Half-Life of Guanfacine Extended Release (ER)
The half-life of guanfacine extended release (ER) is approximately 16-17 hours in adults and 14-18 hours in children and adolescents. 1, 2
Pharmacokinetic Profile of Guanfacine ER
Half-Life by Age Group
- Adults: The mean elimination half-life is approximately 17 hours (range 10-30 hours) 3
- Children (6-12 years): 14.4 ± 2.39 hours 1
- Adolescents (13-17 years): 17.9 ± 5.77 hours 1
Key Pharmacokinetic Properties
- Linear pharmacokinetics across dosage ranges (1-4 mg) 2
- Peak plasma concentrations occur between 1-4 hours after administration, with an average of 2.6 hours 3
- Steady-state blood levels are typically achieved within 4 days of consistent dosing 3
- Bioavailability is approximately 80% relative to intravenous administration 3
Clinical Implications of Half-Life
Dosing Considerations
- The extended half-life allows for once-daily dosing, typically administered at bedtime to minimize daytime somnolence 3
- Younger patients tend to have shorter elimination half-lives (13-14 hours) while older patients tend to have half-lives at the upper end of the range (up to 30 hours) 3
- For patients with renal impairment, the lower end of the dosing range should be used, as clearance is reduced in these patients 3
Discontinuation Considerations
- Due to the relatively long half-life, guanfacine should not be stopped abruptly if it has been taken for more than 9 weeks, as this may cause rebound hypertension 4
- Unlike clonidine (another alpha-2 agonist), guanfacine's longer half-life results in a more gradual return to pretreatment blood pressure levels upon discontinuation 3
Comparison to Other Alpha-2 Agonists
Guanfacine has several pharmacokinetic advantages over clonidine:
- Longer half-life (16-17 hours vs. 8-12 hours for clonidine)
- More selective for alpha-2A receptors (vs. clonidine's action on all alpha-2 subtypes) 4
- Less sedation and more stable blood pressure effects 4
- Reduced risk of rebound hypertension upon discontinuation 4
Common Side Effects Related to Pharmacokinetics
- Somnolence (most common, reported in 63.5% of subjects in clinical trials) 2
- Dizziness and fatigue 5
- Hypotension and bradycardia (dose-dependent) 6
The long half-life of guanfacine ER contributes to its clinical utility as a once-daily medication for conditions like ADHD and hypertension, while also necessitating careful consideration when discontinuing treatment to avoid rebound effects.