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If your dog is itching or has just been diagnosed with allergies, an antihistamine is usually the first thing owners ask me about. They work best for sudden allergic reactions, the swollen face and hives kind. For ongoing seasonal itch they're much less reliable, and a fair number of dogs don't respond at all.1
Key Takeaways
Call us right away if: your dog's face or muzzle is swelling, hives are spreading across the body, they're vomiting, or they're having any trouble breathing. A reaction that affects breathing is an emergency and shouldn't wait on an antihistamine to work. Call (519) 896-0532 or see our emergency care page.
I use antihistamines mainly to treat allergic reactions, though they can help some dogs with seasonal and environmental allergies too.
They work by blocking histamine, which your dog's body releases during an allergic response.1 If you see swelling, especially around the face, histamine is usually part of what's driving it.
The catch with seasonal and environmental allergies is that response is inconsistent. Histamine is only one piece of what's making your dog itch, and the published evidence doesn't support antihistamines as a standalone itch treatment.1 In my experience, dogs with a runny nose alongside itchy skin tend to do better on one than dogs with itch alone.
For type-1 hypersensitivity reactions, the facial and eye swelling and hives you get from a bee sting, a wasp sting or a vaccine reaction, antihistamines are one of the treatments I reach for.
They're also used to prevent motion sickness. Maropitant (Cerenia®) is more effective for that, but diphenhydramine is a common alternative. Give it on an empty stomach, about an hour before travel.
Yes, with most medications. What you want to avoid is doubling up with another antihistamine, Temaril-P® or Vanectyl-P.
Antihistamines are safe alongside allergy medications like Apoquel® and Atopica®, with no known interaction. We also combine them with corticosteroids like prednisone or dexamethasone in patients who need both.
Tell your vet about every medication and supplement your dog is on before you start, so they can check for conflicts.
The ones I reach for most often:
Diphenhydramine is the most sedating of the group. It's a first-generation antihistamine, which means it crosses into the brain more readily than the newer ones do.1 Cetirizine is one of the most commonly used over the counter options, and it's the one with the better experimental showing in dogs.2
Claritin® can work, but many formulations contain pseudoephedrine, a decongestant that's toxic to dogs. Signs of poisoning can appear at 5 to 6 mg/kg.3 If you're picking up Claritin, read the label carefully.
Response is individual. One dog does well on one antihistamine and poorly on another, so degree of sedation and cost are usually what tips the decision.
Most are dosed every 8 to 12 hours. The exceptions are Claritin and Zyrtec/Reactine, which are once every 24 hours.
We dose by body weight in milligrams per kilogram. Here's what I typically use, alongside the ranges published in the 2023 AAHA allergic skin disease guidelines.4
| Drug | Typical dose | Frequency |
|---|---|---|
| Diphenhydramine (Benadryl®) | 2 to 4 mg/kg | Every 8 to 12 hours |
| Loratadine (Claritin®) | 0.5 to 1 mg/kg | Every 24 hours |
| Cetirizine (Zyrtec® or Reactine®) | 1 mg/kg | Every 24 hours |
| Hydroxyzine (Vistaril®) | 2 to 4 mg/kg | Every 8 to 12 hours |
For a weight by weight breakdown of Benadryl on its own, see our Benadryl dosing guide.
Talk to your vet before you start. Getting the right drug and the right dose for your dog matters more than starting something today.
Sedation and drowsiness are the common ones, and they show up more with first-generation antihistamines like diphenhydramine.1 Zyrtec, Reactine and Claritin are newer and less sedating.
At higher doses some dogs go the other way and become agitated or restless rather than sleepy.5 Loss of appetite, nausea, vomiting, constipation and diarrhea are all possible too.
I'm hesitant in dogs with respiratory conditions like asthma or bronchitis, liver disease, or seizure disorders like epilepsy. First-generation antihistamines also carry anticholinergic effects, which is part of why I'm careful about which dogs get them.1
Since they can be sedating, skip them ahead of anything strenuous. Swimming is the one I worry about most.
The recommended dose for Benadryl in dogs is 2-4 mg/kg of body weight every 8-12 hours. Higher doses greater than 5 mg/kg can have toxic side effects such as seizures, respiratory depression, and cardiovascular effects.
Antihistamines are generally very safe for dogs when the correct dose is used.
As some antihistamines may also contain a decongestant such as pseudoephedrine, it is important to read the label to ensure you do not accidentally give this to your dog.
Yes, while not as effective as maropitant (Cerenia®), antihistamines can be used to prevent motion sickness in dogs. The most common antihistamine used for this reason is diphenhydramine (Benadryl®). It is best to administer Benadryl® on an empty stomach and 1 hour prior to car travel.
Yes, they can be. Antihistamines seem to have the best effect when used for an acute allergy flare-up or when started at the beginning of the allergy season. Starting antihistamines when symptoms have been present for many weeks may not be helpful to your dog.
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