My dog ate one 10 mg Zyrtec tablet.

Merck describes cetirizine at 1-4 mg/kg daily as well tolerated in dogs, and 10 mg works out to about 2 mg/kg for an 11 lb dog. Lethargy, GI upset, and hyperactivity are still reported, so call your vet with the weight and dose — and check whether it was actually Zyrtec-D, which changes everything.

Can I give Benadryl for my dog's itching or a bee sting?

Ask your veterinarian first, by phone if needed. The dose has to be calculated per kilogram, the product must be plain diphenhydramine with no decongestant or acetaminophen, and some conditions need a different drug entirely. Guessing on the label's human dose is how overdoses happen.

She took an allergy pill and now she's wired, pacing, and shaking.

Agitation and tremors instead of sedation are recognized reactions, and they can precede seizures — this is not a dog to leave overnight. Check whether the product contained a decongestant, then call your vet or an emergency clinic now with the ingredients and the time.

Antihistamines

Toxic

Human antihistamines can leave a dog heavily sedated or, just as often, agitated with a racing heart, tremors, and seizures. The combination "-D" products with pseudoephedrine are far more dangerous than the plain drug.

This food guide has not been reviewed by a veterinarian. It summarises general information from sources and should not be used to decide whether a food is suitable for an individual dog or to assess an emergency.

Antihistamines · Illustration — Illustration

Check before feeding

Do not offer as a treat

There is no recommended treat portion for this hazardous item. If swallowed, contact a vet with the amount, time and product details. Do not induce vomiting yourself.

Signs and health concerns

This list includes both observable signs and conditions identified through examination or testing. It cannot establish the cause or whether an exposure is safe.

Lethargy
Drowsiness or reduced alertness
Agitation
Excessive activity
Fast heartbeat
Dilated pupils
Tremors
Vomiting
Low blood pressure
Overheating
Seizures

More about this food

Plenty of veterinarians do use antihistamines in dogs. That is a decision about which drug, what dose per kilogram, and which formulation — not a green light to split a tablet from your own medicine cabinet. The Merck Veterinary Manual separates them clearly. Diphenhydramine (Benadryl), a first-generation drug, is dosed therapeutically in dogs at 2-4 mg/kg with an onset of 30-60 minutes; in overdose it causes lethargy and sedation or the opposite — agitation — plus tachycardia, vomiting, hypotension, mydriasis, and tremors, and it can cause seizures. Chlorpheniramine produces mild signs below 1 mg/kg with an onset of 2-4 hours. Among second-generation drugs, cetirizine (Zyrtec) at 1-4 mg/kg daily is described as well tolerated by dogs and cats, with lethargy, GI upset, and hyperactivity reported, and loratadine (Claritin) produces mild signs below 10 mg/kg. Same category, very different risk. Three American products cause most of the trouble. First, anything with a "-D" after it: Zyrtec-D, Claritin-D, Allegra-D, and Benadryl-D contain pseudoephedrine or phenylephrine, and Merck notes explicitly that combination products can result in more serious clinical signs. Second, Tylenol PM and similar nighttime products, which pair diphenhydramine with acetaminophen — a second toxin with its own liver and red-cell effects. Third, chewable and liquid children's formulations, which taste good enough that a dog will finish the bottle. So start with the label, not the drug name. Whether this is a single-ingredient antihistamine or a multi-symptom combination decides how urgent it is. Then note the product, every active ingredient, the strengths, the number of doses missing, the time, and your dog's weight, and call your veterinarian, an emergency clinic, or the ASPCA Animal Poison Control Center at (888) 426-4435. Do not induce vomiting at home; Merck states that emesis induction is not appropriate for most animals that have ingested first-generation antihistamines, because they act too quickly. Sedation, heart rate and temperature control, and fluids are veterinary decisions.

Frequently asked questions

My dog ate one 10 mg Zyrtec tablet.: Merck describes cetirizine at 1-4 mg/kg daily as well tolerated in dogs, and 10 mg works out to about 2 mg/kg for an 11 lb dog. Lethargy, GI upset, and hyperactivity are still reported, so call your vet with the weight and dose — and check whether it was actually Zyrtec-D, which changes everything.
Can I give Benadryl for my dog's itching or a bee sting?: Ask your veterinarian first, by phone if needed. The dose has to be calculated per kilogram, the product must be plain diphenhydramine with no decongestant or acetaminophen, and some conditions need a different drug entirely. Guessing on the label's human dose is how overdoses happen.
She took an allergy pill and now she's wired, pacing, and shaking.: Agitation and tremors instead of sedation are recognized reactions, and they can precede seizures — this is not a dog to leave overnight. Check whether the product contained a decongestant, then call your vet or an emergency clinic now with the ingredients and the time.

This information is general guidance and does not replace veterinary care. If your dog has swallowed something, contact a veterinarian immediately.