Why do your pupils dilate when you have taking ecstasy (MDMA)?

Pupil size is determined by two different types of muscles: muscles that contract the pupil and muscles that dilate the pupil. These muscles keep each other in balance. When you take ecstasy (MDMA), substances (neurotransmitters) are released in your body that lead to a so-called ‘fight or flight’ reaction. As part of this response, they stimulate the muscles that dilate your pupils, giving these muscles the upper hand. This will give you large pupils.