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Zyrtec: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on Apr 8, 2020.

1. How it works

  • Zyrtec is a brand (trade) name for cetirizine. Cetirizine is an antihistamine that works selectively on peripheral histamine-1 (H-1) receptors (these are histamine receptors that are located outside of the brain and spinal cord). Because it acts on peripheral histamine receptors, cetirizine is much less likely to cause drowsiness compared with some older antihistamines.
  • Histamine is a chemical that is released by mast cells in response to an allergen, and it is responsible for many of the symptoms of an allergic reaction, such as swelling of the mucous membranes, sneezing, and itching. Zyrtec binds to histamine receptors and prevents histamine from having an effect on those receptors, which reduces the symptoms of an allergic reaction.
  • Zyrtec belongs to the group of drugs known as antihistamines. Zyrtec may also be called an H1-antihistamine, a second-generation antihistamine, or a nonsedating antihistamine.

2. Upsides

  • Used to treat allergic-type reactions due to perennial or seasonal allergic rhinitis (hay fever).
  • Effective at controlling symptoms such as sneezing, itching, watery eyes, and runny nose that occur as a result of other respiratory allergens.
  • Can provide relief from itching that occurs as a result of chronic urticaria (hives). Symptoms of hives include raised, red, itchy bumps, streaks, or blotches on the skin.
  • May be used in the treatment of allergic conjunctivitis, insect bites, and other allergic skin disorders.
  • Is less likely to cause sedation than older antihistamines.
  • Can be taken once a day.
  • May be given daily on a regular basis when allergens are most prevalent (such as during spring or summer).
  • Zyrtec does not appear to interact with many drugs (theophylline slightly decreased the clearance of Zyrtec).
  • Zyrtec has been demonstrated to be safe in children older than 6 months.
  • No dosage adjustment is required in liver disease.
  • Available over-the-counter.
  • Zyrtec is available as a generic under the name of cetirizine.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • A headache, sore throat, abdominal pain, fatigue, drowsiness, or coughing. Other side effects are uncommon.
  • Generally does not cause drowsiness, although more likely to do so at higher dosages. Caution should be exercised before driving or operating machinery until full effects of Zyrtec are known.
  • The dosage of Zyrtec should be reduced in kidney disease and caution should be exercised when Zyrtec is used in people with seizure disorders.
  • Zyrtec, like all other antihistamines, may decrease the response to skin prick tests. Discontinue Zyrtec at least 72 hours prior to skin testing.

Notes: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. For a complete list of all side effects, click here.

4. Bottom Line

Zyrtec is an antihistamine that effectively treats allergic-type reactions such as hay fever or skin reactions caused by insect bites. Although drowsiness with Zyrtec is uncommon, Zyrtec is more likely than loratadine to cause drowsiness.

5. Tips

  • May be taken with or without food. Zyrtec is usually taken once a day. During the pollen season, your doctor may advise you to take it every day if you suffer from seasonal allergic rhinitis.
  • Although Zyrtec is unlikely to make you drowsy, some people are susceptible to this effect, especially if they are taking dosages at the upper end of the recommended dosage range. Do not drive or operate machinery if Zyrtec makes you drowsy. Alcohol and other sedative-like drugs may cause additional reductions in alertness if used with Zyrtec.
  • Seek urgent medical advice if you have hives and develop swelling of the face, throat, or tongue, dizziness, drooling, difficulty speaking, or shortness of breath.
  • Zyrtec is not a substitute for epinephrine which is used for the treatment of severe allergies and anaphylaxis.
  • Zyrtec is available as a tablet, chewable tablet, or a suspension for adults or children who have difficulty swallowing tablets.
  • See your doctor if your mild allergy symptoms have not improved after three days of treatment with Zyrtec, or if your hives persist for more than six weeks despite taking Zyrtec.
  • Discontinue Zyrtec once your allergy symptoms have resolved.
  • Do not take Zyrtec during pregnancy or while breastfeeding unless on the advice of your doctor.

6. Response and Effectiveness

  • Peak concentrations are reached within one hour of administration of Zyrtec tablets or syrup. The onset of effect occurs within 20 minutes in 50% of people and within an hour in 95%.
  • Effects persist for at least 24 hours following a single dose of Zyrtec. No tolerance has been found to Zyrtec's antihistaminic effect. Once discontinued, the skin recovers its normal reactivity to histamine within three days.
  • Taking 10mg of Zyrtec is more effective than taking 5mg; taking 20mg appears not to provide any additional effect.
  • Food has no effect on the overall absorption of Zyrtec; however, it may increase the time it takes for peak levels of Zyrtec to be reached in the blood.

7. Interactions

Medicines that interact with Zyrtec may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with Zyrtec. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with Zyrtec include:

  • anticonvulsants, such as fosphenytoin, lamotrigine, levetiracetam, phenytoin, or valproic acid
  • antidepressants, such as clomipramine, escitalopram, or fluvoxamine
  • antipsychotics, such as aripiprazole, chlorpromazine, or clozapine
  • baclofen
  • barbituates, such as butabarbital
  • benzodiazepines, such as diazepam or temazepam
  • benztropine
  • bupropion
  • buspirone
  • cannabis and cannabinoids
  • clonidine
  • HIV medications such as darunavir, lopinavir, ritonavir, or saquinavir
  • duloxetine
  • ketamine
  • lithium
  • monoamine oxidase antidepressants, such as phenelzine, isocarboxazid
  • mumps skin test antigen
  • opioids, such as codeine, fentanyl, hydrocodone
  • other antihistamines, such as azatadine, azelastine, or brompheniramine
  • sleeping medications, such as zolpidem
  • St John's wort
  • topiramate
  • voriconazole.

Alcohol may increase the risk of sedation with Zyrtec.

Note that this list is not all-inclusive and includes only common medications that may interact with Zyrtec. You should refer to the prescribing information for Zyrtec for a complete list of interactions.


Zyrtec [Package Insert] Revised 12/2019. McNeil Consumer Healthcare Division of McNeil-PPC, Inc.

Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use Zyrtec only for the indication prescribed.

Copyright 1996-2020 Revision date: April 8, 2020.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.