When your dog develops diarrhea after taking Zyrtec, it can be alarming — especially if it comes on suddenly or is accompanied by other symptoms.
While Zyrtec (cetirizine) is generally considered safe for dogs when prescribed by a vet, some pups can experience gastrointestinal side effects, including loose or watery stools.
We outline the common causes of diarrhea in dogs taking Zyrtec, what you can do at home, and when to seek veterinary help.
Table of Contents
Dog Diarrhea and Zyrtec — Why It Happens
Dog diarrhea related to Zyrtec use can happen for several reasons, including mild allergic reactions, stomach irritation, or sensitivity to the medication itself.
Some dogs may also experience diarrhea due to an unrelated digestive issue that just happens to occur while taking Zyrtec — such as dietary changes, bacterial imbalance, or an underlying illness.
Zyrtec is an antihistamine designed to reduce allergic reactions, but in rare cases, it can interfere with normal gut function or trigger mild nausea and diarrhea.

Dog Diarrhea and Zyrtec: Common Causes
1. Gastrointestinal Sensitivity to Zyrtec
Some dogs have naturally sensitive stomachs and may not tolerate certain medications well.
When Zyrtec enters the digestive tract, it can sometimes irritate the stomach lining or alter gut motility, leading to loose stools or diarrhea.
This reaction is usually mild and short-lived — typically resolving once the medication leaves the dog’s system.
However, if diarrhea continues for more than a day or two, it’s best to contact your vet to rule out a more serious underlying issue or confirm whether the Zyrtec dosage needs adjusting.
Read more: Dog Diarrhea Prednisone (What It Means and What to Do)
2. Incorrect Dosage or Dosing Frequency
While Zyrtec is safe for many dogs, the dosage must be precise — typically based on weight and given only once daily.
Accidental overdosing (even slightly) can stress the digestive system and cause watery diarrhea, vomiting, and lethargy. Overmedication can also affect the liver and kidneys over time if left unaddressed.
Always follow your vet’s exact dosage instructions and avoid guessing based on human tablet sizes, since Zyrtec formulations can vary (especially those with added ingredients like pseudoephedrine, which is toxic to dogs).
3. Allergic Reaction or Sensitivity to Ingredients
Ironically, Zyrtec — an allergy medication — can sometimes cause mild allergic or sensitivity reactions in dogs.
The cetirizine itself is usually not the culprit; rather, the inactive fillers, dyes, or coating materials in certain over-the-counter brands can trigger irritation or diarrhea.
If your dog develops additional symptoms such as excessive drooling, paw licking, facial swelling, or itchiness alongside diarrhea, it may indicate an adverse reaction. Your vet can recommend an alternative antihistamine or compounded formula that’s easier on your dog’s system.
4. Interaction with Other Medications or Supplements
If your dog is taking other medications — such as steroids, antibiotics, or supplements — these can interact with Zyrtec and intensify digestive upset.
The liver metabolizes most antihistamines, so combining them with other drugs that affect liver function can increase the chance of side effects, including diarrhea.
Always let your vet know about every medication or supplement your dog takes before introducing Zyrtec. Even natural products like fish oil or probiotics can occasionally alter how drugs are absorbed in the gut.
5. Unrelated Digestive Issues Coinciding with Zyrtec Use
Sometimes the timing of diarrhea is just a coincidence.
Your dog might be experiencing digestive upset from something entirely different — such as eating table scraps, switching food brands, or picking up a mild intestinal infection.
Because Zyrtec is often given for allergies, many dogs are already battling itchy skin, paw chewing, or licking — which can lead to bacterial ingestion from grooming. This means that even if your dog is on Zyrtec, the diarrhea may not be caused by the medication itself, but rather by a secondary digestive problem that occurred at the same time.
6. Underlying Health Conditions
In some cases, dogs develop diarrhea after taking Zyrtec because of underlying illnesses that the medication temporarily irritates.
Conditions like pancreatitis, inflammatory bowel disease (IBD), or liver disease can make dogs more sensitive to medications. These dogs may react with vomiting, fatigue, or diarrhea whenever a new drug is introduced.
If your dog has a history of chronic digestive issues, always discuss it with your vet before adding antihistamines or other allergy medications to their routine.
Read more: Dog Diarrhea Due to Medication (Why it happens and how to help)
What to Do If Your Dog Has Diarrhea After Taking Zyrtec
If your dog has mild diarrhea but is otherwise acting normal, you can try a few at-home care steps to help them recover while monitoring their condition closely.
Start by withholding food for about 12 hours (not water) to give the digestive system a short rest. Then, reintroduce a bland diet of plain boiled chicken and white rice in small, frequent portions.
Keep your dog well-hydrated, as diarrhea can cause rapid fluid loss. You can add a small amount of unflavored Pedialyte to their water for extra electrolytes, but check with your vet first — especially for small or elderly dogs.
If your vet confirms that Zyrtec is the likely cause, they may recommend stopping the medication temporarily or switching to another antihistamine like Benadryl (diphenhydramine) or Apoquel. Never stop or switch medications without veterinary guidance, as your dog’s allergy symptoms may worsen.
You can also ask your vet about adding a probiotic supplement or prescription gastrointestinal support formula to restore gut balance and reduce inflammation.
During recovery, monitor your dog’s stool consistency and frequency. If diarrhea persists beyond 48 hours, or if your dog becomes lethargic, refuses food, or starts vomiting, professional care is essential.
When to Call or Visit Your Vet
While many mild medication-related diarrhea cases resolve within a day, you should call or visit your vet right away if your dog shows any of the following:
Persistent diarrhea lasting more than 48 hours
Vomiting, especially if it’s frequent or contains bile or blood
Weakness, shaking, or lethargy
Loss of appetite or refusal to drink water
Black, tarry, or bloody stools
Signs of dehydration (dry gums, sunken eyes, skin tenting)
Your vet may recommend bloodwork or a stool test to determine whether the diarrhea is medication-related or caused by infection, parasites, or another underlying condition.
If Zyrtec is confirmed as the trigger, your vet can advise a safer alternative or adjust the dosage. They may also prescribe medications to soothe your dog’s digestive tract, such as metronidazole or probiotics, depending on the severity.
Read more: Dog Diarrhea No Other Symptoms (What it means)
Key Takeaway
Most dogs tolerate Zyrtec very well, but for a few, diarrhea can be an uncomfortable side effect — especially if the dose is off or the dog’s stomach is sensitive.
If your dog develops diarrhea after taking Zyrtec, monitor them closely, provide gentle home care, and keep them hydrated. If symptoms persist or worsen, contact your veterinarian to determine whether the medication is to blame or if another underlying condition is at work.
With the right support and adjustments, most dogs bounce back quickly — and can continue finding relief from allergies without digestive discomfort.