number needed to harm

3 min read 20-03-2025

The Number Needed to Harm (NNH) is a critical, yet often overlooked, statistic in evaluating the risks associated with a treatment or intervention. Unlike the more familiar Number Needed to Treat (NNT), which quantifies the number of patients needing treatment to prevent one adverse event, the NNH tells us how many individuals must receive a treatment before one experiences harm. Understanding the NNH is crucial for making informed decisions about medical interventions and assessing the balance between benefits and risks.

What is the Number Needed to Harm (NNH)?

The NNH represents the reciprocal of the absolute risk increase (ARI). The ARI is the difference in the risk of harm between the treatment group and the control group (or placebo group). A smaller NNH indicates a greater risk of harm, meaning fewer individuals need to be treated for one person to experience an adverse event. A larger NNH indicates a lower risk of harm.

Formula: NNH = 1 / ARI

Where ARI = Risk of harm in treatment group - Risk of harm in control group

How to Interpret NNH

Low NNH (e.g., 2-5): Indicates a high risk of harm. This means that for every 2 to 5 individuals treated, one will experience a harmful side effect.
High NNH (e.g., >100): Suggests a low risk of harm. Many individuals would need to be treated before one experiences a harmful side effect.

It is important to consider the magnitude of the harm when interpreting NNH. A low NNH for a minor side effect might be less concerning than a high NNH for a serious adverse event.

Calculating the NNH: A Practical Example

Let's say a study compares a new drug to a placebo for treating hypertension. The study finds that 10% of patients taking the new drug experienced a serious adverse event (e.g., heart problems), while only 2% of patients in the placebo group experienced the same adverse event.

Calculate the Absolute Risk Increase (ARI): ARI = 10% - 2% = 8% or 0.08
Calculate the Number Needed to Harm (NNH): NNH = 1 / ARI = 1 / 0.08 = 12.5

This means approximately 13 individuals would need to be treated with the new drug before one experiences a serious adverse event like the heart problem mentioned above.

NNH vs. NNT: Understanding the Difference

While both NNH and NNT are valuable tools, they address different aspects of treatment outcomes. NNT focuses on the beneficial effects, while NNH concentrates on the harmful effects. Clinicians and patients should carefully consider both metrics when making treatment decisions. A treatment might have a low NNT (meaning it's effective for many), but also a low NNH (meaning it has significant side effects). The decision to use the treatment will depend on the balance between benefits and harms.

Limitations of NNH

Study Design: The NNH is heavily dependent on the quality of the study design. Bias or limitations in the study can affect the accuracy of the NNH calculation.
Specific Harms: NNH calculations usually focus on a single, pre-defined adverse event. It might not capture the full spectrum of potential harms.
Subgroup Analyses: Results may vary significantly across different subgroups of patients (age, sex, other conditions). This should be carefully considered.
Context Matters: The acceptability of a given level of harm varies depending on the context of the treatment. A low NNH might be acceptable for a life-saving treatment, while it might be unacceptable for a treatment of a minor condition.

Conclusion: NNH in Clinical Decision-Making

The Number Needed to Harm is an essential statistic for evaluating the risks associated with medical interventions. While it does have limitations, incorporating NNH into clinical decision-making can empower patients and clinicians to make more informed and balanced decisions regarding the best treatment options. Always consider the NNH alongside the NNT, and critically assess the study's methodology before making conclusions. Remember to consult with a healthcare professional for personalized medical advice.