MDMA interferes with the clearance of serotonin, dopamine and adrenalin/epinephrin from the brain, causing their levels to rise and ultimately leads to the serotonin syndrome.
In line with this, many recreational users of MDMA often use cannabis to soften the come-down of MDMA.
In rats, the after-effects of MDMA (2 x 10 mg/kg) include hyperthermia, increased Anxiety-like behavior and reduced exploration.
Administration of THC reduced these behavioral effects. In addition, THC normalized serotonin levels and prevented MDMA-induced neurotoxicity (Shen et al., 2011).
Shen, E.Y., Ali, S.F., and Meyer, J.S. (2011). Chronic administration of THC prevents the behavioral effects of intermittent adolescent MDMA administration and attenuates MDMA-induced hyperthermia and neurotoxicity in rats. Neuropharmacology 61, 1183–1192.