BACKGROUND: Among the renewed applications of psychedelic medicines in psychiatry, 3,4-methylenedioxymethamphetamine (MDMA)-assisted therapy for posttraumatic stress disorder (PTSD) has demonstrated the most promise in early small-scale studies. Recent exploratory analyses from prior clinical trials of MDMA-assisted therapy for PTSD have suggested that recent use of selective serotonin reuptake inhibitors (SSRIs)-the only medication class with United States Food and Drug Administration (FDA) approval to treat PTSD-can significantly dampen the efficacy of this novel therapy. Although psychedelic medicines are not yet FDA approved, MDMA is very likely to be the first to achieve FDA approval-perhaps within the next 2 years. Given this timeline, the field would benefit from more knowledge about potential interactions between this novel therapy and our current treatments. METHODS: This brief report reviews selected literature in the basic and clinical neurosciences relevant to the interaction of SSRIs and MDMA. FINDINGS: The possibility that SSRI use could dampen future responses to MDMA-assisted therapy for PTSD raises many important questions about the biological mechanisms as well as ethical implications around the most appropriate way to counsel patients. In this brief report, we compare the evidence for SSRIs and MDMA-assisted therapy in the treatment of PTSD and discuss what is known about the neurobiological interactions between these 2 medicines. CONCLUSIONS: There is strong neurobiological plausibility for the hypothesis that chronic SSRI use dampens response to MDMA-assisted therapy, although current knowledge in the field is limited and primarily relates to acute pharmacodynamic interactions. Our commentary highlights the urgent need for future work dedicated to addressing this important clinical topic.