Examples
Worked examples
- Is an instance
A two-period, two-treatment (AB/BA) crossover bioequivalence study administers a single dose of the test drug in period one, then, after a washout interval set at roughly five times the drug's elimination half-life, administers the reference drug in period two -- the washout ensures pharmacokinetic measurements in period two reflect only the reference product, not residual test-drug exposure.
- Is an instance
An N-of-1 trial comparing two chronic pain therapies within a single patient alternates active-drug and comparator periods across several replicated cycles, with a washout interval between each period sized to the treatment's known onset/offset profile so that symptom ratings in each period reflect only the treatment assigned for that period.
Counter-examples
Looks similar, but isn't
- Not an instance
A parallel-group randomized controlled trial, where each participant is assigned to only one arm for the trial's full duration, has no washout period between treatments -- there is no second treatment period for a given participant to wash out before.
- Not an instance
The interval between finishing one clinical trial and becoming eligible to enrol in a different, unrelated trial is sometimes informally called a 'washout,' but it is not the same concept -- the crossover-design washout period is a formal, protocol-specified element of a single trial's own design, not an eligibility rule governing enrollment across separate studies.
Editorial commentary
A washout period is the interval between treatment periods in a crossover trial design during which no study intervention is given, allowing a prior treatment’s effects to dissipate before the next treatment period begins. It exists to prevent a carryover effect — the previous period’s treatment influencing outcome measurements recorded in the following period — from being mistaken for, or masking, the effect of the treatment actually being evaluated in that later period. Washout periods are a defining structural feature of any design where the same participant is exposed to more than one treatment in sequence, most commonly a crossover trial and its single-patient variant, the N-of-1 trial.
Why crossover designs need a washout period
A crossover trial’s statistical efficiency comes from using each participant as their own control: because the same person receives every treatment under comparison, between-subject variability — a major source of noise in a parallel-group trial — is largely removed from the treatment comparison. That efficiency depends entirely on each period’s outcome data reflecting only the treatment assigned to that period. If a treatment’s effect (or its side effects) persists into the next period, the comparison is confounded: a difference measured in period two might reflect period one’s treatment lingering, period two’s treatment, or some mixture of the two, and the trial has no clean way to separate them. A washout period addresses this directly, by inserting enough time between periods that the prior treatment’s effect has genuinely resolved before the next period’s data collection begins.
How washout length is determined
Washout duration is a protocol design decision, not a default or a convenience gap in the schedule. For pharmacological interventions, it is typically set relative to the drug’s elimination half-life — the time required for the amount of drug in the body to fall by half. A commonly cited rule of thumb in bioequivalence and pharmacokinetic crossover studies is a washout of at least five half-lives, by which point roughly 97% of the drug has cleared the system, though the actual figure used in any given protocol should be justified against the specific compound’s known pharmacokinetics rather than assumed. For non-pharmacological or behavioral interventions, where there is no half-life to calculate from, washout length is instead based on the expected duration of the intervention’s physiological or symptomatic effect, informed by prior clinical or mechanistic evidence. Either way, the washout period is specified in the protocol and its rationale documented in the statistical analysis plan, the same way any other design parameter is.
What a washout period cannot fix
A washout period manages carryover risk; it does not eliminate it as a possibility to be checked for. Even a well-justified washout can fail to fully clear an effect — particularly for interventions with a long or poorly characterized half-life, an active metabolite, or a genuinely irreversible physiological change (surgery, most disease-modifying therapies, learning or training effects in a behavioral intervention). This is precisely why crossover designs, including N-of-1 trials, are only appropriate for conditions and treatments with a relatively rapid, reversible onset and offset of effect — if a treatment’s effect cannot plausibly wash out within a practical timeframe, a crossover design is the wrong choice regardless of how long the washout is set. Analysis of crossover trial data commonly includes a formal test for a differential carryover effect (a carryover effect that differs by treatment sequence) as a check on whether the washout period actually achieved its purpose, in addition to — not instead of — the a priori design safeguard of setting an adequately long washout period in the first place.
Washout period vs. related concepts
Washout period vs. run-in period
A run-in period occurs before randomization, often on a placebo or on no treatment, and is used to establish a stable baseline or to screen out non-adherent participants before they’re randomized to a study arm. A washout period occurs between treatment periods that a participant has already been randomized into, within an already-underway crossover trial. The two serve different design purposes even though both involve an interval without the study’s active intervention.
Washout period vs. follow-up period
A follow-up period is the interval after a treatment period (or after the trial’s last treatment period) during which outcomes continue to be observed, typically to capture delayed effects or safety events. A washout period is specifically the gap between treatment periods in a multi-period crossover design, inserted for the purpose of clearing the prior treatment before the next one starts — not for continued observation of the treatment just completed.
Related terms
- N-of-1 Trial Design — a single-patient crossover design in which washout periods separate each replicated treatment cycle.
- Clinical Study Design Types — how crossover designs fit among the broader set of interventional and observational study designs.
- Placebo-Controlled Study Design
- Randomized Controlled Trial (RCT)
- Clinical Trial Protocol — where washout duration and its rationale are specified.
Machine-readable encodings
Use in your systems
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