Wellbutrin SR vs. Wellbutrin XL: Key Differences
No, Wellbutrin sustained release (SR) and Wellbutrin XL are not the same—they differ in dosing frequency and pharmacokinetic profiles, though both formulations are bioequivalent in terms of total systemic exposure to bupropion. 1, 2, 3
Formulation Differences
Dosing Frequency
- Wellbutrin SR (sustained release) is administered twice daily (typically 150 mg BID for a total of 300 mg/day) 4, 1, 2
- Wellbutrin XL (extended release) is administered once daily in the morning (typically 300 mg QD) 4, 1, 2
Pharmacokinetic Profile
- Wellbutrin SR reaches maximum concentration (Cmax) at approximately 3 hours after administration 3
- Wellbutrin XL has prolonged absorption with Cmax occurring at approximately 5 hours after administration 3, 5
- Despite these differences in absorption rates, all three bupropion formulations (IR, SR, and XL) are bioequivalent in terms of total systemic exposure (AUC) to bupropion 2, 3
Clinical Implications
Switching Between Formulations
- When switching from Wellbutrin SR to Wellbutrin XL, use the same total daily dose 1
- For example, a patient on 150 mg SR twice daily (300 mg total) should receive 300 mg XL once daily 1
Choosing Between Formulations
- Wellbutrin XL offers the advantage of once-daily dosing, which may improve adherence and convenience 2, 6
- Wellbutrin SR may be preferred for patients requiring more flexible dosing due to side effects or those needing dose adjustments for hepatic or renal impairment 4
- Both formulations have similar efficacy and tolerability profiles for major depressive disorder 2, 6
Maximum Dosing Differences
- Wellbutrin SR: Maximum dose is 400 mg per day (typically 200 mg BID) 4
- Wellbutrin XL: Maximum dose is 450 mg per day 4, 1
Important Safety Considerations
- Both formulations carry the same seizure risk and should be avoided in patients with seizure disorders 4, 1
- Gradual dose titration is required for both formulations to minimize seizure risk 1
- For hepatic impairment (moderate to severe), the maximum dose is 150 mg daily for either formulation 4, 1
- For renal impairment, dose reduction by half is recommended for both formulations 4, 1
Historical Context on Generic Bioequivalence
- After loss of patent protection for bupropion, several generic versions of bupropion were found not to be bioequivalent to the brand-name version, leading to FDA reports of loss of antidepressant effect or worsening adverse effects 7
- This underscores the importance of monitoring patients when switching between brand and generic formulations, though current FDA-approved generics must demonstrate bioequivalence 7