GUANETHIDINE SULFATE
(gwahn-eth'i-deen)
Ismelin, Apo-Guanethidine 
Classifications: cardiovascular agent; centrally acting antihypertensive
Prototype: Methyldopa
Pregnancy Category: C
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10 mg, 25 mg tablets
Potent, long-acting, adrenergic blocking agent. Competes with norepinephrine for reuptake into adrenergic neurons; displaces
stored norepinephrine, thus exposing it to degradation by MAO. Produces a gradual prolonged fall in BP, usually associated
with bradycardia and decreased pulse pressure. Drug-induced sodium retention and expansion of plasma volume, with resulting
tolerance to antihypertensive effect, may occur unless concomitant diuretic therapy is administered.
It is more effective in lowering orthostatic than supine BP. Antihypertensive effect results from venous dilatation with peripheral
pooling, decreased venous return, and decreased cardiac output.
Stepped care approach to treatment of moderate to severe hypertension either alone or in conjunction with a thiazide diuretic
or hydralazine.
Chronic open-angle glaucoma, endocrine ophthalmopathy. Orphan Drug: Reflex sympathetic dystrophy syndrome; causalgia.
Pheochromocytoma, frank CHF (not due to hypertension). Safe use during pregnancy (category C) is not established.
Diabetes mellitus, impaired renal or hepatic function, sinus bradycardia, limited cardiac reserve, coronary disease with insufficiency,
recent MI, cerebrovascular insufficiency, febrile illnesses, older adults; lactation; history of peptic ulcer, colitis, or
bronchial asthma.
Hypertension
Adult: PO 10 mg once/d, may be increased by 10 mg q5–7d up to 300 mg/d (start with 25–50 mg/d in hospitalized patients, increase
by 25–50 mg q1–3d)
Geriatric: PO Start with 5 mg once daily
Child: PO 0.2 mg/kg/d, may increase by 0.2 mg/kg q1–3wk if needed (max: 1–1.6 mg/kg/d)
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Oral
- Crush tablet before administration if needed to enable swallowing and give with fluid of patient's choice.
- Increase dosage slowly (at intervals of no less than 5–7 d for adults and 1–3 wk in children) and only if there
has been no reduction in standing BP from previous levels. BP should be monitored during dosage adjustment period.
CV:
Marked orthostatic and exertional hypotension with dizziness, light-headedness; bradycardia, symptomatic sick sinus syndrome (weakness, dizziness, blurred vision); angina,
edema with weight gain, CHF, complete heart block. Special Senses: Blurred vision, ptosis of eyelids, parotid tenderness, nasal congestion. GI:
Severe diarrhea, nausea, vomiting, constipation, dry mouth. Urogenital: Nocturia, urinary retention, incontinence, inhibition of ejaculation, impotence. Skin: Skin eruptions, loss of scalp hair. Other: Dyspnea, psychic depression, weakness, fatigue, myalgia, tremor, chest paresthesias, asthma, rise in BUN, polyarteritis nodosa.
Drug:
Alcohol,
levodopa,
diuretics and other hypotensive agents increase hypotensive effects; mao inhibitors may antagonize hypotensive effects; norepinephrine,
pseudoephedrine,
other decongestants, tricyclic antidepressants, phenothiazines block hypotensive effects. Herbal:
Ma-huang,
ephedra may cause enhanced sympathomimetic effects.
Absorption: Completely absorbed, but undergoes significant first pass metabolism by liver; 3–50% of dose reaches systemic circulation. Peak: 1–3 wk. Distribution: Rapidly distributed to adrenergic neuron storage sites; does not cross blood–brain barrier. Metabolism: Metabolized in liver to inactive metabolites. Elimination: Excreted in urine. Half-Life: 5 d.
Assessment & Drug Effects
- Take BP first in supine position and then again after patient has been standing for 10 min. Ideal dosage reduces standing
BP to within normal range without faintness, dizziness, weakness, or fatigue.
- Monitor I&O, especially in older adults and patients with limited cardiac reserve or impaired renal function. Report changes
in I&O ratio.
- Observe for evidence of edema and weight gain. Sudden weight gain of 1 kg (2 lb) in 24 h or more should be reported to physician.
Patients with limited cardiac reserve are particularly susceptible to guanethidine-induced sodium and water retention, with
resulting edema, CHF, and drug resistance.
- Observe patients on antidiabetic therapy closely for signs of hypoglycemia.
Patient & Family Education
- Do not stop drug without consulting physician.
- Ask for assistance with walking; older adults are prone to develop orthostatic hypotension.
- Understand that orthostatic hypotension is most prominent shortly after arising from sleep and when too rapid changes are
made to sitting or upright positions. Move gradually to sitting position and make all position changes slowly and in stages.
Flex arms and legs slowly before standing to augment venous return. Orthostatic hypotension is intensified by prolonged standing,
hot baths or showers, hot weather, alcohol ingestion, and strenuous physical exercise (particularly if followed by immobility).
- Lie down or sit down (in head-low position) immediately at the onset of dizziness, weakness, or faintness.
- Consult physician regarding allowable salt intake.
- Reduced dosage in presence of febrile illnesses. Report fever to physician.
- Consult physician or pharmacist before taking any OTC drug; guanethidine may sensitize the patient to some sympathomimetic
agents found in OTC cold remedies and cause hypertensive crisis.
- Do not breast feed while taking this drug.
1%)
Prototype drug