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Will 25 mg Benadryl hurt my dog?

Zyrtec for Dogs

Zyrtec is an over-the-counter human allergy medication sometimes given to dogs with atopic dermatitis or mild to moderate environmental allergies. Vets may recommend Zyrtec for dogs who scratch excessively, are sensitive to insect bites, or have dermatitis.


Zyrtec is available in in 5- and 10-milligram tablets. It can be given either every 24 hours for atopic dermatitis, or every 12 hours for allergy symptoms.

Here’s the recommended Zyrtec dosage for dogs based on their weight:

  • Less than 10 pounds: 2.5 milligrams (half of a 5-milligram pill or a quarter of a 10-milligram pill)
  • 10–50 pounds: One 10-milligram tablet
  • 50+ pounds: Up to two 10-milligram tablets

Zyrtec also comes in a liquid form that must be dosed with a syringe or dropper. This form can be drizzled over pet food or squirted directly into the pet’s mouth. Liquid Zyrtec may be easier to dose out for smaller dogs since the tablets are small and can be difficult to cut evenly.

Got questions or concerns about the correct dosage of Zyrtec for dogs? Use Wag! Vet Chat to connect with a veterinary expert fast. (Average response time = 6 minutes!)


Zyrtec’s active ingredient, cetirizine, is closely related to hydroxyzine, a powerful antihistamine that’s sometimes used before surgeries to help patients relax. Zyrtec is considered a second-generation antihistamine and doesn’t cause drowsiness like first-generation alternatives.

So how does Zyrtec stack up to other antihistamines? One study of dogs with atopic dermatitis found that Zyrtec reduced itching in 18% of the dogs studied. The only side effects noted in this study were vomiting in two dogs after taking the drug.

In another, more successful study, dogs who took Zyrtec before being injected with histamine had less inflammation around the injection site than those who didn’t receive the drug. This experiment had a 100% efficacy rate in the small 8-dog study group.

A separate study of 12 dogs injected with histamines found that the active ingredient in Zyrtec was notably more effective than Benadryl at minimizing the body’s immune response.

Side effects

Side effects from Zyrtec in dogs are rare, but occasionally some dogs experience:

  • Drowsiness
  • Drooling
  • Vomiting
  • Hyperactivity
  • Constipation
  • Difficulty urinating
  • Impulsiveness


Experts do not recommend Zyrtec for pregnant or breastfeeding dogs since there is little research on its use in these groups. Some veterinarians will still prescribe Zyrtec for pregnant and nursing dogs on an individual basis if the benefits outweigh the risks.

Never give Zyrtec to dogs who are allergic to antihistamines, especially those with cetirizine or hydroxyzine in them. This may cause a life-threatening reaction called anaphylactic shock.

Use this medication carefully in dogs with kidney or liver problems since it can stay in their system for much longer than healthy dogs. Stop treatment immediately if your dog begins showing signs of urine retention or other severe side effects.

Giving an antihistamine inappropriately may mask signs of other disease, so it is best to ask you veterinarian if this medication is right for your dog.

Drug interactions

  • Opioids (such as codeine, hydrocodone, or morphine)
  • Anti-anxiety or sleep medications (such as alprazolam, lorazepam, zolpidem, diazepam, and clonazepam)
  • Muscle relaxers (such as carisoprodol andcyclobenzaprine)
  • Oral or topical antihistamines (such as chlorpheniramine and diphenhydramine)
  • Pseudoephedrine (found in nasal decongestants)

Allergic reactions and sensitivity

It sounds strange, but pets can be allergic to allergy medications, including Zyrtec. Tell your vet if your pet’s allergy symptoms worsen or if they have signs of anaphylaxis like trouble breathing or swelling of the face.

Symptoms of a severe drug reaction include:

  • Hives
  • Swelling of the face, muzzle, tongue, or paws
  • Uncontrollable itching
  • Difficulty breathing
  • Confusion
  • Racing heartbeat
  • Abrupt loss of bladder or bowel control
  • Cold gums and mouth
  • Shock
  • Involuntary muscle twitching

Frequently asked questions about Zyrtec for dogs

Should I give Zyrtec to my dog with a meal?

Food does not affect Zyrtec’s efficacy, so feel free to give it to your pup with or without food.

What should I do if I miss a dose of Zyrtec for my dog?

  • For dogs taking Zyrtec twice daily: give the next dose at the usual time.
  • For dogs taking Zyrtec once daily: give the medication when you think of it, and resume the normal dosage time the next day.
  • Avoid «doubling up» to make up for missed doses.

Is it normal for Zyrtec to make my dog tired?

Zyrtec is one of the few antihistamines that do not cause drowsiness for most dogs, though there are some exceptions. If Zyrtec makes your dog sleepy, try giving it to them before bedtime to see if that helps their energy levels during the day.

How should I store Zyrtec for dogs?

Keep your pup’s Zyrtec in a cool, dry area away from bright light. Liquid Zyrtec is shelf-stable while sealed. While it may be tempting to put liquid Zyrtec in the refrigerator after opening, doing so can actually make it less effective.

Can I give my dog Zyrtec-D?

Never give your dog Zyrtec-D since it contains powerful decongestants that can harm dogs. While one pill might seem innocuous, there’s data that shows a single dose of Zyrtec-D can kill or inflict irreversible harm to small dogs.

