Ecstasy/MDMA and alcohol

Alcohol lessens the effects of MDMA. Also the desired effects. So you feel less of the MDMA. There is a chance that you will then take more MDMA to get a bit more effect.
You may also be less aware that you have had too much to drink. When your MDMA wears off, you are suddenly very drunk. The hangover will also be much bigger the next day.