Tarry stool in dogs is dark, sticky, almost black feces caused by digested blood from somewhere in the upper gastrointestinal tract. Veterinarians call it melena. Unlike bright red blood in stool (which signals bleeding in the lower intestine or colon), tarry stool gets its distinctive color and texture because blood has been partially broken down by stomach acid and digestive enzymes as it moves through the gut. If your dog’s stool looks like black tar or coffee grounds, that’s a sign of bleeding that started in the stomach or upper small intestine.
Why the Stool Turns Black
When blood enters the stomach or upper small intestine, it’s exposed to hydrochloric acid and digestive enzymes that break down hemoglobin, the oxygen-carrying protein in red blood cells. This chemical process converts the iron in hemoglobin into a dark compound, turning the blood from red to black. The longer blood sits in the digestive tract, the darker and stickier it becomes. By the time it reaches the colon and exits the body, it has that unmistakable tarry appearance and a notably foul smell, different from normal stool odor.
This is different from a dog that eats something dark-colored (like activated charcoal treats or certain foods containing iron supplements), which can also darken stool temporarily. True melena has a glossy, tar-like consistency and a particularly strong, metallic odor.
Common Causes
NSAIDs (non-steroidal anti-inflammatory drugs) and liver disease are the two most commonly reported risk factors for stomach ulcers in dogs, and ulcers are the leading cause of upper GI bleeding. Dogs that accidentally swallow human NSAIDs like ibuprofen or naproxen are at high risk. Even veterinary-prescribed pain medications in this class can cause erosions. One study using endoscopy found that 83% of dogs on chronic NSAID therapy had GI erosions, and 75% had visible stomach erosions specifically. Dogs receiving NSAIDs had roughly three times the risk of developing an erosion compared to untreated dogs.
Corticosteroids (like prednisone) also increase the risk of GI ulcers, particularly when combined with NSAIDs. Other common causes include:
- Foreign bodies: Sharp objects like bone fragments or sticks can scrape or puncture the stomach lining.
- GI tumors: Several tumor types can cause bleeding in the stomach or intestines, including lymphoma, adenocarcinoma, and tumors of the smooth muscle or blood vessels.
- Liver disease: The liver produces many of the proteins responsible for blood clotting. Dogs with liver disease often develop clotting problems, making them vulnerable to GI bleeding and ulcers.
- Clotting disorders: Conditions that impair the blood’s ability to clot, whether inherited or caused by toxin exposure (like rat poison), can lead to bleeding anywhere in the body, including the GI tract.
- Ischemic events: Conditions that cut off blood supply to part of the intestine, such as gastric dilatation-volvulus (bloat) or intestinal twisting, can cause tissue damage and bleeding.
Signs That Accompany Tarry Stool
Tarry stool on its own tells you bleeding is happening. What matters next is how much blood your dog is losing and how quickly. Mild, intermittent bleeding may produce occasional dark stools without other obvious symptoms. Significant or ongoing blood loss, however, triggers visible changes in your dog’s behavior and appearance.
Pale gums are one of the most reliable indicators of serious blood loss. Healthy dog gums are pink and moist. If they look white, gray, or bluish, that signals dangerously low blood volume. You can also check capillary refill time by pressing a finger briefly against the gum and watching the color return. If it takes longer than two seconds, circulation is compromised. A normal packed cell volume (a basic measure of red blood cells) in dogs is 35 to 57%. When it drops below that range, your dog is anemic.
Other warning signs of significant blood loss include weakness or collapse, rapid breathing, a racing heart rate, cold paws and ears, loss of appetite, vomiting (sometimes with blood that looks like coffee grounds), and unusual low energy. A dog showing any combination of these signs alongside dark stool needs emergency veterinary care.
How Vets Find the Bleeding Source
Unexplained GI blood loss is a recognized cause of regenerative anemia in dogs, so identifying the bleeding source is critical. Your vet will typically start with bloodwork, checking red blood cell levels, clotting function, kidney and liver values, and albumin (a protein that drops when blood is being lost). Low albumin or a low red blood cell count with signs the body is trying to make new red blood cells points strongly toward active bleeding.
A fecal occult blood test can confirm the presence of hidden blood in stool, which is useful when the tarry appearance is subtle or when your vet suspects ongoing low-grade bleeding. The standard veterinary version uses guaiac-impregnated paper that reacts to the peroxidase activity of hemoglobin. Fecal samples can come from naturally voided stool or from a direct rectal exam.
Imaging comes next if the cause isn’t obvious from the history (like recent NSAID use). Abdominal ultrasound can reveal masses, thickened intestinal walls, or free fluid. Endoscopy, where a small camera is passed into the stomach and upper intestine, gives a direct view of ulcers, erosions, or tumors and allows biopsies to be taken at the same time.
Treatment and Recovery
Treatment depends entirely on the underlying cause, but nearly all cases of upper GI bleeding involve acid suppression to help the damaged tissue heal. Proton pump inhibitors, the same class of acid-blocking drugs used for human heartburn, are considered the standard of care for stomach ulcers and erosions in dogs. An expert consensus statement from the American College of Veterinary Internal Medicine concluded that these medications are superior to older acid blockers, coating agents, and other protective drugs for most causes of GI ulceration. Twice-daily dosing is more effective than once daily for achieving adequate acid suppression.
If NSAIDs caused the bleeding, stopping the medication is the first and most important step. For dogs that were on anti-inflammatory drugs for pain management, your vet will find an alternative approach. If a tumor is responsible, treatment may involve surgery, chemotherapy, or both depending on the tumor type and location. Liver disease and clotting disorders require their own targeted treatments alongside GI support.
Dogs with severe blood loss may need fluid support or blood transfusions to stabilize them before the underlying problem can be addressed.
Feeding During Recovery
Nutrition plays a supporting role in GI recovery. Veterinary therapeutic diets formulated for gastrointestinal problems are the preferred option. These are designed to be highly digestible so your dog absorbs more nutrients from less food, which reduces the workload on a healing gut. Depending on the situation, your vet may recommend a low-fat version, a high-fiber formula, or a hypoallergenic diet if food sensitivity is a factor.
How you feed matters as much as what you feed. Smaller, more frequent meals are easier on a recovering stomach. If your dog normally eats twice a day, splitting that into four or more smaller portions reduces the volume hitting the stomach at once. The traditional advice of boiled chicken and white rice is falling out of favor. While it’s gentle on the stomach short-term, it’s deficient in more than 10 essential nutrients for dogs and isn’t a good choice beyond a day or two without veterinary guidance.
What to Watch For at Home
If you notice a single episode of slightly dark stool and your dog is otherwise acting normally, eating well, and has pink gums, it’s worth calling your vet for guidance but may not be an emergency. Some dietary factors can darken stool temporarily.
Multiple episodes of tarry stool, or even one episode paired with lethargy, pale gums, vomiting, or loss of appetite, is a different situation. Dogs can lose a significant amount of blood into the GI tract before external signs become obvious, so tarry stool should never be dismissed as “probably nothing.” If your dog is currently taking any anti-inflammatory or steroid medication and you notice dark stool, that combination is especially worth urgent attention, given how frequently these drugs cause GI erosions.