Wellbutrin is a norepinephrine and dopamine reuptake inhibitor and releaser It is also a nicotinic antagonist. This is relevant to bupropion's effects on lowering cravings for cigarettes and smoking reduction. I cannot handle the SR ,much less the straight Wellbutrin IR. It's quicker release causing more norepinephrine activity makes me jittery and anxious. The XL works well for me . I still feel the NE effects, but in a much smoother way. It is more of an activating and wakeful agent. It definitely helps with my co-morbid depression and the sexual, appetite side-effects that I get from Lexapro. Though Lexapro side effects for myself have been lower than any other SSRI.
Here is a link to excellent studies about Wellbutrin, which covers all three forms. The figures below show the concentrations and peak plasma times within the body by hrs.