Washout Period
A period between treatment phases (particularly in crossover studies) during which no study drug is administered, allowing previous treatment effects to clear from the body. Washout duration is typically 5-7 half-lives of the drug to ensure minimal residual effect before starting the next phase.
Technical Context
Washout duration = 5-7 × drug half-life (to reduce residual drug concentration to <3% of steady-state levels). For weekly semaglutide (t1/2 ~1 week): washout would require 5-7 weeks for pharmacokinetic clearance, though pharmacodynamic effects (appetite suppression, GI adaptation) may persist longer. Carryover effects (persistent drug effects after washout) complicate crossover trial interpretation and are assessed statistically. In practice, washout periods must balance completeness of drug elimination against participant burden and disease management needs. For drugs with irreversible effects (e.g. GnRH agonist-induced receptor downregulation), the washout must be long enough for receptor recovery and hormonal normalisation, which may take weeks beyond drug elimination.