Tablet, 4 mg (hydrogen maleate)

General information

Chlorphenamine is an antihistamine that reversibly and competitively inhibits the binding of histamine to H₁ receptors.

It is well absorbed following oral administration and is widely distributed throughout the body including the central nervous system. Plasma concentrations peak after 2-3 hours. It is metabolized in the liver and excreted in the urine, largely as metabolites.

Clinical information

Uses

Symptomatic treatment of:

• urticaria
• severe and intractable pruritus.

Dosage and administration

The dosage should be adjusted according to the patient’s response and tolerance.

Adults and children over 12 years: 4 mg every 6 hours.

Children under 12 years: 0.35 mg/kg daily in three or four divided doses.

Contraindications

Age under 2 years.

Precautions

Drowsiness, dizziness, blurred vision and psychomotor impairment can occur. These effects can seriously impair the patient’s ability to drive and use machinery.

Use in pregnancy

Safe use in pregnancy has not been established. Chlorphenamine should be used only when the need of the mother outweighs any possible risk to the fetus.

Adverse effects

Sedation, which can vary in degree from mild drowsiness to deep sleep is common, but patients rapidly develop tolerance. This effect may be of benefit in patients with pruritus. Other adverse effects on the central nervous system include dizziness, lassitude, incoordination and blurred vision. These effects are rarely observed with the newer H₁ antagonists, which do not cross the blood-brain barrier.

Paradoxical excitation in children and confusional states in the elderly have been reported.

Gastrointestinal symptoms including anorexia, nausea, vomiting, epigastric pain and constipation or diarrhoea also occur.

Drug interactions

Alcohol and other drugs acting on the brain have an additive sedative effect. Phenytoin toxicity has resulted from inhibition of its metabolism.

Overdosage

Drowsiness, dizziness and ataxia are the most common symptoms of acute overdosage. Anticholinergic effects including flushing, dilated pupils and hyperthermia occur within 2 hours of ingestion. In serious cases, seizures are followed by respiratory and cardiovascular depression.

Induction of emesis or gastric lavage followed by administration of activated charcoal is of value when undertaken within a few hours of ingestion. Treatment is otherwise symptomatic and aims to maintain respiration and control seizures and cardiovascular abnormalities.

Storage

Tablets should be stored in well-closed containers, protected from light.