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How long does it take for Zyrtec to work in dogs?

Generally, Zyrtec begins working within an hour of ingestion
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By | Aug. 8, 2024 Medically reviewed by Emma Ryan, DVM
A small black dog sitting: How long does it take for Zyrtec to work in dogs?

Key takeaways

  • Zyrtec (cetirizine) is an antihistamine typically safe for dogs, but Zyrtec-D is not.

  • Zyrtec has shown some effectiveness in lowering histamine levels and treating allergy symptoms in dogs. However, some studies have concluded that it’s not the most effective option for symptoms of atopic dermatitis.

  • Drowsiness is the most common side effect for dogs, but it’s not as severe as the drowsiness from Benadryl or other first-generation antihistamines.

  • The typical dose for dogs is 0.45 mg per pound of body weight. They can take it at any time of day, with or without food, usually once every 12–24 hours.

In this article:

Canine companions instinctively enjoy outdoor activities, but allergies can dampen their fun. Fortunately, pet owners can now turn to Zyrtec (cetirizine) to help their furry friends. This popular over-the-counter antihistamine can help treat various dog allergy symptoms and is often a safe choice for dogs. But how fast will your pup start feeling better? While relief typically begins within an hour, other factors influence how quickly your dog feels better. Here are all the important details.

Can I give my dog Zyrtec?

Standard Zyrtec, with no decongestants, can be given to dogs. Not only does the Zyrtec website say it’s okay to give the medication to pets, but research backs up its efficacy. A study from 2018 confirmed its antihistamine effect in dogs. Since histamines cause many allergy symptoms, they could be a good option for treating everything from runny noses to itchy skin, sneezing, hives, and watery eyes. 

“It’s important to note that dog allergies can range in severity,” says Dr. Jessica Apted, DVM, the regional medical director at Sploot Veterinary Care. “Generally, dogs have manageable allergic reactions that can be treated with lifestyle changes, dietary prescriptions, and medications like antihistamines. However, if an allergy escalates to life-threatening anaphylaxis, antihistamines like Zyrtec are not effective.” 

She also explains that “Zyrtec’s use for dogs is ‘off-label.'” This means that proper vet guidance and consultation are necessary for safe use. Talk to your dog’s vet before administering Zyrtec or other drugs.

Important Note: While standard Zyrtec is safe for dogs, never give a dog Zyrtec-D. The “D” in its name stands for decongestant—in this case, one called pseudoephedrine, which is toxic for dogs. Check the drug’s packaging and label carefully before giving it to your pet.

How quickly does Zyrtec work in dogs?

Allergy medications like Zyrtec bind to histamine receptors, stopping the chemicals from acting. This doesn’t happen immediately, but it doesn’t take all day either. Per the Zyrtec website, the medication typically does its job in around one hour for dogs. According to Dr. Dwight Alleyne, DVM, a veterinarian adviser at Betterpet, “Zyrtec usually starts to work within a couple of hours of a dog taking the antihistamine.”

Of course, Dr. Apted says that Zyrtec’s efficiency can vary depending on the dog, the dosage, underlying conditions, gastrointestinal issues, the dog’s metabolism, and other medications the dog is taking.

Zyrtec works for dogs the same way it does for humans: blocking histamines. A chemical created by the immune system, histamines react when they encounter an allergen like pollen, dust, or mold. As they work to rid a human or dog’s body of these foreign substances, they might overreact and cause symptoms.

The FDA says Zyrtec’s effects typically last around 24 hours for humans, and studies show that its elimination half-life is around 8.3 hours. These timelines are similar for animals.

Benefits of Zyrtec for dogs

It’s no use giving a dog Zyrtec unless it can provide some allergy relief. Fortunately, it can handle a wide range of them. For example, a study from 2020 found that Zyrtec “significantly reduced histamine,” which suggests that it may treat other symptoms of seasonal allergies, insect bites, and other allergic reactions.

However, Zyrtec might not be as effective for chronic atopic dermatitis (eczema). One 2016 study concluded that it performed about the same as a placebo in treating the condition, and an older study showed it reduced eczema symptoms in only 18% of dogs. It may or may not be the best option if your dog has itchy skin.

However, newer antihistamines like Zyrtec and Claritin (loratadine) don’t cross the blood-brain barrier, which means they’re less sedating than first-generation antihistamines like Benadryl (diphenhydramine) or hydroxyzine. So, they won’t necessarily calm a dog down if they’re anxious, but they may still cause some sleepiness for dogs that are hypersensitive to them.

Second-generation antihistamines are less likely to have serious drug interactions than first-generation ones, so Zyrtec may be safer to take with other medications—specifically corticosteroids, one of the most effective treatments for itching.

Side effects of Zyrtec in dogs

Even though newer antihistamines like Zyrtec pose less of a risk for serious side effects, they may still cause milder ones. Specifically, they include:

  • Drowsiness
  • Excessive salivation
  • Vomiting

There are currently no reported serious side effects. But it’s still crucial to administer it in appropriate doses and follow the veterinarian’s instructions.

If your dog has kidney or liver problems, Zyrtec might exacerbate them, so consult a vet before administering it. Dogs allergic to cetirizine shouldn’t take it since it may cause a severe allergic reaction. There’s very little data on Zyrtec for pregnant or nursing dogs, so it’s best to avoid it since it’s unclear how it might affect young puppies.

“If Zyrtec is given an appropriate dose, it can be safe for most dogs, but there are contraindications if combined with other medications,” Dr. Alleyne says. Specifically, Zyrtec can interact with opioids, anxiety medications, sleep aids, muscle relaxants, central nervous system depressants, and other antihistamines. But it’s best to play it safe and inform the veterinarian about any medications or supplements the dog is taking.

Dosage of Zyrtec for dogs

Giving a dog the correct dosage is critical for Zyrtec’s effectiveness and safety. But since dog breeds vary wildly in size, there’s no one-size-fits-all dose. After all, you wouldn’t want to give a Toy Poodle the same dose as a Bull Mastiff. That’s why the recommended dosage for Zyrtec is based on the dog’s weight. According to the Merck Veterinary Manual, it’s typically one milligram per kilogram of body weight (0.45 mg/lb). Do not exceed 20 mg unless the veterinarian recommends it.

Yet certain factors can influence that dosage. It “depends on the dog’s health condition, age, weight, and the severity of the dog’s allergy symptoms,” Dr. Apted says. “It’s crucial to consult a veterinarian before administering Zyrtec to ensure the correct dosage and to rule out any potential contraindications or interactions.”

Zyrtec is fine for dogs to take with or without food. However, if the dog vomits after taking it, give it with food. If you miss a dose, don’t give the dog a double dose to compensate for it. Instead, just provide the next dose when you would normally.

Dogs can typically take Zyrtec once every 24 hours, although the vet might recommend two daily doses in certain situations. It’s often a short-term treatment until the allergy symptoms subside, but it is also safe to take long-term if the dog needs it. However, that’s up to the veterinarian, and you should always follow their instructions about how and when to administer Zyrtec.