Benadryl for Dogs

Adrienne Kruzer, RVT, LVT

Adrienne Kruzer is a veterinary technician with more than 15 years of experience providing healthcare to domestic and exotic animals. She is trained as a Fear Free Certified Professional to prevent and alleviate fear, anxiety, and stress in pets.

Updated on 01/04/22
Reviewed by

Monica Tarantino, DVM

Dr. Monica Tarantino, DVM, is a small animal veterinarian and writer with five years of general practice, emergency medicine, and geriatric pet health experience. She is certified as a Fear Free doctor. Dr. Tarantino is part of The Spruce Pets’ Veterinary Review Board.

Fact checked by

Emily Estep

Emily Estep is a biologist and fact checker focused on environmental sciences. She received a Bachelor of Arts in Journalism and a Master of Science in Plant Biology from Ohio University. Emily has been a proofreader and editor at a variety of online media outlets over the past decade and has reviewed more than 200 articles for The Spruce Pets for factual accuracy.

Jack Russell terrier itching ear in grass

Benadryl is often recommended to be given to dogs. It can be safely administered to most, but it is important to know what exactly Benadryl is, how much to give your dog, and when to know if your dog should take some.

What Is Benadryl?

Benadryl is actually the brand name for a drug called diphenhydramine. Benadryl is an antihistamine medication and does not require a prescription. It comes in tablets, capsules, and liquid gels in multiple strengths or milligrams. It is sometimes combined with other medications, so you will need to be sure to read the package carefully to ensure that diphenhydramine is the only drug in the product you are purchasing for your dog.

How Does Benadryl Work?

As an antihistamine, Benadryl works by blocking the release of histamine and acetylcholine that the body makes in response to an allergic reaction. Benadryl dries up watery eyes, runny noses, and decreases swelling, sneezing, itching, and other symptoms of allergies. It also is often used to make a dog sleepy or help with motion sickness.

What to Treat With Benadryl

Dogs that may benefit from receiving Benadryl are typically ones that may have allergies or are traveling. Some specific examples of when Benadryl may be used are:

  • When a dog is stung or bit by an insect.
  • When a dog is bit by a venomous reptile or insect.
  • When a dog is itching or scratching from environmental or food allergies.
  • When a dog is travelling and may experience motion sickness.
  • When a dog is in need of something to make it a little sleepy so that it doesn’t get impatient, bored, or bark when it needs to be calm and quiet.
  • When pre-treatment of an antihistamine is needed prior to a vaccination that is known to cause an allergic reaction in that dog.

Benadryl is great to keep on hand if you have a dog, even if you never plan on travelling with it or it doesn’t have any known allergies. You never know if a spider will bite or a bee will sting your dog while at home and you will need to give it some Benadryl to prevent or lessen an allergic reaction.

Some dogs react so severely to insects or venomous reptiles that they cannot breathe without antihistamines and immediate veterinary attention. If you can give a dog that has been bit by a snake or stung by an insect some Benadryl at home, you will increase its odds of making it to the veterinarian alive and still breathing. This is especially true for brachycephalic breeds such as English Bulldogs and Pugs that already naturally have compromised airways without the added airway constriction that an allergic reaction can cause.

What Not to Use Benadryl For

Benadryl may help with allergic reactions, motion sickness, and sleepiness, but that doesn’t mean it is always appropriate to use. Some reasons why you may not want to use Benadryl include:

  • If you have a highly stressed or anxious dog and want to make it sleepy. Its anxiety and stress need to be addressed and treated instead of just sedating it with Benadryl.
  • If your dog has heart disease.
  • If your dog has high blood pressure.
  • If your dog has glaucoma.
  • If your dog is taking specific medications, you should discuss the safety of Benadryl with your veterinarian prior to administering it.
  • As a sole remedy after a venomous bite. Your dog needs immediate veterinary attention.

Your veterinarian is the best source of guidance and information for your dog’s health. If you are considering giving your dog Benadryl then ask your vet first to ensure you don’t do more harm than good.

Benadryl Dosage for Dogs

Typically dogs take 1 mg of Benadryl for each pound of body weight two to three times a day. This means a 25 pound dog will take one 25 mg Benadryl tablet up to three times a day depending on your veterinarian’s recommendation. Smaller dogs may be more easily dosed using children’s Benadryl liquid.

Potential Side Effects and Risks of Benadryl

Benadryl can make a dog very sleepy, cause dry mouth or urinary retention, and potentially cause some gastrointestinal upset such as diarrhea and/or vomiting.

Benadryl Overdose

Benadryl is historically very safe but as with anything, too much can be bad. If your pet receives too much Benadryl, it may be extremely lethargic or very excited. Occasionally it may get dry mouth, have respiratory depression, seizures, go into a coma, or even be fatal if a large enough amount is administered.

Article Sources

The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

  1. Diphenhydramine. VCA Hospitals.
  2. Emergencies in Dogs. VCA Hospitals.
  3. Buchweitz, John P et al. Fatal diphenhydramine poisoning in a dog. The Canadian veterinary journal = La revue veterinaire canadienne vol. 55,11 (2014): 1089-92.
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