Came here to pose the exact same question. I also always understood that one of the risks of repeated mdma intake is serotonin depletion. Also the effect where one feels like shit 1-3 days after an mdma trip is very real.<p>So while it is possible serotonin is not <i>the</i> happy chemical, it either does play a role (possibly or even likely if you ask me <i>in combination with</i> other chemicals) or else all other research so far has been very much wrong.