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Sedating Antihistamine (H1 Antagonist — First-generation)

Chlorphenamine (Chlorpheniramine)

Brand names: Piriton, Allerief

Chlorphenamine is a sedating (first-generation) antihistamine used for allergic conditions such as urticaria, hay fever and as an adjunct in the emergency treatment of anaphylaxis.

Dosing — being independently re-sourced

ClinCalc Pro is rebuilding its dose data from primary open sources — the manufacturer SmPC (eMC), the WHO Model Formulary and other official references — under clinician review. This drug's structured dose is not yet published here. Confirm all doses against the product SmPC and your local formulary before prescribing.

Clinical monograph

How it works

It is a competitive antagonist at histamine H1 receptors and, owing to its central penetration and antimuscarinic actions, also commonly causes sedation.

Prescribing in practice

  • It causes sedation that can impair driving and skilled tasks and is additive with alcohol and other CNS depressants, so warn patients accordingly.
  • Its antimuscarinic effects mean it should be used with caution in conditions such as urinary retention, angle-closure glaucoma and prostatic enlargement.
  • It is available in oral and injectable forms, with the injection used as an adjunct in the management of anaphylaxis.

Monitoring

Routine monitoring is not generally required; assess for excessive sedation and antimuscarinic effects, particularly in older patients.

Counselling the patient

  • Be aware it may cause drowsiness and do not drive or operate machinery if affected.
  • Avoid alcohol while taking it.
  • It may cause dry mouth, blurred vision or constipation.

Evidence & guidelines

Chlorphenamine is a long-established antihistamine with extensive clinical use in allergic disorders and anaphylaxis.

Reference: RCUK Anaphylaxis Guidelines 2021; Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. Monograph status: clinician-reviewed (2026-07-04).

Related

Curated clinical cross-links plus same-class fallbacks.

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