Zyrtec for Dogs: Dosage Chart, Safety and Side Effects

Plain Zyrtec (cetirizine) is generally well tolerated by dogs, and vets do use it. Two catches before you reach for the box in your cabinet. In dogs it is a weak itch medicine, far weaker than it is for your hay fever. And the wrong box can poison your dog: Zyrtec-D contains pseudoephedrine, a decongestant that is toxic to dogs. Never give Zyrtec-D.

So yes, sometimes: plain cetirizine only, with your vet's OK, at the dose they set. The rest of this page covers when it actually helps, the published dose range (with a weight calculator), how to tell a safe box from a dangerous one, side effects, and the signs that mean you call your vet now.

What Is Zyrtec for Dogs?

Zyrtec is the brand name for cetirizine, a second-generation antihistamine. It blocks histamine at H1 receptors, the switch behind much of the redness, swelling and itch of an allergic reaction. Cetirizine is also what the body makes from hydroxyzine, an older antihistamine vets have used for years.

Cetirizine isn't FDA-approved for dogs. When a vet recommends it, that is "extra-label" use: common in veterinary medicine, but it means the directions on the human box don't apply to your dog. Store-brand cetirizine contains the same active ingredient as Zyrtec, so the brand name matters less than what else is in the box (more on that below).

Does Zyrtec Work for Dogs?

Honestly? Not very well for the itch most dog owners are fighting.

People get hay fever mostly through histamine, which is why antihistamines work so well for us. Dogs with skin allergies have many other itch signals in play, so blocking histamine alone often isn't enough. The studies show it:

• In a placebo-controlled trial of 23 dogs with atopic dermatitis (environmental skin allergies), cetirizine satisfactorily reduced itching in 4 of 22 dogs (18%) that finished the trial (Cook et al., 2004).
• In a randomized, double-blind trial of 50 dogs with long-term atopic dermatitis, cetirizine had no effect on itching after 14 days compared with a placebo (Hsiao et al., 2016).
• The International Committee on Allergic Diseases of Animals concludes that oral antihistamines "might provide a small and limited benefit in some dogs" with atopic dermatitis, and are more likely to help in mild cases (ICADA 2015 guidelines).

That doesn't make cetirizine useless. In two small studies of healthy research beagles, it blocked histamine skin reactions without making the dogs sleepy. Blocking histamine is a narrow job, but some allergic problems are exactly that job.

When Zyrtec can help, and when it won't

Sources: Merck Veterinary Manual, ICADA guidelines, Veterinary Partner (VIN).

Zyrtec vs Apoquel (and Cytopoint) for an Itchy Dog

If your dog's itch is from atopic dermatitis, Zyrtec is a supporting player at best. The main options are medicines made and licensed for dogs:

• Apoquel (oclacitinib): a prescription tablet that controls itch within about a day. The Merck Veterinary Manual lists it for both sudden flares and long-term control in dogs 12 months or older.
• Cytopoint (lokivetmab): an itch-control injection given by your vet. It starts working in 1 to 3 days and suits both flares and long-term control. Our guide to the Cytopoint injection for dogs covers how it works and its side effects.
• Allergy shots (immunotherapy): a series of injections of the allergens your dog reacts to, which aims at the allergy itself rather than the itch. Your vet or a veterinary dermatologist can tell you whether your dog is a candidate.

Where does Zyrtec fit? Merck describes antihistamines as helpful for mild itch, best as part of combination therapy and in a preventive role, and not suitable for acute flares. Your vet may also try more than one antihistamine before finding one that suits your dog.

Not sure it's atopic dermatitis at all? Fleas are the classic trigger people miss: see flea allergy dermatitis, and our broader guide to allergic dermatitis in dogs.

Zyrtec Dosage Chart for Dogs

For information only. These are typical published dose ranges, not a prescription and not medically verified for your pet. Your veterinarian sets the dose after examining your pet. Never start, stop or change a medicine without their OK.

Only give what your vet prescribed, at the dose they set.

