I’ve found caffeine, and especially whatever they put in energy drinks, only interacts with my body in a helpful way when I’m not dehydrated. A Monster on a sweaty day does not get my ditches dug unless I’m chugging water too. Sorry I have no insight why, but maybe your med is a little like that.

Or perhaps it will cause the buildup of something toxic in your system if you aren’t drinking enough water to flush it or its metabolic byproducts out.