Guanfacine Peak Plasma Concentration
Guanfacine immediate-release reaches peak plasma concentration at 1-4 hours (average 2.6 hours) after oral administration 1.
Immediate-Release Formulation
The FDA label for guanfacine immediate-release provides definitive pharmacokinetic data:
- Time to peak (Tmax): 1 to 4 hours after oral administration
- Average Tmax: 2.6 hours
- Bioavailability: Approximately 80% (relative to IV administration)
- Absorption: Rapid and well-absorbed orally 1
This was confirmed in a study of patients with renal insufficiency, where peak plasma levels were reached within 2 hours after a single 4 mg oral dose 2.
Extended-Release Formulation
For guanfacine extended-release (used in ADHD treatment):
- Time to peak: Approximately 5.0 hours (median) after oral administration
- This was demonstrated in a thorough QT study where maximum plasma concentration following a supra-therapeutic dose occurred at 5.0 hours post-dose 3
Clinical Implications
The pharmacokinetic profile differs significantly between formulations:
- Immediate-release: Faster onset with peak at ~2.6 hours, suitable for hypertension management with dosing at bedtime 1
- Extended-release: Delayed peak at ~5 hours, designed for once-daily dosing in ADHD treatment 4
The elimination half-life is approximately 17 hours (range 10-30 hours) for both formulations, with steady-state achieved within 4 days 1. Younger patients tend toward shorter half-lives (13-14 hours), while older patients approach the upper range.