Generic Name

Cetirizine

Mechanism

• Cetirizine competitively binds the H1 receptor at the same site as histamine, preventing histamine‑mediated vasodilation, increased vascular permeability, and nerve stimulation.
• It exhibits high affinity and selectivity for peripheral H1 receptors, while undergoes limited brain penetration, thereby reducing sedative effects common to first‑generation antihistamines.
• The drug also inhibits low‑grade neurogenic inflammation by blocking histamine‑induced activation of mast cells.

Pharmacokinetics

• Route & Absorption: Oral tablets or syrup; rapid absorption with t_max ≈ 1 h; bioavailability > 70 % after a 10‑mg dose.
• Distribution: Volume of distribution ~ 0.5 L/kg; plasma protein binding ≈ 40 % (primarily to albumin).
• Metabolism: Hepatic metabolism via CYP2D6 and glucuronidation; minimal involvement of CYP3A4.
• Elimination: Excreted mainly unchanged in urine (≈ 48 %); terminal half‑life 7–10 h, supporting once‑daily dosing.
• Special Populations: No dose adjustment required for mild–moderate hepatic impairment; renal dysfunction necessitates reduced dosing (0.5 mg per 24 h in severe eGFR < 30 mL/min/1.73 m²).

Indications

• Allergic rhinitis (seasonal/perennial)
• Chronic spontaneous urticaria (≥ 2 weeks)
• Acute allergic conjunctivitis
• Dermatological pruritus secondary to allergic dermatitis
• Eosinophilic angioedema (under specialist guidance)

Contraindications

• Known hypersensitivity to cetirizine or any excipients.
• Pregnancy: Category B; caution in lactation.
• Pediatric: ≤ 6 y requires 2.5 mg dose; > 6 y standard 5 mg or 10 mg regimens.
• Renal failure: severe impairment demands dose reduction.
• Alcohol or CNS depressants: no additive sedation but avoid in clinically sensitive patients.

*Warnings*:
• Evaluate hepatic/renal function before initiating prolonged therapy (> 4 weeks).
• Monitor for potentials of dizziness or orthostatic hypotension in elderly.

Dosing

• Adults & Adolescents (≥ 12 y): 10 mg PO once daily (preferred).

*Alternative*: 5 mg twice daily if rapid control is needed.
• Pediatrics (6–11 y): 5 mg PO once daily.
• Pediatrics (≤ 6 y): 2.5 mg PO once daily.
• Renal impairment (eGFR 30–50 mL/min/1.73 m²): 5 mg PO each 48 h.

Severe renal impairment (eGFR < 30): 2.5 mg PO twice weekly.

*Administration tips*:
• Take with or without food; absorption unchanged.
• Swallow whole tablet; crushing interferes with prolonged‑release formulation.

Adverse Effects

Common (≤ 5 % incidence):
• Dry mouth, mild headache, fatigue, nausea.
• Dizziness in > 18 % of elderly patients.

Serious (≤ 1 % incidence):
• Severe hypersensitivity rash or anaphylaxis.
• Rare CNS depression or visual hallucinations in predisposed individuals.
• Transient increase in liver enzymes; monitor if therapy > 12 weeks.

*Adverse effect pearls*:
• Clinicians should counsel patients on the ↓ likelihood of sedation versus first‑generation antihistamines.
• Educate on potential for transient constipation despite typical dry mouth.

Monitoring

• Baseline: Liver function tests (AST/ALT), renal function (serum creatinine), electrolytes.
• During prolonged use: Repeat LFTs every 3–6 months; assess renal status if eGFR < 60 mL/min/1.73 m².
• Adherence: Monitor therapeutic response; adjust to “Bid” dosing if inadequate control.
• Pregnant/Lactating: Review infant feeding; observe for any neonatal side effects.

Clinical Pearls

> Key takeaway: Cetirizine provides robust antihistamine efficacy with a high safety profile, making it a first‑line choice for both acute and chronic allergic conditions across adult and pediatric populations.