3,4-methylenedioxymethamphetamine (MDMA, "ecstasy") is an amphetamine derivative with psychostimulant properties. This substance is widely used around the world by young adults in recreational settings. One of the most remarkable characteristic of ecstasy users is the concurrent consumption of several other drugs of abuse including psychostimulants, alcohol, tobacco, LSD, cannabis and opioids. This poly-drug pattern of use is now prompting research towards understanding how the combination of MDMA with cannabis and opioids could affect neuropsychobiological processes related to addiction. As with other drugs of abuse, behavioural evidence has been presented supporting the role of the endocannabinoid system as a modulator of the rewarding/reinforcing properties of MDMA. On the other hand, the neurochemical substrate for the complex interactions between the endocannabinoid system and MDMA is poorly understood. MDMA also modulates the activity of the dynorphinergic and enkephalinergic systems in several brain structures related to addiction, as it has been shown for other psychostimulants. The work regarding the contribution of micro- and delta-opioid receptors in the rewarding effects of MDMA shows differential results in pharmacological studies in rats, with respect to studies using knock-out mice. The present review describes the behavioural and neurochemical interactions between MDMA, cannabinoids and opioids with respect to addiction processes.