What is the equivalent dose of Lamictal (lamotrigine) XR when converting from Lamictal 300mg?

Converting from Lamictal 300mg to Lamictal XR

When converting from immediate-release Lamictal (lamotrigine) 300mg to Lamictal XR, the equivalent dose is 300mg of Lamictal XR. 1

Conversion Rationale

• Lamictal XR is designed to provide the same total daily dose as immediate-release Lamictal but with a once-daily dosing schedule and more stable serum concentrations 2, 3
• The FDA-approved labeling for Lamictal XR includes a 300mg tablet strength that provides direct 1:1 conversion from immediate-release formulation 1
• Extended-release formulations help reduce peak-related side effects while maintaining therapeutic efficacy 2

Benefits of Converting to Extended-Release Formulation

• Reduced fluctuations in serum drug levels, potentially leading to:
  • Lower peak concentrations that may reduce adverse effects such as dizziness, blurred vision, and unsteadiness 2
  • Higher trough levels that may improve seizure control 2
• Improved medication adherence due to once-daily dosing 3
• Potential for better seizure control (46% median reduction in seizure frequency observed in one study when converting from immediate-release to extended-release) 2

Important Considerations

• The conversion should be done as a single step, switching directly from 300mg immediate-release to 300mg extended-release 2, 3
• Lamictal XR should be taken once daily, regardless of concomitant antiepileptic medications 3
• Monitor for any changes in efficacy or side effects after conversion 2
• Be aware that missing a dose of extended-release formulation may have more significant consequences due to the once-daily dosing schedule 4

Potential Pitfalls and Caveats

• Some patients with rapid metabolism may benefit from twice-daily dosing of Lamictal XR despite its approval for once-daily use 4
• Concomitant medications that induce or inhibit hepatic glucuronidation can affect lamotrigine metabolism and may require dose adjustments 4
• The risk of serious rash with Lamictal XR (approximately 0.1%) appears similar to immediate-release formulation 3
• If switching formulations due to side effects, allow sufficient time (2-4 weeks) to evaluate the impact of the extended-release formulation 2

Follow-up Recommendations

• Assess seizure control and side effects 4-6 weeks after conversion 2
• Consider checking serum lamotrigine levels before and after conversion if there are concerns about therapeutic efficacy 2
• Be prepared to make minor dose adjustments if needed based on clinical response 2