What is the purpose of a washout period in cohort exposure definitions?

• A. Drug-free period to define new users
• B. To compare groups
• C. To measure outcome
• D. To recruit more participants

Answer: (A)

A washout period is used to ensure you identify true new users of an exposure rather than people who are continuing exposure from before. By requiring a drug-free interval before the start of follow-up, you confirm that initiation has occurred anew, not just ongoing use carried over from a prior period. This reduces misclassification of prevalent users as new users and minimizes biases from prior exposure or lingering effects, which helps you attribute subsequent outcomes more accurately to the initiation of the exposure. It’s not intended for comparing groups, measuring outcomes, or recruiting participants; its purpose is specifically to define incident exposure status at the start of follow-up.