction (ARR).
The ARR is the actual difference in absolute risk between the control group and the intervention group.

N N T = \frac{1}{A R R}

If the calculation yields a fraction, it must be rounded up to the next whole patient.
For example, if the ARR is 0.036 (or 3.6%), the calculation is $1 / 0.036 = 27.77$ .
Therefore, at least 28 patients would need to be treated to prevent a single adverse event.

The smaller the NNT, the more effective the intervention.
For example, an NNT of 15 implies that 15 patients must undergo a specific therapy to prevent one patient from developing a negative outcome like neuropathy.
NNT will inherently vary according to the baseline risk of the specific population being studied.
Therefore, when quoting an NNT, the exact comparator used must always be explicitly stated to provide proper context.

The concept of NNT can be equally applied to harmful outcomes instead of beneficial ones.
When an intervention or exposure produces a harmful effect, the measure is termed the Number Needed to Harm (NNH).
NNH indicates how many patients must be exposed to an intervention or risk factor to observe one single adverse event taking place in the exposed group.

NNH is calculated as the inverse of the Absolute Risk Increase (or attributable risk).

N N H = \frac{1}{A b s o l u t e R i s k I n c r e a s e}

The larger the NNH, the safer the intervention is considered to be.
A very large NNH indicates that many people must be exposed before one person is harmed, implying low toxicity or a high safety profile.
A very small NNH indicates a highly toxic exposure or a dangerous side effect profile.

Parameter	Number Needed To Treat (NNT)	Number Needed To Harm (NNH)
Primary Focus	Beneficial interventions and preventive treatments.	Harmful exposures, risk factors, or side effects.
Definition	Patients needed to treat to prevent one adverse event.	Patients needed to expose to cause one adverse event.
Calculation	$1 / A b s o l u t e R i s k R e d u c t i o n$ .	$1 / A b s o l u t e R i s k I n c r e a s e$ .
Indicator Of Success	Smaller numbers indicate a highly effective intervention.	Larger numbers indicate a highly safe intervention.
Clinical Value	Helps make decisions between competing treatment options.	Helps assess acceptable toxicity and safety thresholds.