Cetirizine

Cetirizine is a second-generation H1 antihistamine used for allergic rhinitis, allergic conjunctivitis and chronic spontaneous urticaria. It is more sedating than other second-generation agents and is not a treatment for anaphylaxis, which needs intramuscular adrenaline.

Classification. Second-generation H1 antihistamine, piperazine derivative and active metabolite of hydroxyzine.

Mechanism. Selective peripheral H1 receptor inverse agonist with some inhibition of eosinophil chemotaxis.

Pharmacokinetics. Onset within 1 hour, duration 24 hours. Bioavailability over 70%. Little metabolism. Half-life about 8 hours in adults. About 70% is excreted unchanged in urine.

First-line, allergic rhinitis, urticaria, adults and children from 12 years. 10 mg once daily, or 5 mg twice daily.

Children. 6 to 11 years: 5 mg twice daily or 10 mg once daily if over 30 kg. 2 to 5 years: 2.5 mg twice daily, maximum 5 mg daily. 12 to 23 months: 2.5 mg once daily, increased to twice daily if needed. 6 to 11 months: 2.5 mg once daily. Under 6 months: not advised.

Specialist, chronic spontaneous urticaria not controlled at the usual dose. Up to 4 times the usual dose (up to 40 mg daily, divided) for specialist-directed up-dosing.

Alternative. Intranasal corticosteroid is more effective for moderate to severe rhinitis.

Emergency. Not used.

Titration and loading. None.

Contraindications. Hypersensitivity to cetirizine or hydroxyzine. Severe kidney impairment with GFR under 10 mL/min.

Serious adverse effects. Severe itching after stopping prolonged use (withdrawal pruritus), rare angioedema, urinary retention, seizures at overdose.

Common adverse effects. Drowsiness (about 1 in 10), dry mouth, fatigue, headache, dizziness.

Interactions. Alcohol and other sedatives add drowsiness. Ritonavir raises cetirizine exposure modestly. No clinically relevant QT effect.

Common prescribing errors. Using as the only treatment for anaphylaxis. Prescribing in the evening for night driving. Using high doses without specialist review.

Special groups.

• Pregnancy: acceptable; loratadine is the usual first choice. Breastfeeding: small amounts in milk and may cause infant drowsiness; loratadine is preferred.

• Older adults: start at 5 mg once daily.

• Kidney impairment: GFR 30 to 49 mL/min, 5 mg once daily. GFR under 30 mL/min or dialysis: 5 mg once daily at most, often every other day. Avoid when GFR is under 10 mL/min. Liver impairment: 5 mg once daily.

• East Africa: widely available and low cost. Warn about drowsiness when driving, riding motorcycles or operating machinery.

Formulations. Tablets 10 mg, syrup 5 mg/5 mL, drops.

Administration. Once daily, with or without food, in the evening if drowsy.

Baseline. Symptom and urticaria activity score, kidney function in older adults.

Monitoring. Control of symptoms, sedation, urinary symptoms. Review after 2 to 4 weeks.

Stop and reduce rules. Reduce the dose in kidney or liver impairment. When stopping after long use, taper if itching appears. Therapeutic drug monitoring is not used.

Referral and escalation. Refer for urticaria that does not settle at 4 times the usual dose, angioedema, or suspected anaphylaxis.