Diarrhea after starting a new medication can be alarming for dog owners. While many medications help treat illness or manage pain, they can sometimes cause digestive upset as an unwanted side effect.
We outline the common causes of diarrhea in dogs due to medication, what you can do at home, and when to seek veterinary help.
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Dog Diarrhea Due to Medication — Why It Happens
When a dog develops diarrhea after taking medication, it’s often because the drug is irritating the gastrointestinal tract or altering the gut’s normal balance.
Some medicines can increase gut motility, disrupt the microbiome, or cause inflammation of the intestinal lining. Pain medications, antibiotics, and certain anti-inflammatories are common causes.
Other times, diarrhea may not be directly caused by the drug itself but by the illness being treated, changes in diet, or stress from a vet visit or hospitalization.
Understanding Dog Diarrhea Due to Medication
Antibiotics
Antibiotics are designed to kill harmful bacteria, but they can also disrupt the healthy bacteria in your dog’s gut.
When beneficial bacteria levels drop, harmful bacteria may grow unchecked, leading to loose stools, bloating, and discomfort.
Some dogs are more sensitive than others, and diarrhea can range from mild and temporary to severe and watery.
In rare cases, antibiotic-associated diarrhea can progress to more dangerous infections, especially if dehydration sets in.
Read more: Dog Diarrhea Due To Stress (Here’s why)
Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)
NSAIDs such as carprofen, meloxicam, or deracoxib are often prescribed for pain and inflammation.
While effective, they can sometimes irritate the stomach and intestines, leading to diarrhea, vomiting, or loss of appetite. This irritation is partly due to the way NSAIDs reduce protective stomach lining secretions.
If diarrhea is severe, persistent, or accompanied by blood, this can signal ulceration or gastrointestinal bleeding, which requires urgent veterinary care.
Steroids
Steroids like prednisone can cause increased thirst, appetite changes, and sometimes diarrhea.
These drugs affect many body systems, including the digestive tract, and may alter fluid balance in the intestines.
For some dogs, this side effect is mild and temporary, but for others—especially those on long-term steroid therapy—diarrhea can become chronic without supportive care.
Dewormers and Parasitic Medications
Deworming medications are generally safe, but as they kill off parasites, the digestive tract may react with loose stools or even mild cramping.
This reaction is often short-lived and a sign that the medication is working. However, if diarrhea lasts more than a couple of days or becomes severe, it could mean your dog is reacting poorly to the drug or that there’s another underlying infection.
Chemotherapy Drugs
Chemotherapy medications are strong and can damage rapidly dividing cells, including those lining the intestines.
This can lead to nausea, vomiting, and diarrhea. In some cases, the diarrhea may be severe enough to cause dehydration and electrolyte imbalances.
Because chemotherapy already puts stress on your dog’s immune system, any digestive upset should be addressed quickly to avoid secondary infections.
Sudden Change in Medication or Dosage
Changing a dog’s medication routine too quickly can upset their stomach.
This might happen if a new drug is introduced, the dosage is increased, or multiple medications are given at once. Even if each drug is safe individually, the combination or rapid change can overwhelm the digestive system.
Introducing medications gradually—when possible—and monitoring for side effects can help reduce the risk.
What to Do If Your Dog Has Diarrhea Due to Medication
If your dog develops mild diarrhea after starting a medication, you can often manage it at home for a short period while monitoring their condition.
Make sure your dog has constant access to fresh water, as diarrhea can cause dehydration quickly. Offering an electrolyte solution recommended by your vet may help maintain fluid balance.
You can temporarily switch to a bland diet, such as boiled chicken and rice or a prescription gastrointestinal food, to give the digestive system a break.
Probiotics designed for dogs can help restore healthy gut bacteria, especially if the diarrhea is related to antibiotics. Always check with your vet before starting supplements, as some may interfere with medications.
If the diarrhea is mild, your vet may suggest continuing the medication while managing symptoms. But if the side effects are impacting your dog’s well-being, a dosage adjustment or alternative drug might be necessary.
When to Call or Visit Your Vet
Contact your veterinarian promptly if your dog’s diarrhea is severe, watery, or persistent for more than 24–48 hours.
Seek immediate veterinary attention if you notice blood in the stool, black tarry stools, repeated vomiting, lethargy, or refusal to drink water. These may indicate gastrointestinal bleeding, severe dehydration, or a dangerous reaction to the medication.
If your dog is very young, elderly, or has an underlying health condition, do not wait—these dogs can deteriorate much faster from fluid loss.
Also let your vet know if the diarrhea started right after beginning a new drug, as they may need to change the medication or adjust the dose. Never stop a prescription abruptly without veterinary guidance, especially if the medication is treating a serious condition.
Read more: Dog Diarrhea and Not Eating (When to be concerned)
Key Takeaway
Diarrhea in dogs due to medication is a relatively common side effect, but it should never be ignored.
While mild cases may resolve on their own with supportive care, persistent or severe diarrhea can lead to dehydration and other complications.
Work closely with your veterinarian to manage symptoms, adjust treatment if needed, and ensure your dog stays comfortable and safe while getting the benefits of their prescribed medication.