Diarrhoea in Dogs: Causes, What to Do and What Tests

Diarrhoea in a dog is a common problem: sometimes it is a one-off reaction to a food change, and sometimes a sign of infection, poisoning or internal organ disease. Let's break down why a dog has diarrhoea, when it is dangerous, what to do at home and what tests help find the causes.

Common Causes of Diarrhoea in Dogs

• Diet errors: a food change, overeating, spoiled food, table scraps.
• Parasites: worms and protozoa (especially in puppies).
• Infections: viral (parvovirus in puppies — dangerous) and bacterial.
• Stress: travel, a move.
• Organ disease: pancreas, liver, inflammatory bowel disease.
• Toxins and foreign bodies.

When Diarrhoea Is Dangerous (Red Flags)

See a vet urgently if diarrhoea comes with:

• blood (bright red or black tarry stool);
• repeated vomiting, refusing food, severe lethargy;
• lasts more than 1–2 days or is very frequent/watery (dehydration risk);
• in a puppy, an old or small dog — seek help sooner.

Bloody Diarrhoea and Diarrhoea with Vomiting

Blood in the stool always warrants a vet visit: causes range from colitis to infections and foreign bodies. Diarrhoea together with vomiting is dangerous for fast dehydration and may indicate poisoning, infection or obstruction — see also vomiting in dogs.

What to Do at Home (No Red Flags)

If the dog is active, without blood or vomiting: ensure water (dehydration is the main risk), a light fast of up to 12 hours (not for puppies!), then feed small portions of a bland diet. Do not give human anti-diarrhoea medicines without a prescription. No improvement in 1–2 days — see a vet.

What Tests to Run

Most often: a stool test (parasites, infections), a complete blood count and biochemistry — to assess inflammation, dehydration and organs; their interpretation is in dog blood test. If a foreign body or obstruction is suspected — ultrasound for dogs and cats. If the dog is also not eating — dog not eating.

To understand your pet's tests in plain language, upload the form (PDF or photo) to the pet results interpretation service. This helps you understand the result but does not replace a vet.

This article is informational. Diagnosis and treatment of your pet are the job of a veterinarian.