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Drug Reference

chlorphenamine

first-generation antihistamine· Brands: Piriton, Chlor-Tripolon, Chlor-TimetonIMIVPO

Mechanism of action
Chlorphenamine is a first-generation antihistamine that competitively antagonises histamine at H1 receptors, thereby preventing the effects of histamine in allergic reactions. It also has anticholinergic and sedative properties, which contribute to its therapeutic and adverse effects.
Pharmacokinetics
Chlorphenamine is well absorbed from the gastrointestinal tract after oral administration, with peak plasma concentrations reached within 2 hours. It undergoes extensive first-pass hepatic metabolism, primarily by cytochrome P450 enzymes, and has a half-life of approximately 20 hours. Metabolites are excreted in the urine.
Indications & dosing

Allergic rhinitis

For adults and children over 12 years. Sedation is common.

  • adult · PO: 4 mg every 4-6 hours · For adults and children over 12 years. Sedation is common.

Urticaria

May be used for symptom relief of urticaria.

  • adult · PO: 4 mg every 4-6 hours · May be used for symptom relief of urticaria.

Allergic rhinitis

For children aged 2-5 years.

  • pediatric · PO: 1 mg every 4-6 hours · For children aged 2-5 years.

Allergic rhinitis

For children aged 6-12 years.

  • pediatric · PO: 2 mg every 4-6 hours · For children aged 6-12 years.

Urticaria

For children aged 2-5 years.

  • pediatric · PO: 1 mg every 4-6 hours · For children aged 2-5 years.

Urticaria

For children aged 6-12 years.

  • pediatric · PO: 2 mg every 4-6 hours · For children aged 6-12 years.

Anaphylaxis (adjunct)

Adjunct to adrenaline for anaphylaxis. Administer by slow intravenous injection.

  • adult · IV: 10 mg every 6 hours · Adjunct to adrenaline for anaphylaxis. Administer by slow intravenous injection.
Contraindications & precautions
  • Hypersensitivity to chlorphenamine or any component of the formulation · Risk of anaphylaxis or severe allergic reaction.
  • Concomitant use with monoamine oxidase inhibitors (MAOIs) · Increased risk of severe anticholinergic and CNS effects.
  • Neonates and premature infants · Increased susceptibility to anticholinergic effects and possible apnoea.
  • Angle-closure glaucoma · Anticholinergic effects may precipitate acute glaucoma.
  • Prostatic hypertrophy with urinary retention · Anticholinergic effects may worsen urinary retention.
  • Avoid concomitant alcohol and other CNS depressants as sedation is enhanced.
  • Use with caution in epilepsy, hepatic or renal impairment as chlorphenamine may exacerbate symptoms.
  • May impair ability to drive or operate machinery; patients should be warned.
  • Caution in elderly, who are more susceptible to anticholinergic effects and confusion.
  • Caution in patients with urinary retention, prostatic hypertrophy, or angle-closure glaucoma.
  • In children, paradoxical excitation may occur; monitor for restlessness and agitation.
Adverse effects
  • Sedation and drowsiness
  • Anticholinergic effects (dry mouth, blurred vision, urinary retention, constipation)
  • Paradoxical excitation, especially in children
  • Hypotensionserious
  • Tachycardia
  • Arrhythmiasserious
  • Allergic reactions, including anaphylaxisserious
  • Blood dyscrasiasserious
Renal & hepatic adjustment
  • eGFR 0+: No dose adjustment required in renal impairment; use with caution in severe renal impairment.
  • Hepatic impairment (mild): No dose adjustment required.
  • Hepatic impairment (moderate): Use caution; consider reducing the dose or frequency.
  • Hepatic impairment (severe): Avoid use, or use with extreme caution at a reduced dose. (avoid)
Monitoring, pregnancy & toxicity
  • Symptom control · At each review
  • Sedation · During therapy, especially in elderly and children
  • Anticholinergic effects (dry mouth, constipation, urinary retention) · During therapy
  • CNS effects (paradoxical excitement in children, confusion in elderly) · During therapy
  • Pregnancy: Chlorphenamine may be used in pregnancy when treatment is necessary. It is generally considered suitable for short-term use, but should be avoided in the first trimester unless the benefits outweigh the risks.
  • Toxicity: CNS excitation or depression (e.g., agitation, seizures, coma) · Supportive care; treat seizures with benzodiazepines; avoid stimulants.
  • Toxicity: Anticholinergic effects (e.g., dry mouth, blurred vision, tachycardia, urinary retention) · Symptomatic management; monitor ECG; consider physostigmine in severe cases under specialist advice.
  • Toxicity: Hypotension · Intravenous fluids; vasopressors if required.
  • Toxicity: Cardiac arrhythmias · Monitor ECG; treat according to local ALS algorithms.
Clinical notes
Chlorphenamine is used for symptomatic relief of allergic conditions such as hay fever, urticaria, and insect bites. Sedation is a common adverse effect; therefore, patients should be advised to avoid driving or operating machinery. It should be used with caution in elderly patients and those with glaucoma, prostatic hypertrophy, or cardiovascular disease. In children, paradoxical excitation may occur. Avoid concurrent use of other CNS depressants. Doses should be tailored to the individual; refer to local prescribing information.

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BNF; local formulary