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Sedating Antihistamine (H1 Antagonist — First-generation) Pregnancy: Avoid in third trimester — neonatal withdrawal and paradoxical excitability reported. Short courses in first/second trimester acceptable if clearly needed. Loratadine preferred.

Chlorphenamine (Chlorpheniramine)

Brand names: Piriton, Allerief

Adult dose

Dose: Allergic reactions / urticaria: 4mg every 4–6h (oral) or 10–20mg IV/IM (acute allergic reaction / anaphylaxis). Pruritus: 4mg TDS–QDS. Maximum: 24mg daily (oral).
Route: Oral / IV / IM
Frequency: Every 4–6 hours (oral); single dose (IV/IM for acute reactions)
Max: 24mg daily (oral); 40mg/24h (parenteral)
First-generation antihistamine — sedating (crosses BBB). Administered IV/IM in anaphylaxis as adjunct to adrenaline (not a substitute). IV injection must be given slowly over 1 minute (risk of CNS stimulation, hypotension). Available OTC. Causes significant sedation and anticholinergic effects — do not drive.

Paediatric dose

Route: Oral / IV / IM
Frequency: Every 4–6 hours
Max: Varies by age
BNFc: 1–23 months: 1mg BD. 2–5 years: 1mg every 4–6h (max 6mg/day). 6–11 years: 2mg every 4–6h (max 12mg/day). 12–17 years: 4mg every 4–6h (max 24mg/day). IV (anaphylaxis): 250 micrograms/kg (max 10mg) — inject slowly.

Dose adjustments

Renal

Use with caution — some accumulation; reduce dose frequency in severe impairment.

Hepatic

Severe hepatic impairment: avoid — anticholinergic and sedating effects may precipitate hepatic encephalopathy.

Clinical pearls

  • Anaphylaxis: chlorphenamine is adjunct ONLY — adrenaline (epinephrine) 500 micrograms IM (0.5mL 1:1000) is the life-saving first-line treatment; chlorphenamine 10mg IV helps reduce urticarial component but does not reverse bronchospasm or cardiovascular collapse
  • Driving: chlorphenamine causes significant sedation — advise patients not to drive or operate machinery. Recommend loratadine or cetirizine if sedation is problematic
  • Paradoxical excitation in children: some children become agitated and hyperactive rather than sedated — particularly under 2 years; use with caution
  • Night-time pruritus: sedating antihistamine at night is therapeutically useful — use chlorphenamine 4mg at night and loratadine in the morning

Contraindications

  • Neonates (not recommended)
  • Narrow-angle glaucoma
  • Urinary retention
  • Prostatic hypertrophy
  • Severe hepatic impairment
  • MAOIs (avoid — CNS excitation)

Side effects

  • Sedation (very common — warn about driving)
  • Dry mouth, urinary retention, constipation (anticholinergic)
  • Dizziness, headache
  • Paradoxical CNS excitation (children)
  • Thickened bronchial secretions

Interactions

  • CNS depressants, alcohol — additive sedation
  • MAOIs — avoid (anticholinergic crisis, CNS excitation)
  • Anticholinergic drugs — additive effects

Monitoring

  • Symptom response
  • Sedation (caution in elderly — falls risk)
  • Anticholinergic side effects (urinary symptoms)

Reference: BNFc; BNF 90; RCUK Anaphylaxis Guidelines 2021. Verify against your local formulary and the latest BNF before prescribing.

Related

Curated clinical cross-links plus same-class fallbacks.

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