1. Cetirizine

• Second-generation, low-sedating H1-antihistamine.
• Piperazine derivative and active metabolite of hydroxyzine.
• Has relatively high skin concentrations and low affinity for cardiac tissue.

• Blocks H1 receptors and also has anti-inflammatory effects, including reduced eosinophil recruitment/chemotaxis and effects on inflammatory mediators.
• Rapid oral absorption, with peak serum level at about 1 hour.
• Onset: within 1 hour. Duration: about 24 hours.
• About 70% is excreted unchanged in urine, so renal function matters.
• Adult half-life: 7-10 hours, prolonged in older adults and renal impairment.

• Urticaria in adults: 10 mg once daily.
• Children aged 2-5 years: 2.5-5 mg/day.
• In renal impairment, haemodialysis, or hepatic impairment: 5 mg/day is stated in the text.

• Sedation or somnolence, headache, fatigue, dry mouth, nausea, dizziness.
• Alcohol and other CNS depressants can increase sedation and impair driving.
• Reduce dose in significant renal impairment.
• The PDF notes no QT prolongation with cetirizine.

• A key non-sedating oral antihistamine for chronic idiopathic urticaria/chronic spontaneous urticaria and acute urticaria.
• Useful when pruritus is histamine-mediated, including urticaria, allergic skin reactions, and insect-bite itch.
• Its once-daily action and relatively low sedation make it suitable for daytime treatment. The text describes it as highly potent and particularly useful in chronic idiopathic urticaria.

2. Hydroxyzine

• First-generation, sedating H1-antihistamine.
• Piperazine derivative.

• Competitive H1-receptor antagonist.
• Has additional central sedative and anxiolytic action through inhibition of subcortical neuronal activity.
• Oral onset: 15-30 minutes.
• It has a long-lasting suppressive effect on histamine-induced wheal and flare responses.
• Half-life is prolonged, especially in older adults and liver disease.

• Adults with pruritus: 25 mg three or four times daily.
• Older adults: start lower, such as 10 mg three or four times daily, then increase only if necessary.
• Paediatric doses are given in divided doses and should be prescribed according to age and weight.

• Common: drowsiness, dry mouth, dizziness, irritability.
• Avoid alcohol, opioid analgesics, benzodiazepines, barbiturates, and other sedatives because of additive CNS depression.
• MAO inhibitors can increase/prolong anticholinergic effects.
• Use cautiously in glaucoma, prostatic enlargement, cardiovascular disease, asthma, thyroid disease, diabetes, and in older people.
• Dose interval may need extension in liver disease.

• Particularly useful for intense pruritus and urticaria, especially if itch disturbs sleep.
• Its sedation can be useful for breaking the nighttime itch-scratch cycle.
• The PDF states that it suppresses wheal and flare responses more strongly and for longer than other antihistamines, including newer less-sedating agents.
• Its major limitation in dermatology is daytime sedation and anticholinergic adverse effects, so it is generally more appropriate as a short-term or night-time option than as a routine daytime drug.

3. Bilastine

• Second-generation, non-sedating H1-antihistamine.
• Piperidine derivative.
• Not structurally derived from other available antihistamines.

• Potent and highly selective H1-receptor antagonist.
• The PDF reports higher H1-receptor affinity than cetirizine and fexofenadine.
• It also has anti-allergic and anti-inflammatory actions, including inhibition of histamine, IL-4, and TNF-alpha release from mast cells and granulocytes.
• Rapidly absorbed, with peak level at about 1 hour.
• Bioavailability is approximately 60%.
• Minimal hepatic metabolism. Most drug is excreted unchanged in faeces and urine.
• P-glycoprotein substrate, limiting blood-brain barrier entry and helping explain its low sedative effect.
• Mean elimination half-life: approximately 14.5 hours.

• 20 mg once daily.

• Headache, somnolence, and fatigue may occur.
• The PDF reports these effects less often than with cetirizine 10 mg.
• Positron-emission tomography data cited in the text support minimal CNS H1-receptor occupancy, consistent with a non-sedating profile.
• The PDF states no significant drug interactions and no CYP450 enzyme induction.

• Used for symptomatic treatment of chronic urticaria, particularly in patients aged over 12 years according to the PDF.
• A useful choice when controlling wheals and itch while minimizing sedation is important, for example in people who drive, study, or need to remain alert.
• The text reports efficacy in urticaria comparable to levocetirizine 5 mg, with better tolerability than cetirizine in terms of sleepiness.

Quick comparison for dermatology