Cetirizine, is a second-generation antihistamine used to treat allergic rhinitis, dermatitis, and urticaria (hives).

Trade name Zyrtec

Pregnancy category AU: B2

Routes of administration By mouth

Bioavailability Well-absorbed >70%

Protein binding 88–96%

Metabolism Minimal 

Onset of action 20–42 minutes

Elimination half-life Mean: 8.3 hours

Range: 6.5–10 hours

Duration of action ≥24 hours

Excretion Urine: 70–85%

Feces: 10–13%

Effects generally begin within thirty minutes and last for about a day.

The degree of benefit is similar to other antihistamines such as diphenhydramine, which is a first-generation antihistamine.

Common side effects include sleepiness, dry mouth, headache, and abdominal pain.

The degree of sleepiness that occurs is generally less than with first-generation antihistamines because second-generation antihistamines are more selective for the H1 receptor.

Compared to other second-generation anti-histamines, cetirizine can cause drowsiness.

Second-generation antihistamines that do not cause drowsiness are fexofenadine, and loratadine.

Use in pregnancy appears safe.

Use during breastfeeding is not recommended.

The medication works by blocking histamine H1 receptors, mostly outside the brain.

Cetirizine can be used for paediatric patients, with the main side effect to be cautious about is somnolence.

Its primary indication is for hay fever and other allergies. 

Cetirizine is also commonly prescribed to treat acute and chronic urticaria, more efficiently than any other second-generation antihistamine.

Cetirizine is available over-the-counter in the US in the form of 5 and 10 mg tablets. 

A 20 mg strength is available by prescription only.

Side effects of cetirizine include headache, dry mouth, drowsiness, and fatigue, while more serious, but rare, adverse effects reported include tachycardia and edema.

Discontinuing cetirizine after prolonged use may result in pruritus.

Cetirizine acts as a highly selective antagonist of the histamine H1 receptor.

Cetirizine has 600-fold or greater selectivity for the H1 receptor over a wide variety of other sites, including muscarinic acetylcholine, serotonin, dopamine, and α-adrenergic receptors, among many others.

It shows 20,000-fold or greater selectivity for the H1 receptor over the five muscarinic acetylcholine receptors, and hence does not exhibit anticholinergic effects.

It shows no cardiotoxicity at doses of up to 60 mg/day, six times the normal recommended dose.

Cetirizine crosses the blood–brain barrier only slightly, and for this reason, produces minimal sedation compared to many other antihistamines.

A positron emission tomography (PET) study found the H1 receptor was 12.6% for 10 mg cetirizine, 25.2% for 20 mg cetirizine, and 67.6% for 30 mg hydroxyzine.

A 10 mg dose of cetirizine equals about a 30 mg dose of hydroxyzine in terms of peripheral antihistamine effect.

Brain penetration and brain H1 receptor occupancy by cetirizine are dose-dependent.

Cetirizine at doses of 5 to 10 mg have been reported to be non-sedating or mildly sedating, a higher dose of 20 mg has been found to induce significant drowsiness.

Cetirizine has anti-inflammatory properties independent of H1 receptors, suppressing the NF-κB pathway, and by regulating the release of cytokines and chemokines, thereby regulating the recruitment of inflammatory cells.

It has been shown to inhibit eosinophil chemotaxis.

Cetirizine is rapidly absorbed with oral administration in tablet or syrup form.

The oral bioavailability of cetirizine is at least 70% and of levocetirizine is at least 85%.

The Tmax of cetirizine is approximately 1.0 hour.

Food has no effect on the bioavailability of cetirizine but has been found to delay the Tmax by 1.7 hours, to approximately 2.7 hours and to decrease the Cmax by 23%.

Steady-state levels of cetirizine occur within 3 days and there is no accumulation of the drug with chronic administration.

The mean plasma protein binding of cetirizine has been found to be 93 to 96%.

The drug is bound to albumin with high affinity.

The volume of distribution of cetirizine is unknown but is estimated to be 0.3 to 0.45 L/kg.

It poorly and slowly crosses the blood–brain barrier.

Cetirizine does not undergo extensive metabolism.

It is notably not metabolized by the cytochrome P450 system: 

it therefore does not interact significantly with drugs that inhibit or induce cytochrome P450 enzymes such as theophylline, erythromycin, clarithromycin, cimetidine, or alcohol.

Plasma radioactivity attributed to unchanged cetirizine is more than 90% at 2 hours, 80% at 10 hours, and 70% at 24 hours, indicating limited and slow metabolism.

Cetirizine is eliminated approximately 70 to 85% in the urine and 10 to 13% in the feces.

About 50 or 60% of cetirizine eliminated in the urine is unchanged.

The elimination half-life of cetirizine ranges from 6.5 to 10 hours in adults, with a mean across studies of approximately 8.3 hours.

Its duration of action is at least 24 hours.

The elimination half-life of cetirizine is increased in hepatic and renal impairment.

10 mg cetirizine tablets.

Zyrtec-D, a combination of cetirizine and pseudoephedrine.

Cetirizine is available without a prescription. 

Cetirizine is available as a combination medication with pseudoephedrine, a decongestant.

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