GUANETHIDINE MONOSULFATE
Clinical safety rating
cautionComprehensive clinical and safety monograph for GUANETHIDINE MONOSULFATE (GUANETHIDINE MONOSULFATE).
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.
| 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. |
| Molecular Weight | 148.19 |
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. |
| 1st trimester | Avoid; risk of maternal hypotension and reduced uteroplacental perfusion. |
| 2nd trimester | Avoid; potential for fetal harm due to noradrenaline depletion. |
| 3rd trimester | Avoid; may cause neonatal hypotension or bradycardia at delivery. |
Clinical note
Comprehensive clinical and safety monograph for GUANETHIDINE MONOSULFATE (GUANETHIDINE MONOSULFATE).
| Placental transfer | Crosses placenta; documented in animal studies and limited human data. |
| Breastfeeding | Excreted in breast milk; not recommended due to potential for hypotension and gastrointestinal disturbances in the nursing infant. |
| Lactation Rating | L4 |
| 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. |
| Fetal Monitoring | Monitor maternal blood pressure, heart rate, and signs of orthostatic hypotension. Fetal heart rate monitoring is recommended during labor and delivery due to potential for fetal bradycardia. Assess fetal growth via ultrasound if prolonged use. |
| Fertility Effects | Guanethidine can cause inhibition of ejaculation in males, leading to oligospermia or aspermia. This effect is reversible upon discontinuation. No direct effects on female fertility have been reported. |
■ 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.
| Serious Effects |
PheochromocytomaSevere renal impairment (CrCl <10 mL/min)Concomitant use with MAO inhibitors
| 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. |
| Food/Dietary | Avoid tyramine-rich foods (e.g., aged cheese, cured meats, fermented products) as guanethidine may potentiate pressor effects. Limit alcohol intake due to additive hypotensive effects. No other significant food interactions known. |
| Clinical Pearls | Guanethidine monosulfate is an adrenergic neuron blocking agent used primarily for severe hypertension unresponsive to other agents. Its use is limited due to orthostatic hypotension, syncope, and drug interactions. Avoid concurrent use with MAOIs, tricyclic antidepressants, and sympathomimetics as they can reverse or block its effect. Monitor for orthostatic BP changes; advise patient to rise slowly. Discontinue gradually to avoid rebound hypertension. |
| Patient Advice | Take this medication exactly as prescribed, usually once daily. · Do not stop taking this medication suddenly; withdrawal can cause a dangerous rise in blood pressure. · Avoid alcohol, as it can worsen orthostatic hypotension. · Stand up slowly from sitting or lying down to prevent dizziness and fainting. · Inform your doctor if you experience fainting, severe dizziness, or palpitations. |
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