Drug information of Phenoxybenzamine

Mechanism of effect

Phenoxybenzamine is an alpha-blocker. It works by relaxing blood vessels, causing blood pressure to decrease. Produces long-lasting noncompetitive alpha-adrenergic blockade of postganglionic synapses in exocrine glands and smooth muscle; relaxes urethra and increases opening of the bladder.

Pharmacodynamic

Phenoxybenzamine is indicated for the control of episodes of hypertension and sweating that occur with a disease called pheochromocytoma. If tachycardia is excessive, it may be necessary to use a beta-blocking agent concomitantly. Phenoxybenzamine is a long-acting, adrenergic, alpha-receptor blocking agent which can produce and maintain "chemical sympathectomy" by oral administration. It increases blood flow to the skin, mucosa and abdominal viscera, and lowers both supine and erect blood pressures.
It has no effect on the parasympathetic system. Phenoxybenzamine works by blocking alpha receptors in certain parts of the body. Alpha receptors are present in the muscle that lines the walls of blood vessels. When the receptors are blocked by Phenoxybenzamine, the muscle relaxes and the blood vessels widen. This widening of the blood vessels results in a lowering of blood pressure.

Pharmacokinetics

• Onset of action : Within 2 hours; Maximum effect: Within 4 to 6 hours
• Duration : ≥3 to 4 days
• Half-Life : 24 hr
• Bioavailability : 20-30%
• Excretion : Urine & feces
• Metabolism : Extensively metabolized in liver
• Metabolite : N-phenoxyisopropyl-benzylamine
• Absorption • : Twenty to 30 percent of orally administered phenoxybenzamine appears to be absorbed in the active form.

Drug indications

Dosage

Aduts:
Pheochromocytoma:
Initial dose: 10 mg orally twice daily
Maintenance dose: 20 to 40 mg orally 2 to 3 times daily until an optimal dosage (as judged by blood pressure control) is obtained.
Dose Adjustments:
Increases in dosage should be made every other day, with careful observation of the patient after each increase. The dosage should be adjusted to a point where symptomatic relief and/or objective improvement are obtained, or until the side effects from blockade cannot be tolerated.
Micturition disorders (off-label use) :
Oral: 10 to 20 mg 1 to 2 times/day
Pediatric: Pheochromocytoma (off-label uses) :
Oral: Initial: 0.25 to 1 mg/kg/day; increase slowly to blood pressure control .

Drug contraindications

Side effects

Interactions

Alerts

-Concomitant use of drugs that stimulate alpha/beta receptors (may increase hypotension/tachycardia)
-First dose effect may occur, causing a sudden and drastic fall in blood pressure after administering the first dose.
-Hypotension/syncope may occur with first few doses or with increase in dose.
-Minimize by using small first dose at bedtime
-Increase dose slowly
-Phenoxybenzamineinduced alpha-adrenergic blockade leaves beta-adrenergic receptors unopposed. Compounds that stimulate both types of receptors may, therefore, produce an exaggerated hypotensive response and tachycardia.
-Carcinogenesis and Mutagenesis: Case reports of carcinoma in humans after long-term treatment with phenoxybenzamine have been reported. Hence long-term use of phenoxybenzamine is not recommended.
- Carefully weigh the benefits and risks before prescribing this drug.

Points of recommendation

You should not use this medication if you are allergic to phenoxybenzamine orAny conditions in which fall in BP is undesirable
Before taking phenoxybenzamine,tell your doctor if you have coronary artery disease, a lung infection, or kidney disease.
Avoid getting up too fast from a sitting or lying position. Get up slowly and steady yourself to prevent a fall.
Avoid drinking alcohol. It can increase drowsiness caused by phenoxybenzamine.
This medication can cause side effects that may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be awake and alert.

Pregnancy level