Why does Zyrtec cause drowsiness

CetirizineDrug groupsAntihistamines Cetirizine is an antiallergic active ingredient from the group of antihistamines of the 2nd generation. It is used to treat hay fever, allergic rhinitis, allergic conjunctivitis and hives. Compared to 1st generation antihistamines, cetirizine causes less fatigue and drowsiness, as it hardly gets into the brain. The effect occurs quickly and lasts for a long time, so that only one dose per day is necessary.

synonymous: Cetirizine, Cetirizini dihydrochloridumPhEur, Cetirizine dihydrochloride, Cetirizine hydrochloride


Cetirizine is available in the form of film-coated tablets, lozenges and drops (Zyrtec®, generics). It has been approved in Switzerland since 1989.

Structure and properties

Cetirizine (C.21H25ClN2O3, Mr = 388.9 g / mol) is a racemate consisting of R.-Levocetirizine and S.-Dextrocetirizine. It is found in medicines as cetirizine dihydrochloride, a white powder that is easily soluble in water. Cetirizine is a piperazine derivative and a carboxylic acid metabolite of hydroxyzine (Atarax®), a 1st generation antihistamine. The R.-EnantiomerLevocetirizine is also commercially available (Xyzal®, generics).


Cetirizine (ATC R06AE07) has antihistamine, antiallergic and anti-inflammatory properties. It has a high selectivity for the H1 receptor and is not anticholinergic or cardiotoxic. Cetirizine crosses the blood-brain barrier only insignificantly and therefore causes less tiredness and drowsiness than antihistamines of the 1st generation; however, both undesirable effects can still occur. The effect occurs quickly and lasts for a long time.

Mechanism of action of antihistamines, click to enlarge. Illustration © PharmaWiki

Indications Dosage

According to the specialist information. The medicines are usually taken once a day with or without meals. In the case of children, the intake can also be divided into two doses (half a dose each) in the morning and in the evening, if necessary.

  • Hypersensitivity
  • Terminal renal failure
  • Children under 2 years

The complete precautionary measures can be found in the medicinal product information sheet.


Interactions have been reported with theophylline, glipizide and ritonavir. A potentiation of the effect of alcohol could not be proven, nevertheless, as a precaution, cetirizine should not be combined with alcohol.

unwanted effects

Possible adverse effects include tiredness and drowsiness, headache, dizziness, agitation, indigestion, diarrhea, dry mouth, urinary retention, tachycardia and visual disturbances.

see also

Levocetirizine, antihistamines, hay fever

  • Arlette J.P. Cetirizine: a piperazine antihistamine. Clin Dermatol, 1991, 9 (4), 511-3 Pubmed
  • Medicinal product information (CH, D)
  • Curran M.P., Scott L.J., Perry C.M. Cetirizine: a review of its use in allergic disorders. Drugs, 2004, 64 (5), 523-61 Pubmed
  • European Pharmacopoeia PhEur
  • Portnoy J.M., Dinakar C. Review of cetirizine hydrochloride for the treatment of allergic disorders. Expert Opin Pharmacother, 2004, 5 (1), 125-35 Pubmed
  • Tillement J.P., Testa B., Brée F. Compared pharmacological characteristics in humans of racemic cetirizine and levocetirizine, two histamine H1-receptor antagonists. Biochem Pharmacol, 2003, 66 (7), 1123-6 Pubmed

Conflicts of Interest: None / Independent. The author has no relationships with the manufacturers and is not involved in the sale of the products mentioned.

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This article was last changed on February 20, 2020.
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