Cymbalta is (among other things) a norepinephrine affector, kind of like Effexor. The way it was explained to me was that people don't just have norepinephrine receptors in their brains - they've got them in their upper GI tract, too. So when you start messing around with those types of medications, it's not just firing off the receptors in your head, it's firing off the ones in your esophagus (etc.). That's why Effexor (and to a lesser extent Cymbalta) can make people nauseous when they start or change it.