The short answer
A washout period is a set time before a trial begins when a person stops a previous treatment so it clears the body. It keeps the earlier treatment from affecting the trial results.
A washout period is a wait before a trial after stopping a prior treatment.
It lets the earlier treatment leave the body.
This keeps old treatment effects from clouding the results.
The length depends on the previous treatment.
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A pause before starting
A washout period is a set amount of time before a clinical trial begins during which a person stops a previous treatment. The goal is to let that earlier treatment clear from the body before the trial's treatment starts.
Why it matters
Say a previous treatment were still active in your system. It could change how you respond, and make it hard to tell what the trial treatment actually did. A washout period keeps the results clean. That way the trial can measure its own treatment fairly.
Where it fits in the rules
Every trial has eligibility criteria. These are the rules that set out who can join. They cover things like your health, your age, your treatment history and the genetic changes in your tumor. A washout requirement is part of that treatment-history rule. The point of these rules is to lower the risk of harm and to make the results meaningful.
How long it lasts
The length varies. It depends on how long the previous treatment stays active in the body. Some drugs clear quickly, others take longer. The trial team works out the washout and supervises it as part of the study plan.
Safety comes first
The team weighs the safety of pausing a treatment carefully. If stopping a treatment would be unsafe for you, you may not be eligible for that trial. Trials also have safety systems around them. An ethics panel reviews the plan before anyone joins, and a monitoring board can stop a trial early if concerns come up. It is always worth raising any worries about a pause with the trial team before you agree.
When to get help sooner
- Call 911 or go to an emergency department if during a treatment pause you have sudden severe pain, trouble breathing, chest pain, heavy bleeding, confusion, or a seizure. Do not drive yourself.
- Call your trial team immediately, day or night, if you have a temperature of 100.4°F (38°C) or higher and you have had chemotherapy in the past few weeks. The CDC treats fever after chemotherapy as a medical emergency, and a washout does not end that risk straight away. If no one picks up quickly, go to an emergency department and say you have had chemotherapy.
- Call your trial team right away if you run a fever and are not currently on chemotherapy or within a few weeks of a dose (see above if you are), or if symptoms your treatment had been controlling come back quickly or get worse.
- Call your trial team within a day or two if you have new or growing pain, unexplained weight loss, or you are unsure whether to take another medicine during the washout. Do not start or stop any medicine on your own.
Sources
Common questions
What is a washout period?
It is a defined stretch of time before starting a trial during which a person stops taking a previous treatment, so that treatment can clear from the body before the study begins.
Why is it needed?
If a previous treatment were still active, it could affect how a person responds and make it hard to tell what the trial treatment actually did. A washout keeps the results clean.
How long does it last?
It varies. The length depends on how long the previous treatment stays active in the body, and the trial team sets and supervises it.
Is it safe to pause treatment?
The trial team weighs this carefully and monitors participants. If pausing a treatment would be unsafe, a person may not be eligible for that trial. Discuss any concerns with the team.
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Fever of 100.4°F (38°C) or higher during chemo, severe pain, or trouble breathing: call your cancer care team right away, day or night. Call 911 for an emergency.
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Written by: Cancer ExplainedSources last checked: 2026-08-13 what this meansLast updated: 2026-09-29Next planned review: 2027-01-14
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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