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Sunday, July 27, 2025

Chlorphenamine (Piriton)


Chlorphenamine (also known as chlorpheniramine in some countries and marketed under brand names such as Piriton®, Alleramin, and others) is a first-generation antihistamine used for the relief of allergic conditions including hay fever, urticaria, insect bites, drug allergies, and anaphylaxis (as adjunct). Due to its sedating effects, it is frequently found in over-the-counter cold and flu remedies, and is occasionally used off-label for its anticholinergic and antipruritic properties.

This profile provides a comprehensive, professional overview of chlorphenamine’s pharmacology, indications, usage, precautions, and interactions.

Pharmacological Classification

  • Therapeutic Class: Antiallergic / Antihistamine

  • Pharmacologic Class: First-generation alkylamine derivative H1 receptor antagonist

  • ATC Code: R06AB04

Brand Names

  • Piriton® (UK, Middle East)

  • Chlor-Trimeton® (US)

  • Alleramin, Histal, Chlorphenamine Maleate (various generics)

  • Available as tablet, oral syrup, injectable, and combination formulations

Mechanism of Action

Chlorphenamine is a competitive antagonist of histamine H1 receptors, effectively blocking histamine-mediated responses in:

  • Capillaries: reducing vascular permeability, edema, and erythema

  • Sensory nerves: decreasing itching and sneezing

  • Smooth muscles: preventing bronchoconstriction (to a minor extent)

Its central action due to crossing the blood-brain barrier also causes sedation, which can be beneficial in pruritic or severe allergic reactions.

Additional anticholinergic activity may contribute to drying of nasal and ocular secretions but also underlies many side effects.

Indications

Primary Indications

  • Allergic rhinitis (hay fever)

  • Urticaria (hives)

  • Angioedema

  • Insect bites and stings

  • Food allergies

  • Allergic conjunctivitis

  • Cold and flu symptoms (as part of combination therapy)

  • Pruritus (e.g., eczema, chickenpox-related itching)

  • Adjunct in anaphylaxis (after adrenaline)

Off-label or Adjunctive Uses

  • Drug-induced rashes or reactions

  • Sleep aid (due to sedative effects; off-label)

  • Motion sickness (limited efficacy)

  • Cough suppressant adjunct (in cold medications)

Dosage and Administration

Adults and Adolescents (over 12 years)

  • Oral tablets/syrup: 4 mg every 4–6 hours

  • Maximum: 24 mg per 24 hours

Children (per UK BNF and product license)

  • Age 6–12 years: 2 mg every 4–6 hours

  • Age 1–5 years: 1 mg every 4–6 hours

  • Not recommended under 1 year

Elderly

  • Start at lower end of dosing range due to increased CNS sensitivity

  • Monitor closely for confusion, sedation, and anticholinergic effects

Parenteral use (emergency settings)

  • IV/IM injection: 10–20 mg slowly (used in anaphylaxis adjunct therapy)

  • Must follow adrenaline administration

  • Not for routine outpatient use

Pharmacokinetics

  • Absorption: Rapid and well absorbed orally

  • Onset: Within 30 minutes

  • Peak: 2–4 hours

  • Duration: 4–6 hours (up to 8 hours in some individuals)

  • Half-life: 12–15 hours

  • Metabolism: Liver (hepatic oxidation and demethylation)

  • Excretion: Renal (both as unchanged drug and metabolites)

Contraindications

  • Hypersensitivity to chlorphenamine or excipients

  • Neonates and premature infants

  • MAO inhibitor use within past 14 days (risk of hypertensive crisis)

  • Narrow-angle glaucoma

  • Prostatic hypertrophy with urinary retention

  • Severe liver disease (relative contraindication)

Precautions and Warnings

  • Sedation: Avoid activities requiring mental alertness (driving, machinery)

  • CNS Depression: Potentiated with alcohol and sedatives

  • Elderly: Greater sensitivity to confusion, falls, urinary retention

  • Children: May experience paradoxical excitation rather than sedation

  • Asthma: Caution due to thickening of bronchial secretions

  • Renal/Hepatic impairment: Use cautiously and consider dosage adjustment

  • Pregnancy/Lactation: Limited safety data; use if clearly needed

Adverse Effects

Common

  • Drowsiness, fatigue, dizziness

  • Dry mouth, blurred vision, urinary retention

  • Constipation, nausea, abdominal discomfort

  • Headache

Less Common

  • Confusion, hallucinations (especially in elderly)

  • Tachycardia, palpitations

  • Hypotension

  • Paradoxical CNS stimulation in children (restlessness, insomnia)

  • Photosensitivity

Rare

  • Seizures

  • Agranulocytosis, thrombocytopenia

  • Anaphylaxis

  • Jaundice

  • Extrapyramidal symptoms (very rare)

Drug Interactions

CNS Depressants

  • Additive sedative effect with:

    • Alcohol, benzodiazepines, opioids, hypnotics, anxiolytics

Monoamine Oxidase Inhibitors (MAOIs)

  • Risk of hypertensive crisis, anticholinergic effects, and CNS toxicity

  • Avoid concomitant use or within 14 days of discontinuation

Anticholinergic Agents

  • Additive effects: atropine, tricyclic antidepressants, antipsychotics

  • Increases risk of blurred vision, urinary retention, dry mouth

Phenytoin

  • Chlorphenamine may inhibit phenytoin metabolism, leading to toxicity

QT-Prolonging Drugs

  • Although not a major offender, caution advised in polypharmacy

Pregnancy and Lactation

Pregnancy

  • Category B (US) / C (UK) – limited human data

  • Generally considered safe in short-term use, especially in 2nd and 3rd trimester

  • Risk-benefit assessment required; avoid in 1st trimester if alternatives exist

Lactation

  • Present in breast milk; may cause sedation or irritability in infants

  • Avoid chronic use while breastfeeding

  • Short-term use likely acceptable under medical supervision

Use in Special Populations

  • Children: Effective, but monitor for paradoxical effects

  • Elderly: Increased risk of cognitive impairment, sedation, and falls

  • Hepatic/Renal impairment: Caution advised due to prolonged clearance

Counseling Points

  • May cause drowsiness: avoid alcohol, driving, or operating machinery

  • Not suitable for long-term use in allergic rhinitis—prefer non-sedating antihistamines (e.g., loratadine)

  • Take with or without food; with food may reduce gastric upset

  • Inform physician if urinary problems, glaucoma, or liver disease exist

  • Do not exceed recommended dose; toxicity is dose-dependent

  • Combination cold/flu preparations may contain duplicate antihistamines

Comparison with Other Antihistamines (Qualitative)

  • More sedating than newer second-generation antihistamines (e.g., cetirizine, loratadine)

  • Faster onset, but shorter duration of action

  • More anticholinergic side effects, especially in elderly

  • Preferred when sedation is desired, such as at night or in acute allergic reactions



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