Generic Name

Zyrtec

Mechanism

Zyrtec competitively blocks peripheral H1 receptors on mast cells and basophils, preventing histamine‑induced vasodilation, increased vascular permeability, and nerve sensitization.
• Rapid onset (≈1 h) and duration of 24 h.
• Minimal affinity for M3, α1, or M1 receptors → low anticholinergic or sedative effects.

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Pharmacokinetics

• Absorption: Oral bioavailability ≈63 %; peak plasma at 1–3 h.
• Distribution: 95 % protein bound; crosses placenta (but no dose adjustment needed in pregnancy Q2‑3).
• Metabolism: Hepatic via glucuronidation and CYP1A2.
• Excretion: Primarily renal (≈70 % unchanged); clearance 4–6 mL/min/1.73 m² in adults.
• Half‑life: 8–10 h; longer in CKD stage 3–4 (up to 12 h).

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Indications

• Seasonal allergic rhinitis (hay fever)
• Perennial allergic rhinitis
• Chronic spontaneous urticaria (≥6 weeks)
• Allergic conjunctivitis (ocular itching, tearing)

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Contraindications

• Allergy to cetirizine or any excipients.
• Hypersensitivity to other second‑generation antihistamines.
• Severe renal impairment (CrCl < 30 mL/min): dose reduction advised.
• Liver dysfunction: monitor liver function tests.
• Pregnancy: category B; use only if benefit outweighs risk.
• Breastfeeding: evidence for safe use; monitor infant for sedation.

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Dosing

• Extended‑release formulation for nighttime symptoms (10 mg QHS) is available.
• Re‑dosing: if inadequate symptom control, repeat dose up to 20 mg/day (max 10 mg BID).

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Adverse Effects

• Common (≤5 %): dry mouth, drowsiness (rare), headache, fatigue, nausea.
• Serious (≤0.1 %): QT prolongation (rare), arrhythmias in severe CKD, hypersensitivity rash, angioedema.
• Drug interactions: CYP1A2 inhibitors (cimetidine, fluvoxamine) ↑ serum levels; avoid concurrent use of other sedating antihistamines.

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Monitoring

• Renal function: baseline and every 3–6 months or after acute decline.
• Liver enzymes: baseline if chronic liver disease, repeat if symptoms arise.
• QT interval: check in patients on multiple QT‑prolonging drugs or with heart disease.
• Efficacy: symptom diary for 1–2 weeks to assess response.
• Adverse effects: report sedation, dizziness, or rash promptly.

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Clinical Pearls

• “No‑sedation” badge: Zyrtec has <10 % CNS penetration, making it preferable over first‑generation antihistamines in patients requiring alertness (e.g., shift workers).
• Renal dose flexibility: Even in CKD stage 3, a 10 mg QD dose is generally safe; only consider 5 mg QD if CrCl < 15 mL/min.
• Pregnancy safety: Category B; the FDA recommends continued therapy for severe symptoms if the benefit outweighs any theoretical risk.
• Pediatric use: The liquid formulation is available in 1 mg/mL; split a 5 mg tablet into 5 mL portions for doses of 5 mg.
• Combination therapy: Pair Zyrtec with a low‑dose intranasal steroid for refractory rhinitis; additive benefit without increased sedation.
• Drug screening: Note that Zyrtec is detected in routine urinalysis immunoassays for antihistamine metabolites; be aware when interpreting false‑positive results.

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