NNH, number needed to harm, can be calculated by counting the number of people treated to get 1 bad outcome.

The Number Needed to Harm (NNH) is calculated by determining the number of patients who need to be treated for one additional patient to experience a specific adverse effect. It essentially quantifies the risk of harm associated with a treatment.
Here’s a breakdown of how it works:
NNH is the inverse of the absolute risk increase (ARI):
NNH = 1 / ARI.
Absolute Risk Increase (ARI):
This is the difference in the rate of adverse events between the treatment group and the control (or placebo) group.
Lower NNH = Worse Risk Factor:
A lower NNH indicates that fewer patients need to be treated to cause harm, meaning the treatment has a greater risk of causing adverse effects.
Higher NNH = Safer Treatment:
A higher NNH indicates that more patients need to be treated before one experiences an adverse effect, suggesting the treatment is relatively safer.
In simpler terms: If a treatment has an NNH of 20, it means that, on average, 20 patients need to be treated for one additional person to experience a specific adverse effect.