Fluoxetine-psychedelic interaction data will be incorporated into FDA guidance requiring standardized SSRI washout criteria for psychedelic NDA submissions within 24 months

The FDA's guidance infrastructure for novel CNS therapies has a documented pattern: mechanistic preclinical data on drug-drug interactions, when clinically relevant to a large patient population, moves into guidance documents within 18-30 months of first publication. The fluoxetine-psychedelic interaction is not a minor edge case — SSRIs are prescribed to approximately 13% of the U.S. adult population (NCHS 2020 data), and treatment-resistant MDD patients, the primary psychedelic therapy target, have often cycled through multiple SSRI regimens. If acute vs. chronic fluoxetine exposure produces differential effects on psychedelic pharmacodynamics (likely via 5-HT2A receptor downregulation), the FDA faces a direct validity problem: existing trial populations may systematically differ from real-world patients in SSRI washout status. This is precisely the kind of methodological gap the FDA's