number needed to harm

3 min read 19-03-2025

The Number Needed to Harm (NNH) is a critical but often overlooked metric in evaluating the risks associated with a treatment or intervention. Unlike the more widely known 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 need to be exposed to a treatment before one additional person experiences harm. Understanding NNH is crucial for making informed decisions about healthcare choices, especially when weighing the benefits against the potential risks.

What is the Number Needed to Harm (NNH)?

The NNH represents the number of patients who need to receive a specific treatment or intervention for one additional person to experience a harmful effect compared to a control group. A lower NNH indicates a higher risk of harm. For example, an NNH of 5 means that for every 5 people treated, 1 will experience an additional harmful event compared to those who didn't receive the treatment. Conversely, a high NNH suggests a lower risk of harm.

How is NNH Calculated?

The calculation of NNH is relatively straightforward:

NNH = 1 / (Risk in treated group - Risk in control group)

Where:

Risk in treated group: The proportion of patients in the treatment group who experienced the adverse event.
Risk in control group: The proportion of patients in the control group who experienced the adverse event.

It's vital to remember that the risk difference should be expressed as a positive number; if the risk is lower in the treated group, the result will be negative and the NNH is not applicable.

Interpreting the NNH: High vs. Low Values

The interpretation of the NNH is crucial for clinical decision-making. A lower NNH signifies a higher risk, while a higher NNH suggests a lower risk. Clinicians and patients should consider the NNH alongside the NNT and other relevant factors when making treatment choices.

Low NNH (e.g., 2-5): This indicates a substantial risk of harm associated with the treatment. The potential benefits of the treatment must be carefully weighed against this significant risk.
High NNH (e.g., 100+): This suggests a low risk of harm. The likelihood of experiencing an adverse event is relatively small compared to the potential benefits.

Limitations of NNH

While the NNH is a valuable tool, it's not without its limitations:

Dependence on study design and population: The NNH is highly dependent on the specific study design, the characteristics of the participants, and the definition of the adverse event. Results from one study may not be directly generalizable to other populations or contexts.
Multiple adverse events: Many treatments can have multiple potential adverse effects. The NNH should be calculated and considered for each individual adverse event.
Subjectivity in defining harm: Defining what constitutes "harm" can be subjective and context-dependent. This variability can influence the calculation and interpretation of the NNH.
Rare adverse events: The NNH becomes less reliable when dealing with rare adverse events, as larger sample sizes are necessary to accurately estimate the risk.

NNH in Practice: Examples and Applications

The NNH is used across various medical fields to assess the risk-benefit profile of treatments and interventions. For instance, it can be employed in evaluating the safety of medications, assessing the risks associated with surgical procedures, and comparing the efficacy and safety of different therapies. Clinicians use this data to provide patients with a comprehensive understanding of both the potential benefits and the potential harms of a particular treatment.

Conclusion: NNH as a Complementary Metric

The Number Needed to Harm is a valuable, yet often underutilized, tool for assessing the risks associated with medical interventions. By considering the NNH alongside the NNT, clinicians and patients can make more informed decisions that balance potential benefits with the possibility of adverse effects. It's essential to remember the limitations of the NNH and to interpret its results within the context of the study design, population, and definition of harm. Utilizing the NNH empowers individuals to participate more actively in their healthcare decisions.