Drug Half-Life Calculator

How the drug half-life calculator works

Most drugs are cleared by first-order kinetics, where the amount eliminated per unit time is proportional to the concentration present. That produces exponential decay with a constant half-life, and five half-lives is the usual rule for practical clearance.

Ethanol is the notable exception at normal doses: its elimination is zero-order, a fixed amount per hour regardless of concentration, which is why the Widmark calculator uses a linear term instead.

Frequently asked questions

How long until a drug is fully eliminated?

After five half-lives about 97 percent is gone, which is the conventional threshold for practical clearance. Detection in urine or hair can extend far past that, since those matrices reflect accumulated metabolites.

Why does ethanol not follow this model?

Alcohol dehydrogenase saturates at low concentrations, so ethanol is eliminated at a roughly fixed rate per hour rather than a fixed proportion. That is zero-order kinetics and it needs the Widmark linear model.

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