DrugindexOnline^{2nd edition}

Pronouncation: (GWAHN-fay-seen HIGH-droe-KLOR-ide)
Class: Antiadrenergic, centrally acting

Trade Names:
Tenex
- Tablets 1 mg
- Tablets 2 mg

Mechanism of Action

Pharmacology

Appears to stimulate central alpha ₂ -adrenergic receptors, with decreased sympathetic outflow causing decrease in peripheral vascular resistance and reduction in heart rate.

Pharmacokinetics

Absorption

Absolute bioavailability is about 80%. T _max is 1 to 4 h.

Distribution

Protein binding is about 70%. Mean Vd is 6.3 L/kg.

Elimination

T _{1/ 2} is 10 to 30 h. About 50% is excreted in the urine as unchanged, the remainder as metabolites.

Special Populations

Renal Function Impairment

Cl is reduced; plasma levels are only slightly increased.

Indications and Usage

Treatment of hypertension.

Unlabeled Uses

Amelioration of heroin withdrawal symptoms.

Contraindications

Standard considerations.

Dosage and Administration

Adults

PO 1 mg daily at bedtime; may increase gradually up to 3 mg daily.

General Advice

Administer medication at bedtime to decrease daytime sedation.

Storage/Stability

Store in tightly closed container and protect from light.

Drug Interactions

Alcohol, CNS depressants

Increased CNS depression.

Barbiturates, phenytoin

Decreased guanfacine levels with loss of antihypertensive effect.

Laboratory Test Interactions

None well documented.

Adverse Reactions

Cardiovascular

Chest pain; bradycardia; palpitations.

CNS

Somnolence; drowsiness; dizziness; headache; sleep disturbances; insomnia; confusion; depression.

Dermatologic

Dermatitis; pruritus; sweating.

EENT

Conjunctivitis; visual disturbance; tinnitus; rhinitis; taste perversion.

GI

Dry mouth; constipation; diarrhea; nausea; abdominal discomfort; dyspnea.

Genitourinary

Urinary incontinence; testicular disorder; decreased libido; impotence.

Miscellaneous

Paresthesia; paresis; leg cramps; hypokinesia.

Precautions

Pregnancy

Category B .

Lactation

Excreted in breast milk.

Children

Safety and efficacy in children younger than 12 yr of age not established.

Special Risk Patients

Use with caution in patients with severe coronary insufficiency, recent MI, cerebrovascular disease, or chronic renal or hepatic function impairment.

Sedation

Occurs in a large percentage of patients.

Withdrawal

Do not discontinue therapy without consulting health care provider; drug must be withdrawn gradually to avoid rapid rise in BP (rebound hypertension).

Overdosage

Symptoms

Severe drowsiness, hypotension, bradycardia.

Patient Information

• Instruct patient to take medication at bedtime.
• Teach patient proper technique for taking BP. Advise patient to check BP weekly.
• Instruct patient not to discontinue drug abruptly.
• Advise patient on benefits of weight loss, exercise, reduction of alcohol and sodium intake and cessation of smoking.
• Instruct patient to lie down if dizziness or blurred vision occurs.
• Explain that impotence may occur but is reversible. Tell patient to report to health care provider.
• Instruct patient to report these symptoms to health care provider: dizziness, constipation, headache, insomnia, nausea, sweating, or weakness.
• Advise patient to take sips of water frequently, suck on ice chips or sugarless hard candy, or chew sugarless gum if dry mouth occurs.
• 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.