Guanethidine Monosulfate

Trade Names:
Ismelin
- Tablets 10 mg
- Tablets 25 mg

Pharmacology

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Interferes with release or distribution of norepinephrine from nerve endings, resulting in reduction in total peripheral resistance and diastolic and systolic BP.

Pharmacokinetics

Metabolism

Converted by the liver to 3 metabolites that are less active than the parent drug.

Elimination

Renal Cl is 56 mL/min. T _{1/ 2} is 1.5 to 8 d. Renal excretion.

Indications and Usage

Treatment of moderate and severe hypertension and renal hypertension, including that secondary to pyelonephritis, renal amyloidosis, and renal artery stenosis.

Unlabeled Uses

Reflex sympathetic dystrophy and causalgia.

Contraindications

Known or suspected pheochromocytoma; frank CHF not related to hypertension; use of MAOIs.

Dosage and Administration

PO 10 mg every day initially; may increase by about 10 mg at 5 to 7 days; increase only if no decrease in standing BP is observed. Maintenance dose: 25 to 50 mg every day.

PO 25 to 50 mg initially; increase by 25 or 50 mg/day or every other day until desired response is obtained. Loading dose (for severe hypertension): Give at 6 h intervals over 1 to 3 days, omitting nighttime dose.

PO 0.2 mg/kg/24 h (6 mg/m ² /24 h) as single oral dose initially; increase by increment of 0.2 mg/kg/24 h every 7 to 10 days. Max: 3 mg/kg/24 h.

Storage/Stability

Store in tightly closed container at room temperature.

Drug Interactions

May reverse hypotensive effect of drug.

May decrease effectiveness of guanethidine; discontinue MAOIs more than 1 wk before starting guanethidine therapy.

May inhibit hypotensive effect.

May reverse hypotensive effect of guanethidine; guanethidine may potentiate effects of sympathomimetics.

May inhibit hypotensive effect of drug.

Laboratory Test Interactions

None well documented.

Adverse Reactions

Cardiovascular

Bradycardia; orthostatic fluid retention; edema; angina.

CNS

Dizziness; weakness; lassitude; syncope; fatigue; muscle tremor; mental depression; chest paresthesias; ptosis; headache; confusion.

EENT

Blurred vision; nasal congestion.

GI

Nausea; vomiting; dry mouth; parotid tenderness; diarrhea (may be severe, requiring discontinuation of therapy); increase in bowel movements.

Genitourinary

Inhibition of ejaculation; nocturia; urinary incontinence; priapism.

Hematologic

Anemia; thrombocytopenia.

Respiratory

Dyspnea; asthma in susceptible individuals.

Miscellaneous

Myalgia; weight gain; dermatitis; scalp hair loss; leg cramps.

Precautions

Pregnancy

Category C .

Lactation

Excreted in breast milk.

Children

Safety and efficacy not established.

Elderly

More prone to side reactions of guanethidine therapy, especially orthostatic hypotension.

Renal Function

Use very cautiously, because hypotension may worsen renal function impairment.

Bronchial asthma

May aggravate the hypersensitive condition of asthmatics because of further catecholamine depletion.

Cardiovascular disease

Use cautiously in patients with coronary disease, recent MI, or cerebral vascular disease, especially with encephalopathy; avoid use in patients with severe cardiac failure.

Fever

May decrease dosage requirements.

Orthostatic hypotension

Occurs frequently, especially during initial treatment and with postural changes.

Peptic ulcer

Ulcers may be aggravated by relative increase in parasympathetic tone.

Preoperative withdrawal

Withdrawal is recommended 2 wk prior to surgery to reduce risk of vascular collapse and cardiac arrest during anesthesia; during emergency surgery administer preanesthetic and anesthetic agents cautiously in reduced dosages and prepare for possible vascular collapse.

Overdosage

Symptoms

Severe drowsiness, hypotension, bradycardia, severe diarrhea, nausea, vomiting, syncope.

Patient Information

• Instruct patient in proper technique for taking BP. Advise patient to check BP weekly.
• Caution patient not to get out of bed without help during period of dosage adjustment.
• Advise patient to lie down if dizziness or blurred vision occurs.
• Warn patient not to double up on doses.
• Instruct patient not to discontinue drug abruptly and not to stop taking drug because of improvement in symptoms.
• Counsel patient about benefits of weight reduction, exercise, reduction of alcohol and sodium intake, and cessation of smoking.
• Explain that impotence and ejaculation disturbances may occur but is reversible. Tell patient to report to health care provider.
• Instruct patient to report these symptoms to health care provider: dizziness, diarrhea, confusion, depression, fever, sore throat.
• Caution patient to avoid sudden position changes to avoid orthostatic hypotension.
• Instruct patient to avoid intake of alcoholic beverages or other CNS depressants.
• Advise patient that drug may cause drowsiness and to use caution while driving or performing other tasks requiring mental alertness.

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