1. Clinical Overview
Cetirizine is a second-generation, piperazine-derivative antihistamine, classified as a selective, competitive peripheral H1-receptor antagonist. It is a major active metabolite of hydroxyzine. It is widely used for its potent anti-allergic properties with minimal sedation due to poor penetration of the blood-brain barrier. It is a first-line therapy for allergic conditions in the Indian population due to its efficacy, safety, and cost-effectiveness.
| Onset | Duration | Bioavailability |
|---|---|---|
| Within 20-60 minutes after oral administration. | Approximately 24 hours, allowing for once-daily dosing. | High and rapid, approximately 70%. |
2. Mechanism of Action
Cetirizine competitively and selectively inhibits histamine H1 receptors on effector cells in the gastrointestinal tract, blood vessels, and respiratory tract. This blockade prevents the binding of histamine released from mast cells and basophils during an allergic reaction, thereby inhibiting the downstream effects such as vasodilation, increased vascular permeability, and smooth muscle contraction.
3. Indications & Uses
- Allergic Rhinitis (Seasonal and Perennial)
- Chronic Idiopathic Urticaria
- Allergic Conjunctivitis (when associated with allergic rhinitis)
4. Dosage & Administration
Adult Dosage: 10 mg once daily. May be taken in the evening if drowsiness occurs.
Administration: Can be taken with or without food. Swallow the tablet whole with a glass of water. Do not crush or chew unless it is a scored tablet intended for splitting. For faster onset, take on an empty stomach.
5. Side Effects
Common side effects may include:
- Somnolence (drowsiness)
- Fatigue
- Dry mouth
- Headache
- Pharyngitis
- Mild dizziness
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| CNS Depressants (Alcohol, Benzodiazepines, Opioids) | Additive sedative effect, increased impairment of psychomotor performance. | Major |
| Theophylline | May decrease Cetirizine clearance, increasing its levels. Clinical significance is minimal. | Moderate |
| Ritonavir | May increase Cetirizine plasma concentration. | Moderate |
| Anticholinergic drugs (e.g., Atropine, TCAs) | Potential additive anticholinergic effects like dry mouth, urinary retention. | Moderate |
7. Patient Counselling
- DO take the medication as prescribed, usually once daily.
- DO inform your doctor about all other medications you are taking.
- DO inform your doctor if you are pregnant, planning pregnancy, or breastfeeding.
- DO NOT consume alcohol while on this medication.
- DO NOT drive or operate heavy machinery until you know how the medicine affects you.
- DO NOT take a double dose to make up for a missed one.
8. Toxicology & Storage
Overdose: Drowsiness, agitation, restlessness, confusion, tachycardia, headache, malaise, urinary retention, dry mouth. In massive overdose: hypotension, cardiac arrhythmias, convulsions, coma.
Storage: Store below 30°C, in a cool, dry place. Protect from light and moisture. Keep out of reach of children. Do not use after the expiry date printed on the pack.