What it is
Cetirizine is a widely used second-generation H1 antihistamine. It is the active metabolite of hydroxyzine. It acts quickly (15–30 minutes), crosses the blood–brain barrier poorly —which is why it sedates far less than the old ones— and its effect lasts 12–24 hours.
It is one of the reference oral antihistamines for rhinitis, urticaria and allergic conjunctivitis. Among second-generation antihistamines, it is one of those that cause a little more drowsiness in some people: it is worth checking how it affects each person before driving. In pregnancy it is one of those with the most data, but the decision is the doctor's.
Mechanism of action
It blocks:
- Peripheral histamine H1 receptors, with high selectivity
- It does not cross the blood–brain barrier to any great extent (peripheral selectivity)
- A fast antihistamine effect with little sedation
At the counter
When to recommend it
- Allergic rhinitis (seasonal or perennial)
- Allergic urticaria
- Allergic conjunctivitis
- Mild to moderate systemic allergic reactions
When not to
- Allergy to cetirizine, hydroxyzine or other piperazine derivatives
- Severe kidney failure: the dose must be adjusted
- Young children: according to the age and presentation in the product information
- With alcohol or other nervous system depressants: drowsiness can add up
Warnings
- Important Hypersensitivity: very rare, but anaphylactic reactions have been reported in susceptible people.
- Caution Dry mouth: very common but tolerable.
- Caution Occasional headaches.
- Caution Drowsiness: uncommon, but more frequent than with loratadine or bilastine. In drivers and older people, check the response.
Pharmacokinetics
- Absorption
- Rapid absorption after oral administration. Bioavailability of 70%. Onset of action at 15–30 minutes, maximum at 45–60 minutes.
- Distribution
- Mainly peripheral distribution. Protein binding of 93%. It crosses the blood–brain barrier poorly (hence its low sedation).
- Metabolism
- Minimal hepatic metabolism. It is mostly eliminated unchanged.
- Elimination
- 80–90% renal elimination. Half-life of 8–9 hours, but the clinical effect lasts longer because of its receptor affinity.
Cetirizine vs diphenhydramine
| Aspect | Cetirizine | Diphenhydramine (old H1) |
|---|---|---|
| Sedation | Nil or minimal | Significant |
| Speed of action | Fast (15–30 min) | Fast (20–30 min) |
| Duration | Long (12–24 h) | Short (4–6 h) |
| Doses a day | 1 dose | 3–4 doses |
| Work / driving safety | Generally compatible (check the response) | NOT safe |
Cetirizine for allergies in people who need to stay alert. Diphenhydramine if sedation is wanted (at night).
Self-assessment
Three questions. When you check your answers you will see the explanation for each one.
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Cetirizine penetrates the central nervous system poorly, so it blocks few of the brain's H1 receptors (the ones that cause sleepiness). That is why it sedates far less than the old ones.
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In chronic allergy (rhinitis, urticaria), a regular daily dose works better than taking it 'as needed'. One dose a day is standard.
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Cetirizine sedates much less than the old antihistamines, but not to zero in everyone. If drowsiness is a problem, loratadine or bilastine are even less sedating alternatives.
Training content. It does not replace the summary of product characteristics or clinical judgement.