The medication is available as Wellbutrin, Wellbutrin XL and Wellbutrin SR, and the generic bupropion. They vary in the way the chemical enters the body. For instance, Wellbutrin, Zyban and generic bupropion are classified under immediate release bupropion, Wellbutrin SR as intermediate release and Wellbutrin XL as extended release.What do these various release mean? Immediate release means that the formulation of the drug is released into the bloodstream immediately, for a short span. The SR formulation mixes in the blood gradually in 12 hours while XL takes 24 hours.