If You Take Effexor with MDMA Can You Get Serotonin Syndrome?

The answer to your question is somewhat complex.  The way in which MDMA works is not entirely clear because it affects multiple neurotransmitters besides serotonin, including dopamine and norepinephrine.  It is known that MDMA causes an increase serotonin in the synapse.  It is also known that taking serotonin affecting antidepressants (SRIs), such as Effexor, increase serotonin in the synapse by blocking reuptake of serotonin in the synapse. It turns out that the SRIs and MDMA both work to affect serotonin through the reuptake mechanism.  The SRIs are thought to have a greater affinity for the reuptake mechanism and therefore block the effects of MDMA on this mechanism.  Alternately, it is thought that the SRIs have already depleted the stores of serotonin and the MDMA cannot elicit a robust release of the neurotransmitter.  Pretreatment with SRIs in people taking MDMA  have shown that the MDMA has a less intense effect on people who have been taking SRIs.  To my knowledge combining a drug like Effexor and MDMA does not result in serotonin syndrome.  However, this is not to say that there might be other adverse effects and I do not endorse taking MDMA alone or with a SRI.