Half-Life Washout Calculator

This calculator applies first-order elimination pharmacokinetics -- the standard assumption that a drug's concentration falls by a constant fraction with each elapsed half-life -- to estimate how long it takes a medication to clear the body, accounting for how close to steady state the patient actually was when the drug was stopped. Days at Steady State (how many days the patient was dosed consistently before stopping) determines what fraction of full steady-state concentration was actually reached, using F = 1 - 0.5^(24 x Days at Steady State / Half-Life): F approaches 1 once Days at Steady State covers roughly 5 half-lives (the same rule of thumb behind Time to Steady State below), but short of that, the patient never fully accumulated, and Washout Time and Half-Lives to Clear (95%) are calculated from that ACTUAL starting fraction rather than assuming a full steady-state start -- Half-Lives to Clear (95%) is the number of half-lives from that starting fraction down to 5% of steady state, and Washout Time multiplies it by Half-Life. At a full steady-state start (Days at Steady State well past 5 half-lives), this converges to the classic ln(0.05)/ln(0.5), approximately 4.32, half-lives. If the patient was stopped before ever reaching even 5% of steady state, the calculated washout time floors at 0 rather than going negative -- the drug was already below the washout threshold before it was even stopped.

Time to Steady State uses the standard pharmacology rule of thumb that a drug reaches steady-state concentration after about 5 half-lives, regardless of dose or dosing frequency -- it does NOT depend on Days at Steady State (that input describes how long dosing already ran, not how long it takes to reach steady state in the first place) or on Doses Per Day. Doses Per Day is collected as a reference input but does NOT currently change any calculated output here -- none of Washout Time, Time to Steady State, or the percent-remaining figures depend on dosing frequency, since none of the outputs this calculator reports are a function of tau. This is a first-order, single-compartment approximation: it does not model nonlinear (capacity-limited) elimination, active metabolites with their own half-lives, or organ dysfunction that can prolong a drug's true half-life beyond its typical reference value -- always use the half-life documented for your specific patient population and clinical situation, and confirm any procedure, drug interaction, or pregnancy-related washout timing with a pharmacist or prescriber rather than relying on this estimate alone.