I recently refilled a prescription for bupropion. When I picked it up, the pharmacist told me that the original prescription was written for bupropion hcl sr 150 mg but was mistakenly filled with bupropion hcl xl 150 mg. She apologized profusely and said that the refill was what had actually been prescribed. She had contacted my physician and discussed this with her. What is the difference and is there anything I should be concerned about?