Terminal vs effective half-life

Many substances do not behave like a single, well-mixed compartment. After a dose, concentration may drop quickly at first (distribution) and then more slowly later (elimination).

Terminal half-life

The terminal half-life is estimated from the last, slowest part of the concentration-time curve. It often reflects the slowest process governing decline in the sampled compartment during the terminal phase.

Effective half-life

The effective half-life is a practical summary that reflects how concentration changes across the period of interest. In repeated dosing contexts, some clinicians use an effective half-life concept to reason about accumulation and timing between doses.

Why one number can confuse

• A long terminal half-life can coexist with a relatively short “felt” duration if distribution dominates early effects.
• A substance with a short terminal half-life can still produce long-lasting effects if metabolites or downstream biology matter.
• Different studies may report different half-lives depending on sampling window and analytic model.

How HalfLifeDB handles this

HalfLifeDB intentionally uses a single half-life estimate to illustrate exponential decay. When a substance has multi-phase kinetics, a single curve is a simplification. Use the curve to understand the idea of a half-life process, not to make timing decisions.

Next: why half-life varies.