What the numbers don't tell you:

• The same Merck table gives the dog dose as "1 mg/kg or 10-20 mg per dog". Ask your vet which applies to your dog, especially a large one.
• Other figures are out there. Merck's toxicology chapter lists 1 to 4 mg per kg once a day, and a study in healthy research beagles used 2 to 4 mg per kg once a day. They stay out of the chart because neither is tied to treating a skin condition, and a higher dose hasn't been shown to calm itch better: the 14-day trial above found no benefit for chronic allergic itch.
• The per-pound figures are conversions of the per-kg dose (1 kg is about 2.2 lb), rounded.
• Tablet size is not a dose. Zyrtec tablets are sized for people, so your vet works out which tablet, chewable or liquid gives the right amount for your dog's weight.
• Liver or kidney disease can make cetirizine last longer, so your vet may adjust the plan.
• Plain cetirizine only. Never a "-D" or "allergy and congestion" product (next section).
• Too much, or got into the bottle? Call now: your vet, the ASPCA Animal Poison Control Center at (888) 426-4435 or the Pet Poison Helpline at (855) 764-7661. Overdose signs include sleepiness or the opposite (agitation, hyperactivity), vomiting or diarrhea, dilated pupils and a fast heart rate. Signs can start within about 5 hours.

Which Zyrtec Is Safe for Dogs? Read the Box First

The Zyrtec name covers several products, and they are not interchangeable for a dog:

Zyrtec-D is the dangerous one. Pseudoephedrine overdose in dogs causes agitation, hyperactivity, dilated pupils, a racing heart, high blood pressure, tremors, overheating and vomiting, according to the Merck Veterinary Manual. Zyrtec-D is an extended-release tablet, so signs can be delayed by hours and last longer. If your dog swallowed any, call your vet or a poison helpline now; don't wait for signs.

Xylitol: check every label, every time. The current Zyrtec-brand labels we checked on DailyMed don't list xylitol, but formulas change and products vary. Merck notes that sugar-free syrups and elixirs used as medicine bases frequently contain xylitol, and Vetster's vet reviewers warn that dissolvable or flavored allergy products may too. Read the inactive ingredients on the exact box, and ask your vet or pharmacist if a liquid is compounded. More in our guide to xylitol toxicity in dogs.

What Vets Use Cetirizine for in Dogs

Veterinary Partner (VIN) lists these uses:

• Vaccine reactions
• Blood transfusion reactions
• Bee stings and insect bites
• Itchy skin, usually alongside other treatment
• Stabilizing mast cell tumors, which release histamine (a decision for your vet or veterinary oncologist)

How it works

Mast cells are immune cells packed with histamine. When an allergic dog meets something it is sensitized to (a sting, pollen, dust mites), mast cells release histamine, which binds H1 receptors and causes redness, swelling and itch. Cetirizine sits on those receptors and blocks histamine from switching them on. It does nothing to the many other itch signals in an allergic dog's skin, which is the whole reason it underperforms.

By mouth, it usually starts working within 1 to 2 hours and wears off within about 24 hours, longer in dogs with liver or kidney disease. That timing is why vets favor giving antihistamines before a reaction rather than chasing one.

Zyrtec Side Effects in Dogs

Side effects are uncommon. The ones reported:

• Vomiting or drooling. Two of 23 dogs vomited in the 2004 trial.
• Sleepiness, mainly in small dogs given high doses. Cetirizine was designed not to cross into the brain, so drowsiness is rarer than with older antihistamines like diphenhydramine (Benadryl).

Tell your vet about anything new or worsening. If cetirizine seems to upset the stomach, ask whether to give it with a small amount of food.

Dogs that need extra caution

Your vet will think twice, or choose something else, if your dog:

• has ever reacted to cetirizine or hydroxyzine (don't use it again)
• has seizures (epilepsy)
• has trouble urinating, or seems constipated
• has thyroid or prostate problems
• has heart, kidney or liver disease
• has glaucoma
• is a senior
• is a working dog (even mild sleepiness matters on the job)
• is pregnant or nursing (use with caution; your vet decides)

Drug interactions and allergy tests

Cetirizine adds to the effect of sedatives and other drugs that slow the nervous system, such as diazepam or alprazolam. Tell your vet about every medicine and supplement your dog takes.

It also blunts allergy skin tests. If your dog is booked for intradermal allergy testing, tell the dermatologist: VCA advises stopping cetirizine 2 weeks before, and Veterinary Partner says to ask your veterinary dermatologist how far ahead to stop.

