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

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

[22] Cetirizine is also commonly prescribed to treat acute and (in particular cases) chronic urticaria (hives), more efficiently than any other second-generation antihistamine.

In the UK, up to 30 tablets of 10 mg are on the general sales list (of pharmaceuticals) and can be purchased without a prescription and pharmacist supervision.

Their report noted that some patients indicated the itchiness impacted their ability to work, sleep or perform normal daily activities.

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

[35] As such, brain penetration and brain H1 receptor occupancy by cetirizine are dose-dependent, and in accordance, while 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 in other studies.

[44] At a dosage of 20 mg, Boone et al. found that it inhibited the expression of VCAM-1 in patients with atopic dermatitis.

[44] Cetirizine is rapidly and extensively absorbed upon oral administration in tablet or syrup form.

[9][8][45] Similar findings were reported for levocetirizine, which had its Tmax delayed by 1.25 hours and its Cmax decreased by about 36% when administered with a high-fat meal.

[46] The mean plasma protein binding of cetirizine has been found to be 93 to 96% across a range of 25 to 1,000 ng/mL independent of concentration.

[7] The drug is bound to albumin with high affinity, while α1-acid glycoprotein and lipoproteins contribute much less to total plasma protein binding.

[9][7] Cetirizine poorly and slowly crosses the blood–brain barrier, which is thought to be due to its chemical properties and its activity as a P-glycoprotein substrate.

[49] Because of this, it does not interact significantly with drugs that inhibit or induce cytochrome P450 enzymes such as theophylline, erythromycin, clarithromycin, cimetidine, or alcohol.

[50] The 1-(4-chlorophenylmethyl)-piperazine is alkylated with methyl (2-chloroethoxy)-acetate in the presence of sodium carbonate and xylene solvent to produce the Sn2 substitution product in 28% yield.

[56][57] Cetirizine is sold under various brand names including Alatrol, Alerid, Allacan, Allercet, Alzene, Cerchio, Cetirin, Cetizin, Cetriz, Cetzine, Cezin, Cetgel, Cirrus, Histec, Histazine, Humex, Letizen, Okacet (Cipla), Piriteze, Reactine, Razene, Rigix, Sensahist (Oethmann, South Africa), Triz, Zetop, Zirtec, Zirtek, Zodac, Zyllergy, Zynor, Zyrlek, and Zyrtec (Johnson & Johnson), inter alios.