A washout period is a defined period of time in a clinical trial during which participants receive either no treatment or only a placebo. The aim is to ensure that the effects of previously taken medication have been completely eliminated from the body before the new study treatment begins.
A washout period is typically used before the start of a trial to bring all participants to the same treatment-free baseline state. It is also an essential feature of crossover trials. In a crossover design, each participant receives different treatments in sequence. A washout period is inserted between treatment periods to prevent ‘carry-over effects’, i.e. the residual effect of the first treatment on the measurement of the effect of the second treatment. The duration of the washout period is determined by the pharmacokinetic half-life of the medicinal product.