When to Call Your Vet Now

Hives and a puffy face after a sting or a vaccine are usually not life-threatening. Still, call your vet the same day, even if your dog seems bright: they decide whether an antihistamine is needed and which one. Book a visit soon, too, for itching that keeps coming back, red ears or paws and constant licking, or no change on the plan your vet already gave you.

In dogs, a severe allergic reaction (anaphylaxis) shows up mostly as gut signs rather than breathing signs, and it can start within minutes. Call your vet or an emergency clinic now, and don't wait to see whether Zyrtec helps, if you see:

• facial swelling or hives with vomiting, diarrhea or heavy drooling
• pale gums, cold legs, weakness or collapse
• trouble breathing
• a seizure
• your dog swallowed Zyrtec-D, any cold or allergy pill with a decongestant, or a pile of tablets

Poison lines, 24/7: ASPCA Animal Poison Control Center (888) 426-4435 and Pet Poison Helpline (855) 764-7661.

FAQ About Zyrtec for Dogs

Can I give my dog Zyrtec with food?

Yes, with or without a meal. If it seems to upset your dog's stomach, a small amount of food may help.

What if I miss a dose?

Give it when you remember, unless the next dose is almost due. Then skip the missed one and carry on. Never give two doses at once.

Is it normal for Zyrtec to make my dog tired?

Mild sleepiness can happen, mostly in small dogs at higher doses. Tell your vet if it's more than mild or if your dog is a working dog.

How should I store Zyrtec?

At room temperature, in the original container, out of reach of pets and children. Follow the label for syrups.

Can I give my dog Zyrtec-D?

No. Zyrtec-D contains pseudoephedrine, which is toxic to dogs. If your dog ate some, call your vet or a poison helpline now.

Does my dog need a prescription for Zyrtec?

You can buy Zyrtec without one, because it is sold for people. It isn't approved for dogs, though, so your vet should decide whether it fits, which product, and how much.

How long does Zyrtec take to work in dogs?

Usually 1 to 2 hours. If you see no change in itching, talk to your vet rather than raising the dose yourself: in many itchy dogs, antihistamines simply aren't strong enough.

The Bottom Line

Plain cetirizine is a low-risk drug that does one narrow job well: blocking histamine. That makes it useful for stings, hives and vaccine reactions, and a modest add-on for mildly allergic dogs, given before trouble starts. It is not the answer to a dog scratching itself raw. If your dog reacts to stings or vaccines, or allergy season is coming, ask your vet now whether cetirizine belongs in the plan, and which box and dose to use, so you're not guessing at the medicine cabinet at 10 pm.

Originally written by Dr. Catharina Hjorth. Updated by the I Love Veterinary editorial team.

Sources

• Merck Veterinary Manual: Atopic Dermatitis in Dogs
• Merck Veterinary Manual: Antihistamine Dosages for Integumentary Disease in Animals
• Merck Veterinary Manual: Toxicoses in Animals From Human Cold and Allergy Medications
• Merck Veterinary Manual: Xylitol Toxicosis in Dogs
• Merck Veterinary Manual: Anaphylactic Reactions and Atopy in Dogs
• Olivry et al.: Treatment of canine atopic dermatitis, 2015 updated guidelines from the ICADA (BMC Veterinary Research)
• Cook et al.: Treatment of canine atopic dermatitis with cetirizine (Canadian Veterinary Journal, 2004)
• Hsiao et al.: Effects of cetirizine in dogs with chronic atopic dermatitis (Journal of Veterinary Science, 2016)
• Ekstrand et al.: Cetirizine per os, exposure and antihistamine effect in the dog (Acta Veterinaria Scandinavica, 2018)
• Banovic et al.: Effect of diphenhydramine and cetirizine on immediate and late-phase cutaneous allergic reactions in healthy dogs (Veterinary Dermatology, 2020)
• Veterinary Partner (VIN): Cetirizine Hydrochloride (Zyrtec)
• VCA Animal Hospitals: Cetirizine
• PetMD: Cetirizine for Dogs
• Vetster: Does Zyrtec work for dog allergy symptoms?
• DailyMed: Zyrtec-D (cetirizine and pseudoephedrine) label
• DailyMed: Zyrtec Allergy (cetirizine) tablets label
• ASPCA Animal Poison Control Center
• Pet Poison Helpline

This article is for general information and is not a substitute for advice from your veterinarian.