Re: Zantac and MDMA, there was a post in Health Q & A (now purged alas) about a person who took Zantac regularly, and one night took 2 pills. She then experienced what sounded like a minor MDMA overdose. Her friend who had taken 2 pills was fine. If I remember rightly, she was hallucinating, vomiting, dizzy and had trouble walking etc.
I found one article that stated Zantac was an inhibitor of CYP2D6. As BT stated, this is the enzyme that does most of the metabolising of MDMA, and if you inhibit its action, MDMA will be metabolised more slowly leading to increased blood levels. Other sources stated however that Zantac did not inhibit CYP2D6, including the manufacturer. Which left me a bit perplexed. Perhaps the acid neutralising effect of the Zantac was solely responsible for the overdose? Anyhow, her experience suggests you should proceed carefully if you plan on trying this combination. For example, don't start by taking multiple pills.