Pharmacy Editor: Jay W. Marks, MD

GENERIC NAME: labetalol

BRAND NAMES: Normodyne, Trandate

DRUG CLASS AND MECHANISM: Labetalol is a drug that is used for treating high blood pressure. It is related to carvedilol (Coreg). Nerves that are part of the adrenergic nervous system travel to most arteries where they release an adrenergic chemical norepinephrine. The norepinephrine attaches to receptors on the muscles of the arteries and causes the muscles to contract, narrowing the arteries, and increasing the blood pressure. Labetalol blocks receptors of the adrenergic nervous system. When labetalol attaches to and blocks the receptors, the arterial muscles relax, and the arteries expand, resulting in a fall in blood pressure. The FDA first approved labetalol in 1997.

GENERIC AVAILABLE: Yes

PRESCRIPTION: Yes

PREPARATION: Tablets: of 100, 200, and 300 mg; Injection: 5 mg/ml

STORAGE: Tablets should be stored between 2-30°C (36-86°F).

PRESCRIBED FOR: Labetalol is used alone or in combination with other drugs to reduce blood pressure.

DOSING: Labetalol may be given two or three times daily. The starting oral dose is 100 mg twice daily and the dose can be increased by 100 mg twice daily every 2-3 days based on the response of the blood pressure. Usual maintenance doses are 200-400 mg twice daily. Patients with severe high blood pressure may require 1.2 to 2.4 g daily. Side effects may be less if labetalol is given in three daily doses rather than two. Stopping or changing the dose of the drug should be directed by a physician because serious side effects may occur.

The initial intravenous dose of labetalol is 20 mg injected over 2 minutes. Additional injections of 40 or 80 mg may be administered every 10 minutes as needed up to a total dose of 300 mg. Maximum effect is usually seen within 5 minutes after intravenous administration. Labetalol also may be administered by intravenous infusion.

DRUG INTERACTIONS: Labetalol can mask early warning symptoms of hypoglycemia (low blood sugar) such as tremors and increased heart rate. These symptoms are caused by activation of the adrenergic nervous system which labetalol blocks. Therefore, diabetics taking medications that lower blood sugar such as insulin or oral anti-diabetic medications may need to increase the frequency with which they monitor their blood sugar in order to prevent hypoglycemia.

Labetalol taken with calcium channel blockers such as diltiazem (Cardizem) or verapamil (Calan) may trigger an irregular heart rhythm or an increase in blood pressure.

Labetalol taken with tricyclic antidepressants such as amitriptyline (Elavil), imipramine (Tofranil), or nortriptyline (Pamelor) may lead to an increase in the tremor that is a side effect of the antidepressants.

If taken with adrenergic stimulating drugs used for treating asthma, for example, albuterol (Proventil, Ventolin) or pirbuterol (Maxair), the adrenergic blocking effects of labetalol may counteract the effects of the stimulating drugs and reduce their effectiveness for treating asthmatic attacks. More of the adrenergic drug may be needed.

Glutethimide (Doriden) may decrease the effectiveness of labetalol by increasing its elimination. When both drugs are used concomitantly, more labetalol or less glutethimide may be needed. Cimetidine (Tagamet) may increase the effectiveness of labetalol by blocking its elimination and increasing its levels in the blood. Therefore, less labetalol may be needed when cimetidine and labetalol are used together.

PREGNANCY: It has not been determined if it is safe to use labetalol during pregnancy.

NURSING MOTHERS: Labetalol has been found in human breast milk. Therefore, it should be used cautiously in nursing mothers because of the risk that the infant may develop a slow heart rate.

SIDE EFFECTS AND PRECAUTIONS: The most common side effects of labetalol are fatigue, dizziness, headache, diarrhea, edema (fluid accumulation), dry eyes, and tingling of the scalp and skin. Postural hypotension (a rapid decrease in blood pressure when going from the lying or seated position to the standing position that may cause light-headedness or fainting) occurs rarely. Patients should be observed for this possible side effect within two to four hours of the first labetalol dose and after any changes in dose. Sexual dysfunction, abnormal heart rhythm and difficulty breathing also may occur.

From the Doctors at MedicineNet.com

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