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Sunday, July 27, 2025

Cetirizine


Cetirizine is a second-generation antihistamine commonly used to treat allergic conditions such as hay fever (allergic rhinitis), urticaria (hives), and other symptoms of allergy like sneezing, runny nose, and itching. It is a selective antagonist of the histamine H1-receptor and is preferred over older antihistamines because it causes minimal sedation and has a longer duration of action.

This comprehensive monograph outlines the pharmacological classification, mechanism of action, therapeutic uses, dosage forms, contraindications, adverse effects, precautions, and drug interactions of cetirizine, along with clinical considerations relevant to both prescribers and patients.

Pharmacological Classification

  • Therapeutic class: Antiallergic agent

  • Pharmacological class: Second-generation selective H1-antihistamine

  • ATC Code: R06AE07

Brand Names and Formulations

  • Zyrtec® (global), Reactine® (Canada), Piriteze® (UK)

  • Also available as generic cetirizine hydrochloride

  • Available formulations include:

    • Tablets (10 mg)

    • Oral solution or syrup (1 mg/mL or 5 mg/5 mL)

    • Chewable tablets, orodispersible tablets, and capsules

Mechanism of Action

Cetirizine is a selective antagonist of the peripheral H1 histamine receptors. By blocking histamine activity at these receptors, it inhibits:

  • Capillary dilation and leakage (reduces swelling)

  • Sensory nerve stimulation (reduces itch and sneezing)

  • Mucus secretion in nasal tissues

As a second-generation antihistamine, cetirizine penetrates the blood–brain barrier poorly, resulting in significantly less sedation than first-generation antihistamines (e.g., diphenhydramine or chlorphenamine). It also exhibits anti-inflammatory activity, including suppression of eosinophil migration and cytokine release.

Therapeutic Indications

Approved Uses

  • Allergic rhinitis (seasonal and perennial)

  • Chronic idiopathic urticaria

  • Acute urticaria

  • Allergic conjunctivitis (as adjunct)

  • Eczema-related pruritus

  • Insect bite reactions

  • Food and drug allergies (as part of antihistamine regimen)

  • Dermatographism

Off-label or Adjunctive Uses

  • Anaphylaxis adjunct (after epinephrine)

  • Atopic dermatitis-related itch

  • Cold-induced urticaria

  • Allergy prevention before allergen exposure (e.g., pollen season)

Dosage and Administration

Adults and Children ≥12 years

  • 10 mg once daily (tablet or oral solution)

  • Can be taken with or without food

  • Maximum dose: 10 mg per 24 hours

Children 6–11 years

  • 5 mg twice daily or 10 mg once daily

Children 2–5 years

  • 2.5 mg twice daily or 5 mg once daily (oral solution preferred)

Children 6 months–2 years (only when prescribed)

  • 2.5 mg once daily, increased to 2.5 mg twice daily if needed

Renal Impairment

  • Dose adjustment required:

    • Creatinine clearance 11–31 mL/min: 5 mg once daily

    • Severe impairment or hemodialysis: Avoid or limit to 5 mg every other day

Hepatic Impairment

  • No routine adjustment required, but caution advised in severe impairment

Pharmacokinetics

  • Onset of action: ~20–60 minutes

  • Peak plasma levels: 1 hour post-dose

  • Duration: ≥24 hours

  • Half-life: 8–10 hours in adults, longer in elderly and renal impairment

  • Bioavailability: ~70%

  • Metabolism: Minimal hepatic metabolism

  • Excretion: Primarily renal (urine)

Contraindications

  • Known hypersensitivity to cetirizine, hydroxyzine, or any formulation excipient

  • End-stage renal disease (CrCl <10 mL/min)

  • Breastfeeding (unless benefits outweigh risks; minimal data on infant exposure)

Precautions and Warnings

  • Sedation risk: Though minimal, drowsiness may still occur, especially at higher doses or in sensitive individuals

  • Alcohol consumption: May potentiate CNS depression

  • Concurrent use with other CNS depressants: May enhance sedative effects

  • Driving and machinery use: Warn patients of possible psychomotor effects

  • Seizure disorders: Use with caution; rare reports of seizure exacerbation

  • Renal impairment: Accumulation risk due to renal clearance

Adverse Effects

Common (1–10%)

  • Drowsiness, fatigue, headache

  • Dry mouth, dizziness, nausea

  • Somnolence (notably in children under 6 years)

Uncommon (0.1–1%)

  • Diarrhea, abdominal pain, pharyngitis

  • Agitation or hyperactivity (in children)

Rare (<0.1%)

  • Allergic reactions, including angioedema and anaphylaxis

  • Confusion, hallucinations, tachycardia

  • Liver function abnormalities

  • Urinary retention, especially in patients with spinal cord injury or prostatic hypertrophy

Drug Interactions

Cetirizine has minimal clinically significant drug interactions, due to its lack of hepatic enzyme induction or inhibition. However:

  • Alcohol and sedatives: Increased CNS depressant effects

  • Theophylline (400 mg): May reduce cetirizine clearance slightly

  • Ritonavir: May increase cetirizine plasma levels slightly, but generally safe

  • CNS depressants (e.g., benzodiazepines, opioids): Additive sedation risk

  • Anticholinergics: May increase risk of urinary retention

Cetirizine does not interact significantly with CYP450 isoenzymes.

Pregnancy and Lactation

Pregnancy (Category B)

  • Animal studies show no teratogenicity

  • Human studies: No increased risk of congenital abnormalities

  • Generally considered safe during pregnancy, especially after the first trimester

  • Preferred alternative to first-generation antihistamines (e.g., diphenhydramine)

Lactation

  • Excreted in breast milk, but in small quantities

  • Infant exposure appears minimal and likely safe

  • Monitor infants for sedation or irritability

Use in Special Populations

  • Pediatrics: Approved down to 6 months in certain countries; preferred non-sedating antihistamine for allergic rhinitis

  • Elderly: Increased half-life; start at 5 mg daily to minimize sedation or confusion

  • Renal impairment: Dose adjustment necessary

  • Hepatic impairment: No adjustment usually required

Clinical Considerations

  • Compared to first-generation antihistamines, cetirizine offers:

    • Less sedation

    • Longer duration of action

    • Once-daily dosing

  • Compared to other second-generation antihistamines:

    • Cetirizine may have slightly higher sedation potential than loratadine or fexofenadine

    • Cetirizine shows stronger antihistaminic potency than loratadine in some studies

  • Levocetirizine is the active enantiomer of cetirizine; more potent but also more sedating

Counseling Points

  • Take at the same time each day for consistent control of allergy symptoms

  • Can be taken with or without food

  • Avoid alcohol and other sedatives if drowsiness occurs

  • Safe for seasonal use or long-term management under supervision

  • Do not exceed recommended dose, especially in children

  • Store at room temperature, away from moisture and direct sunlight

  • Report signs of severe rash, swelling, or breathing difficulties

  • May be used as preventive therapy before allergen exposure (e.g., pollen season)




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