Switching from Immediate-Release to Extended-Release Guanfacine
When switching from 1 mg immediate-release guanfacine daily to extended-release with a modest dose increase, use 2 mg of extended-release guanfacine.
Rationale for Dosing Conversion
The formulations are not interchangeable on a milligram-per-milligram basis due to different pharmacokinetic profiles 1. Research directly comparing these formulations demonstrates that:
- 3 mg/day immediate-release is approximately equivalent to 4 mg/day extended-release in terms of plasma medication levels after 24 hours 2
- This suggests roughly a 1.33:1 conversion ratio (ER:IR)
Recommended Conversion with Dose Increase
For your specific scenario of converting 1 mg IR with a slight increase:
- 1 mg IR × 1.33 = approximately 1.3 mg ER equivalent
- Adding a modest increase → 2 mg ER is the appropriate dose
This provides both the conversion equivalency and a clinically meaningful dose increase. The 2 mg ER dose is well-studied and represents a standard therapeutic dose 3.
Important Clinical Considerations
Cardiovascular Monitoring
Both formulations cause:
Monitor vital signs closely during the transition, particularly in the first 1-2 weeks.
Tapering Requirements
Critical pitfall to avoid: Never abruptly discontinue guanfacine. Rebound hypertension can occur 2-4 days after discontinuation 4, 5. Always taper when stopping therapy.
Timing of Administration
Extended-release guanfacine for ADHD is typically dosed once daily (morning or evening based on tolerability), whereas immediate-release for hypertension is traditionally given at bedtime 5. Adjust timing based on the indication and side effect profile.
Common Adverse Effects
Expect somnolence (most common), dry mouth, fatigue, dizziness, and headache 4, 6. These are generally mild and decrease over time with continued dosing.