Wellbutrin XL to SR Dosage Conversion
When converting from Wellbutrin XL (extended-release) to Wellbutrin SR (sustained-release), maintain the same total daily dose but divide it into twice-daily administration instead of once-daily administration.
Conversion Guidelines
According to the FDA drug label for bupropion, when switching patients from Wellbutrin XL to Wellbutrin SR, the same total daily dose should be given when possible 1. This is because all three formulations of bupropion (immediate release, sustained release, and extended release) are bioequivalent in terms of systemic exposure to bupropion 2.
Specific Conversion Instructions:
- Wellbutrin XL 150 mg once daily → Wellbutrin SR 150 mg (as 75 mg twice daily)
- Wellbutrin XL 300 mg once daily → Wellbutrin SR 300 mg (as 150 mg twice daily)
- Wellbutrin XL 450 mg once daily → Wellbutrin SR 450 mg (as 150 mg three times daily)
Pharmacokinetic Considerations
The primary difference between the formulations is their release profile and dosing frequency:
- Wellbutrin XL: Once-daily dosing with absorption prolonged over approximately 5 hours 3
- Wellbutrin SR: Twice-daily dosing with absorption over approximately 3 hours 3
- Wellbutrin IR: Three-times-daily dosing with absorption over approximately 1.5 hours 3
Important Clinical Considerations
Timing of Doses
- Administer Wellbutrin SR doses approximately 8 hours apart
- Morning and afternoon dosing is typically recommended (e.g., 8 AM and 4 PM)
- Avoid evening doses when possible to reduce risk of insomnia
Monitoring After Conversion
- Monitor for changes in efficacy or side effects during the first 1-2 weeks after conversion
- Common side effects include headache, dry mouth, nausea, insomnia, constipation, and dizziness 4
- Seizure risk is dose-dependent; maintain the same total daily dose to avoid increasing this risk 1
Special Populations
- For patients with moderate to severe hepatic impairment, the maximum dose should be 150 mg every other day for XL formulation; when converting to SR, consider 75 mg once daily 1
- For patients with renal impairment, consider reducing the dose and/or frequency 1
Potential Pitfalls to Avoid
- Dosing errors: Ensure patients understand the change from once-daily to twice-daily administration
- Abrupt discontinuation: If discontinuing treatment with Wellbutrin SR 300 mg daily, decrease to 150 mg daily prior to discontinuation 1
- Confusion with different formulations: Verify that the prescription specifies "SR" (sustained release) rather than "XL" (extended release) or immediate release
Remember that bupropion formulations are not interchangeable on a mg-per-mg basis with other bupropion products due to different bioavailability profiles, but the total daily dose should remain the same when converting between formulations.