Pharmacology and actions
Chlorpheniramine is an alkylamine antihistamine with H1-receptor antagonist action. It acts by reversible, competitive inhibition of the interaction of histamine with H1-receptors
Histamine is released from mast cells on their encounter with antigens. It then binds to H1-receptors to initiate the typical symptoms of hypersensitivity and allergic responses. It acts on bronchial smooth muscles and blood vessels to produce bronchospasm and flare, wheal and itch responses to an antigens.
Chlorpheniramine inhibits the constrictor action of histamine and smooth muscles, especially respiratory muscle and blocks the action of histamine that results in capillary permeability and formation of oedema and wheal. It also binds to H1-receptors in the Central Nervous System
Pharmacokinetics
Chlorpheniramine maleate is absorbed relatively slowly from the gastrointestinal tract, peak concentrations occurring about 2.5 to 6 hours after administration by mouth. It appears to undergo considerable first pass metabolism and about 70% of chlorpheniramine in the circulation is bound to plasma proteins. Chlorpheniramine is widely distributed in the body and also crosses into the Central Nervous System.
There is a wide interindividual variation in its pharmacokinetic parameters. Chlorpheniramine is extensively metabolized to desmethyl and didesmethyl chlorpheniramine.
Indications
Chlorpheniramine is used for the symptomatic relief of allergic reactions including urticarial and angioedema, rhinitis and conjunctivitis and in pruritic skin conditions
Side effects
The side effects vary in incidence and severity with each patient. Central Nervous System depression Effects Include sedation, drowsiness, deep-sleep, lassitude, dizziness and inco-ordination
Paradoxical central nervous system stimulation particularly in children may occur with insomnia, nervousness, euphoria, irritability, tremors and rarely nightmares, hallucinations and convulsions.
Antimuscarinic effects include dry mouth, thickened respiratory secretions and tightness of chest, blurred vision, urinary difficulty and retention, constipation. Other gastrointestinal effects include nausea, vomiting, diarrhea and epigastric pain.
Contraindications
Premature infants and neonates
Hypersensitivity to antihistamines
Precautions and warnings
Caution is required when administering to elderly patients
Caution is required in closed-angle glaucoma, urinary retention, prostatic hypertrophy, pyloroduodenal obstruction, epilepsy, severe cardiovascular disorders, and liver disorders
Chlorpheniramine may cause drowsiness. Patient should be warned not to drive or operate machinery if affected.
Antihistamines may suppress positive skin test results and should be stopped several days before the test
Drug interactions
Antihistamines enhance the sedative effects of alcohol, barbiturates, hypnotics, opioid analgesics, anxiolytic sedatives, neuroleptics and should not be taken simultaneously
Monoamine oxidase inhibitors, atropine and tricyclic antidepressants enhance the antimuscarinic effects of antihistamines.
