I have! IME I have always gotten pretty bad nausea from taking 2cb about 4-5 hrs after mdma. And then I kind of regret taking it then. I mean i still get the full experience but the nausea is so distracting it really takes a lot away from the trip for me. I don't particularly get much nausea with 2cb on its own. However, some of my friends really enjoy about 15-20mg of 2cb after the mdma starts to wear off. So i guess it's down to the person.