Concepts
Drug half-life explained
Half-life is the time it takes for the amount of a drug in the body to fall by 50% under a given model. It is a useful simplification, not a guarantee about detection or safety.
Mathematical definition
In a first-order model, concentration over time can be written as C(t) = C₀ × (½)^(t / t½), where t½ is the half-life. After 1 half-life ~50% remains, after 2 half-lives ~25%, after 4 half-lives ~6.25%, and so on.
Why multiple half-lives matter
Many clinicians use 4–5 half-lives as a rough rule of thumb for when most of a dose has been eliminated. This does not guarantee absence of effect or detectability.