GUANETHIDINE MONOSULFATE

Clinical safety rating: caution

Comprehensive clinical and safety monograph for GUANETHIDINE MONOSULFATE (GUANETHIDINE MONOSULFATE).

How it works

Guanethidine is an adrenergic neuron blocking agent that inhibits the release of norepinephrine from postganglionic sympathetic nerve terminals by displacing it from storage vesicles. It also depletes norepinephrine stores, leading to reduced sympathetic tone and vasodilation.

What the body does with it

Metabolism	Primarily hepatic via monoamine oxidase (MAO) with subsequent conjugation; some drug is excreted unchanged in urine. Active metabolite (2,3-dihydroxybenzylguanidine) may contribute to effects.
Excretion	Renal: ~50% unchanged; some biliary/fecal.
Half-life	5-10 days (prolonged due to extensive tissue binding); requires dose adjustment in renal impairment.
Protein binding	~10%; mainly albumin.
Volume of Distribution	~200 L/kg (extensive tissue binding, concentrating in adrenergic neurons).
Bioavailability	Oral: 3-30% (highly variable, extensive first-pass metabolism).
Onset of Action	Oral: 24-48 hours; maximal effect in 1-2 weeks.
Duration of Action	7-14 days after discontinuation due to slow release from adrenergic neurons.

Classification & Brands

Dosing & administration

Initial: 10 mg orally once daily. Increase in increments of 10 mg at weekly intervals until adequate response. Usual maintenance: 25-50 mg once daily. Maximum: 100 mg daily.

Dosage form	TABLET
Renal impairment	GFR 30-50 mL/min: Reduce dose by 25-50%. GFR 10-29 mL/min: Reduce dose by 50-75%. GFR <10 mL/min: Avoid use or administer at 25% of normal dose.
Liver impairment	Child-Pugh A: No adjustment. Child-Pugh B: Reduce dose by 50%. Child-Pugh C: Avoid use (risk of hypotension and accumulation).
Pediatric use	Initial: 0.2 mg/kg orally once daily. Titrate weekly by 0.2 mg/kg. Maximum: 3 mg/kg/day or 100 mg/day, whichever is less.
Geriatric use	Initiate at 5 mg orally once daily due to increased sensitivity. Titrate slowly (2-week intervals) to avoid orthostatic hypotension and falls.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for GUANETHIDINE MONOSULFATE (GUANETHIDINE MONOSULFATE).

Breastfeeding	Guanethidine is excreted into breast milk in small amounts. The M/P ratio is approximately 0.1. At maternal therapeutic doses, the estimated daily infant dose is less than 10% of the maternal weight-adjusted dose, which is considered compatible with breastfeeding. However, monitor infant for hypotension, bradycardia, and diarrhea.
Teratogenic Risk	Guanethidine is an adrenergic neuron blocking agent. Data on human pregnancy are limited. Animal studies have not shown teratogenicity. However, due to its pharmacological action, use in the first trimester should be avoided unless clearly needed. In second and third trimesters, risk is considered low; however, fetal bradycardia and hypotension may occur with maternal use near term.

Warnings & precautions

■ FDA Black Box Warning

WARNING: Orthostatic hypotension, syncope, and bradycardia can occur, especially with rapid dose escalation. Patients should be warned about postural hypotension and encouraged to report symptoms. Avoid use in patients with pheochromocytoma or congestive heart failure.

Side Effect Profile

Serious Effects

Absolute Contraindications

Pheochromocytoma, congestive heart failure (especially due to increased cardiac output), concurrent use with MAO inhibitors, severe renal impairment, and known hypersensitivity.

Clinical Precautions

Precautions

May cause severe orthostatic hypotension, syncope, bradycardia, diarrhea, sexual dysfunction, and sodium/water retention. Use with caution in renal impairment, coronary artery disease, and cerebrovascular insufficiency. Abrupt discontinuation may cause rebound hypertension. Potentiation by MAO inhibitors or anesthetics.

Clinical Tips & Counseling

Drug interactions

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GUANETHIDINE MONOSULFATE

Clinical safety rating: caution

Comprehensive clinical and safety monograph for GUANETHIDINE MONOSULFATE (GUANETHIDINE MONOSULFATE).

How it works

What the body does with it

Metabolism	Primarily hepatic via monoamine oxidase (MAO) with subsequent conjugation; some drug is excreted unchanged in urine. Active metabolite (2,3-dihydroxybenzylguanidine) may contribute to effects.
Excretion	Renal: ~50% unchanged; some biliary/fecal.
Half-life	5-10 days (prolonged due to extensive tissue binding); requires dose adjustment in renal impairment.
Protein binding	~10%; mainly albumin.
Volume of Distribution	~200 L/kg (extensive tissue binding, concentrating in adrenergic neurons).
Bioavailability	Oral: 3-30% (highly variable, extensive first-pass metabolism).
Onset of Action	Oral: 24-48 hours; maximal effect in 1-2 weeks.
Duration of Action	7-14 days after discontinuation due to slow release from adrenergic neurons.

Classification & Brands

Dosing & administration

Initial: 10 mg orally once daily. Increase in increments of 10 mg at weekly intervals until adequate response. Usual maintenance: 25-50 mg once daily. Maximum: 100 mg daily.

Dosage form	TABLET
Renal impairment	GFR 30-50 mL/min: Reduce dose by 25-50%. GFR 10-29 mL/min: Reduce dose by 50-75%. GFR <10 mL/min: Avoid use or administer at 25% of normal dose.
Liver impairment	Child-Pugh A: No adjustment. Child-Pugh B: Reduce dose by 50%. Child-Pugh C: Avoid use (risk of hypotension and accumulation).
Pediatric use	Initial: 0.2 mg/kg orally once daily. Titrate weekly by 0.2 mg/kg. Maximum: 3 mg/kg/day or 100 mg/day, whichever is less.
Geriatric use	Initiate at 5 mg orally once daily due to increased sensitivity. Titrate slowly (2-week intervals) to avoid orthostatic hypotension and falls.

Use during pregnancy

1st trimester	Consult provider
2nd trimester	Consult provider
3rd trimester	Consult provider

Clinical note

Comprehensive clinical and safety monograph for GUANETHIDINE MONOSULFATE (GUANETHIDINE MONOSULFATE).

Breastfeeding	Guanethidine is excreted into breast milk in small amounts. The M/P ratio is approximately 0.1. At maternal therapeutic doses, the estimated daily infant dose is less than 10% of the maternal weight-adjusted dose, which is considered compatible with breastfeeding. However, monitor infant for hypotension, bradycardia, and diarrhea.
Teratogenic Risk	Guanethidine is an adrenergic neuron blocking agent. Data on human pregnancy are limited. Animal studies have not shown teratogenicity. However, due to its pharmacological action, use in the first trimester should be avoided unless clearly needed. In second and third trimesters, risk is considered low; however, fetal bradycardia and hypotension may occur with maternal use near term.

Warnings & precautions

■ FDA Black Box Warning

Side Effect Profile

Serious Effects

Absolute Contraindications

Pheochromocytoma, congestive heart failure (especially due to increased cardiac output), concurrent use with MAO inhibitors, severe renal impairment, and known hypersensitivity.