If they did the FSTs and you were demonstrably impaired, then the obvious conclusion may be that you were impaired by drugs, not alcohol.
Oh, and most people lie about when they last took any drugs. The visible effects of MDMA after 48 hours would be minimal to non-existent, and it is unlikely that they would get a positive test for it after 48 hours (given the standard cutoff of most crime labs), and it is not part of the basic drug screen in any event - at least in my state. So, if you admitted to using in the last 48 hours, they are probably going to assume you are lying and took it more recently.
Consult local counsel.
- Carl