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A trial design where neither participants nor researchers know who received the drug versus placebo — the gold standard for eliminating bias, and a genuine methodological problem for drugs with obvious subjective effects.

Example: The FDA's rejection of MDMA therapy cited exactly this — both pivotal trials (90 and 104 participants) were positive, but reviewers noted "dramatic functional unblinding," since patients could tell whether they'd gotten MDMA. See where that program stands now.