Adult: As immediate release tab: Initially, 1 mg daily at night. May increase to 2 mg daily after 3-4 wk, if necessary. Child: ≥12 yr Same as adult dose.
Child: 6-17 yr As extended-release tab: Initially, 1 mg once daily, increased if necessary by increments of not more than 1 mg per wk. Maintenance: 0.05-0.12 mg/kg daily, according to response and tolerability. Max: 6-12 yr 4 mg daily; 13-17 yr 7 mg daily.
Administration
May be taken with or without food. ER tab: Swallow whole & do not crush/chew/break. Do not take w/ high-fat meal.
Special Precautions
Patient w/ history of hypotension, heart block, bradycardia, CV disease, history of syncope or conditions which may predispose them to syncope (e.g. dehydration), history of QT prolongation, risk factors for torsade de pointes. Avoid abrupt withdrawal. Renal or hepatic impairment. Childn. Pregnancy and lactation.
This drug may cause dizziness, somnolence, and syncope, if affected, do not drive or operate machinery.
Monitoring Parameters
Monitor heart rate, BP and consider ECG prior to initiation.
Overdosage
Symptoms: Drowsiness, lethargy, bradycardia, resp depression, coma and hypotension. Management: Supportive treatment. Perform gastric lavage if ingestion is recent. Admin of activated charcoal may be considered.
Drug Interactions
Additive effect w/ diuretics, other antihypertensives, CNS depressants. Phenobarbital and phenytoin may decrease elimination half-life and plasma concentration of guanfacine for renally impaired patients. Additive sedative effect w/ centrally active depressants (e.g. phenothiazines, barbiturates, benzodiazepines). Increased plasma concentrations w/ CYP3A4/5 inhibitors (e.g. ketoconazole). Decreased rate and extent of exposure w/ CYP3A4 inducers (e.g. rifampicin).
Food Interaction
Avoid high-fat meal due to increased exposure (extended-release). Increased plasma concentrations w/ grapefruit juice. Additive sedative effect w/ alcohol.
Action
Description: Mechanism of Action: Guanfacine is a selective α2A-adrenoceptor agonist that reduces sympathetic nerve impulse from the vasomotor centre to the heart and blood vessels, resulting in a decrease in peripheral vascular resistance, BP and heart rate. Duration: Antihypertensive effect: 24 hr. Pharmacokinetics: Absorption: Rapidly absorbed. Bioavailability: Approx 80%. Time to peak plasma concentration: 1-4 hr. Distribution: Extensively distributed in tissues. Plasma protein binding: Approx 70%. Metabolism: Metabolised rapidly in the liver via CYP3A4/5-mediated oxidation, w/ subsequent sulfation and glucuronidation. Excretion: Via urine, as unchanged drug (approx 50%) and metabolites. Elimination half-life: 10-30 hr.
Chemical Structure
Guanfacine Source: National Center for Biotechnology Information. PubChem Database. Guanfacine, CID=3519, https://pubchem.ncbi.nlm.nih.gov/compound/Guanfacine (accessed on Jan. 23, 2020)
C02AC02 - guanfacine ; Belongs to the class of imidazoline receptor agonists, centrally-acting antiadrenergic agents. Used in the treatment of hypertension.
References
Anon. Guanfacine. Lexicomp Online. Hudson, Ohio. Wolters Kluwer Clinical Drug Information, Inc. https://online.lexi.com. Accessed 27/05/2016.Buckingham R (ed). Guanfacine hydrochloride. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 27/05/2016.Guanfacine Hydrochloride Tablet (Mylan Pharmaceuticals Inc. DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/. Accessed 27/05/2016.Guanfacine Tablet, Extended Release (AvKARE, Inc). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/. Accessed 27/05/2016.McEvoy GK, Snow EK, Miller J et al (eds). Guanfacine. AHFS Drug Information (AHFS DI) [online]. American Society of Health-System Pharmacists (ASHP). https://www.medicinescomplete.com. Accessed 27/05/2016.
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