The most common reason dogs get diarrhea is dietary indiscretion, which is the veterinary term for eating something they shouldn’t have. But the list of possible causes is long, ranging from a simple food switch to parasites, infections, stress, medications, and chronic digestive diseases. Most cases resolve on their own within a day or two, though some signal something more serious.
Eating Something They Shouldn’t Have
Dogs are scavengers by nature, and their stomachs often pay the price. Raiding the trash, scarfing down table scraps, chewing on sticks or toys, or snacking on another animal’s feces can all trigger a bout of loose stool. Fatty foods are especially problematic. Nuts like almonds, pecans, and walnuts contain high levels of fat that can cause vomiting and diarrhea, and in some cases trigger pancreatitis. Dairy products are another frequent offender because dogs lack significant amounts of the enzyme needed to break down lactose.
Several common human foods go beyond simple stomach upset and are genuinely toxic. Chocolate, coffee, and anything containing caffeine can cause diarrhea along with panting, abnormal heart rhythm, and seizures. Alcohol in any form, including food products cooked with it, can cause diarrhea, breathing difficulty, and worse. Coconut flesh and coconut milk contain oils that commonly cause loose stools. Even salty snacks in large quantities can lead to diarrhea, tremors, and salt toxicity.
Switching Foods Too Quickly
Abruptly changing your dog’s food is one of the most overlooked causes of diarrhea. A dog’s gut bacteria are adapted to whatever they’ve been eating, and a sudden switch doesn’t give that microbial community time to adjust. The standard recommendation is to transition over 7 to 10 days, gradually mixing increasing amounts of the new food with the old. This applies to switching brands, protein sources, or even moving between kibble and wet food.
Intestinal Parasites
Parasites are a leading cause of diarrhea, particularly in puppies, dogs from shelters, and dogs that spend time in parks or kennels. In one study of dogs presenting with diarrhea, over 25% tested positive for at least one parasite. Giardia was the most common, found in nearly 13% of cases. Cryptosporidium came in second at about 5%, followed by coccidia and roundworms (Toxocara) at roughly 4% and 3.5% respectively. Whipworms and tapeworms were less common but still present.
Each parasite produces slightly different symptoms. Giardia often causes greasy, foul-smelling stool that may be soft rather than fully liquid. Roundworms can sometimes be visible in the stool as spaghetti-like strands. Whipworms tend to cause mucus-coated stool, sometimes with streaks of blood. Many parasitic infections don’t cause obvious symptoms beyond intermittent loose stool, which is why routine fecal testing matters even if your dog seems otherwise healthy.
Viral and Bacterial Infections
Parvovirus is the most dangerous viral cause of diarrhea in dogs, especially in unvaccinated puppies. The virus targets rapidly dividing cells in the intestinal lining, destroying the tissue that absorbs nutrients and acts as a barrier against bacteria. This damage allows gut bacteria to cross into the bloodstream, which can trigger a life-threatening inflammatory response. Parvovirus diarrhea is typically severe, watery, and often bloody, accompanied by vomiting, lethargy, and fever.
Canine coronavirus (not the same virus that causes COVID-19 in humans) attacks the surface cells of the intestinal lining, causing villous damage and diarrhea that ranges from mild to moderate. Rotavirus works similarly, blunting the absorptive surfaces of the small intestine. Both are generally less severe than parvovirus but can be serious in very young or immunocompromised dogs. Bacterial infections from Salmonella, E. coli, and Campylobacter can also cause acute diarrhea, often picked up from contaminated food, water, or contact with infected animals.
Stress and Environmental Changes
Boarding, traveling, moving to a new home, a visit to the vet, or even a change in household routine can trigger what’s sometimes called stress colitis. The gut and brain are tightly connected, and anxiety or excitement can speed up intestinal contractions, pulling water into the colon faster than it can be absorbed. The result is soft stool or full diarrhea, often with mucus. Stress-related diarrhea usually resolves once the dog returns to a familiar, calm environment, though it can persist for a few days.
Medications
If your dog recently started a new medication, that may be the culprit. Antibiotics are one of the most common drug-related causes of diarrhea because they disrupt the balance of beneficial bacteria in the gut. Giving a probiotic alongside antibiotics can help prevent or reduce this side effect. Nonsteroidal anti-inflammatory drugs (NSAIDs) and steroids like prednisone carry a different risk: they can cause gastrointestinal ulcers, which lead to diarrhea that may contain blood or appear dark and tarry. If you notice these signs while your dog is on any medication, contact your vet promptly.
Chronic Digestive Diseases
When diarrhea lasts more than two to three weeks or keeps recurring, the cause is more likely a chronic condition. In a retrospective study of 136 dogs with chronic diarrhea, 90% had a primary intestinal disease. Chronic inflammatory conditions of the gut were the most frequent diagnosis, accounting for 79% of those cases. About 16% of dogs were ultimately diagnosed with inflammatory bowel disease (IBD), which requires long-term management with dietary changes and sometimes immunosuppressive medication.
The remaining 10% had diseases outside the digestive tract that were causing secondary gut problems. Exocrine pancreatic insufficiency, where the pancreas doesn’t produce enough digestive enzymes, was the most common of these, followed by chronic pancreatitis. Hormonal disorders like Addison’s disease and hypothyroidism, along with liver disease, kidney disease, and even heart failure, accounted for a small number of cases. Intestinal lymphoma, a type of cancer, was also identified and tended to cause severe disease.
How to Check for Dehydration
Diarrhea’s biggest immediate risk is dehydration, especially in small dogs and puppies. You can check hydration at home using the skin tent test: gently pinch the skin along the top of your dog’s head (parallel to the ridge of the skull) between your thumb and index finger for about two seconds, then release. In a well-hydrated dog, the skin snaps back immediately. If it stays tented or returns slowly, your dog may have lost enough body water to need veterinary attention. This test can detect fluid loss as low as 1.8% of body weight. You can also press a finger against your dog’s upper gum and release. The spot should return to its normal pink color within two seconds. A delayed return suggests dehydration, though this sign tends to show up later than the skin test.
Managing Mild Diarrhea at Home
For an otherwise healthy adult dog with mild diarrhea and no other symptoms, a brief period of reduced feeding can give the gut time to recover. Skip rich treats and stick to easily digestible food. The old standby of boiled chicken breast and white rice has fallen somewhat out of favor because it’s deficient in more than 10 essential nutrients for dogs. Commercially prepared gastrointestinal diets are now considered a better option because they’re nutritionally complete while still being gentle on the stomach.
How long you stay on a bland or GI-specific diet depends on the cause. If the trigger was something simple and self-limiting, like a dietary indiscretion or a mild stomach bug, your dog can typically return to regular food as soon as stools firm up. If the diarrhea stemmed from a diagnosed condition like pancreatitis, IBD, or food allergies, the dietary change may need to be longer term or permanent.
Signs That Need Prompt Veterinary Care
Not every case of diarrhea warrants a vet visit, but certain signs raise the urgency. Blood in the stool, whether bright red or dark and tarry, suggests damage to the intestinal lining or ulceration. Vomiting alongside diarrhea accelerates fluid loss and makes dehydration more likely. Lethargy, refusal to eat for more than 24 hours, or a fever all point toward something more than a simple upset stomach. Puppies, senior dogs, and dogs with existing health conditions have less margin for error and should be seen sooner. Any diarrhea lasting more than two days without improvement, or diarrhea that keeps coming back every few weeks, is worth investigating with your vet even if your dog seems fine between episodes.