Introduction
Zyrtec (cetirizine hydrochloride) is a second-generation antihistamine medication primarily used for the management of allergic conditions. As a selective H1-receptor antagonist, it provides effective relief from allergy symptoms with less sedating effects compared to first-generation antihistamines. First approved by the FDA in 1995, Zyrtec has become one of the most widely prescribed and recommended antihistamines worldwide.
Mechanism of Action
Cetirizine competitively antagonizes histamine at peripheral H1 receptors. Unlike first-generation antihistamines, it has minimal affinity for cholinergic, adrenergic, or serotonin receptors, resulting in reduced anticholinergic and sedative effects. Cetirizine inhibits histamine-mediated early phase allergic responses and may also attenuate late phase allergic reactions by inhibiting the release of inflammatory mediators from mast cells, basophils, and eosinophils.
Indications
- Perennial and seasonal allergic rhinitis
- Chronic urticaria (hives)
- Allergic conjunctivitis
- Atopic dermatitis (as adjunctive therapy)
- Other allergic skin conditions characterized by pruritus
Dosage and Administration
Adults and children ≥6 years: 5-10 mg once daily Children 2-5 years: 2.5-5 mg once daily (or 2.5 mg twice daily) Children 6 months-2 years: 2.5 mg once daily Special Populations:- Renal impairment (CrCl 11-31 mL/min): 5 mg once daily
- Hepatic impairment: 5 mg once daily
- Elderly: Consider reduced dosing in those with renal/hepatic impairment
- Pregnancy: Category B - use only if clearly needed
- Lactation: Cetirizine is excreted in breast milk; use with caution
Pharmacokinetics
Absorption: Rapidly absorbed with peak plasma concentrations occurring within 1 hour. Food does not significantly affect absorption but may delay Tmax. Distribution: Volume of distribution is 0.5 L/kg. Plasma protein binding is 93%. Metabolism: Minimally metabolized in the liver via cytochrome P450 enzymes (primarily CYP3A4). Elimination: Half-life is approximately 8.3 hours. Primarily excreted unchanged in urine (70%) with 10% excreted in feces.Contraindications
- Hypersensitivity to cetirizine, hydroxyzine, or any component of the formulation
- Patients with known hypersensitivity to any piperazine derivatives
Warnings and Precautions
- Use with caution in patients with renal or hepatic impairment (dose adjustment required)
- May cause drowsiness; caution patients about driving or operating machinery
- Use with caution in elderly patients due to increased risk of adverse effects
- Monitor for anticholinergic effects in patients with urinary retention, glaucoma, or gastrointestinal obstruction
- Discontinue at least 48 hours before skin allergy testing to avoid false negative results
Drug Interactions
- CNS depressants (alcohol, benzodiazepines, opioids): Additive sedative effects
- Theophylline: Mild increase in cetirizine clearance
- Medications that cause QT prolongation: Theoretical risk of additive effects
- Anticholinergic agents: Potential for additive anticholinergic effects
Adverse Effects
Common (≥1%):- Somnolence (11-14%)
- Fatigue (5.9%)
- Dry mouth (5.0%)
- Pharyngitis (2.9%)
- Dizziness (2.0%)
- Headache
- Nausea
- Abdominal pain
- Diarrhea
- Agitation (in children)
- Severe hypersensitivity reactions
- Seizures
- Hepatitis
- Angioedema
- QT prolongation (at supratherapeutic doses)
Monitoring Parameters
- Therapeutic response and symptom control
- Signs of excessive sedation or impaired cognition
- Renal and hepatic function in patients with impairment
- Adverse effects, particularly in elderly patients
- Signs of hypersensitivity reactions
Patient Education
- Take as directed, usually once daily with or without food
- Report excessive drowsiness, difficulty urinating, or palpitations
- Avoid alcohol and other CNS depressants while taking cetirizine
- Do not drive or operate machinery until effects are known
- Store at room temperature, away from moisture
- Use sugar-free formulations in diabetic patients when available
- Seek immediate medical attention for severe allergic reactions
- Do not crush or chew extended-release formulations
References
1. Simons FER. Advances in H1-antihistamines. N Engl J Med. 2004;351(21):2203-2217. 2. Zyrtec® (cetirizine hydrochloride) prescribing information. Johnson & Johnson Consumer Inc.; 2021. 3. Church MK, Maurer M, Simons FER, et al. Risk of first-generation H1-antihistamines: a GA²LEN position paper. Allergy. 2010;65(4):459-466. 4. Day JH, Briscoe MP, Rafeiro E, et al. Comparative clinical efficacy of cetirizine and fexofenadine for seasonal allergic rhinitis. Allergy Asthma Proc. 2001;22(3):147-153. 5. Golightly LK, Greos LS. Second-generation antihistamines: actions and efficacy in the management of allergic disorders. Drugs. 2005;65(3):341-384. 6. FDA Orange Book: Approved Drug Products with Therapeutic Equivalence Evaluations. Accessed January 2024. 7. UpToDate: Cetirizine drug information. Wolters Kluwer Health. Accessed January 2024.