Black, tarry diarrhea in dogs is a sign of digested blood in the stool, and it typically points to bleeding somewhere in the upper digestive tract (the stomach or small intestine). Veterinarians call this “melena.” It looks different from fresh red blood in stool because the blood has been broken down by digestive enzymes as it travels through the gut, turning it dark and giving it a distinctive tar-like, sticky consistency. This is not something to wait out. A dog passing black diarrhea needs veterinary attention promptly.
Why the Stool Looks Black
When bleeding occurs high up in the digestive tract, hemoglobin (the protein that makes blood red) gets broken down as it moves through the stomach and intestines. By the time it reaches the colon, it has turned dark black. The result is stool that looks like tar: shiny, sticky, and very dark. If you’re seeing bright red blood instead, that bleeding is coming from lower in the digestive tract, closer to the colon or rectum, and represents a different set of concerns.
The black color combined with diarrhea suggests both active bleeding and intestinal irritation. The diarrhea component means the gut is moving contents through faster than normal, which can happen when the lining is inflamed or ulcerated.
Common Causes of Black Diarrhea in Dogs
Pain Medications and Anti-Inflammatory Drugs
One of the most frequent causes of upper GI bleeding in dogs is NSAIDs, the same class of pain relievers many people take. All NSAIDs carry the potential to cause serious stomach and intestinal ulcers in dogs, but some are especially dangerous. Ibuprofen is particularly harmful because of the way a dog’s body recirculates the drug through the liver, amplifying its effect on the gut lining. If your dog got into a bottle of ibuprofen, naproxen, or any human pain reliever, this could be the cause.
Even veterinary-prescribed NSAIDs can cause problems. In one study, over 83% of dogs on long-term NSAID therapy had erosions in their gastrointestinal lining. The risk gets significantly worse when an NSAID is combined with a steroid, or when a dog is given two different anti-inflammatory drugs at the same time. High doses of certain steroids like dexamethasone can also cause ulcers on their own.
Hookworms and Other Parasites
Hookworms are a classic cause of black stool in dogs, especially puppies. These parasites latch onto the wall of the small intestine with sharp, hook-shaped mouths and feed on blood. When they detach and move to a new spot, they leave behind small bleeding ulcers. This ongoing blood loss adds up, and the digested blood turns the stool dark. Even after treatment, the ulcers left behind by dying hookworms can continue to bleed for several days, sometimes temporarily worsening anemia before the dog improves.
Stomach Ulcers and Erosions
Beyond medications, ulcers can develop from liver disease, kidney disease, tumors in the stomach or intestine, and conditions that cause excess stomach acid production. Foreign objects that a dog has swallowed can also scrape or puncture the stomach lining, leading to bleeding. Severe stress or shock that reduces blood flow to the gut can damage the lining as well.
Things That Turn Stool Black Without Bleeding
Before panicking, consider whether your dog recently ingested something that could darken stool without any bleeding at all. Pepto-Bismol (bismuth subsalicylate) is a well-known culprit. It turns stool jet black and can easily be mistaken for melena. Iron supplements, activated charcoal, blueberries in large quantities, and black licorice can do the same thing.
The key difference is texture and consistency. Stool darkened by food or medication tends to be formed and firm, not tarry or sticky. True melena has a characteristic glossy, tar-like appearance and a notably foul smell that’s different from normal stool. If the black stool is also diarrhea, that combination makes a harmless dietary cause much less likely.
Signs Your Dog Is Losing Too Much Blood
Black diarrhea means blood is leaving the body, and if it continues, your dog can become anemic. You can check for this at home by looking at your dog’s gums. Healthy gums are a medium pink color. Pale pink or white gums suggest anemia, shock, or poor circulation, all of which can result from internal bleeding.
You can also test your dog’s circulation with a simple press test. Push your finger gently against the gum until the spot turns white, then release. The color should return to pink in less than two seconds. If it takes longer, blood flow is compromised, and your dog needs emergency care. Other warning signs include lethargy, weakness, rapid breathing, loss of appetite, vomiting (especially if the vomit looks like coffee grounds, which is also digested blood), and collapse.
What Your Vet Will Do
The first priority is figuring out where the bleeding is coming from and how much blood your dog has lost. Your vet will likely run blood work to check for anemia and evaluate organ function, since liver and kidney disease can both contribute to GI bleeding. A fecal test can identify parasites like hookworms. Abdominal imaging with X-rays or ultrasound helps spot foreign objects, masses, or other structural problems. In some cases, an endoscopy (a tiny camera passed into the stomach) is needed to directly visualize ulcers or tumors.
Treatment depends entirely on the cause. If parasites are responsible, deworming medication resolves the problem, though your vet may monitor for worsening anemia in the days right after treatment as the parasites detach. If ulcers from NSAIDs or other causes are the issue, your vet will use medications that reduce stomach acid and protect the gut lining, allowing the ulcers to heal. Any offending medication will be stopped immediately. Dogs with severe blood loss may need IV fluids or, in critical cases, a blood transfusion.
What to Do Right Now
If your dog is actively passing black, tarry diarrhea, stop giving any NSAIDs or over-the-counter pain medications immediately. Don’t offer Pepto-Bismol, as the bismuth component can make it impossible to tell whether the black color is from bleeding or from the medication itself, and the salicylate in it can actually worsen stomach ulcers. Pick up your dog’s food temporarily to give the stomach a rest, but keep fresh water available.
Try to collect a small sample of the stool in a plastic bag or container. Your vet will want to see it, and it can be tested for hidden blood or parasites. Note when the black stool started, how many episodes there have been, whether your dog has eaten anything unusual, and whether they have access to any medications, rat poison, or garbage. All of this information helps your vet narrow down the cause faster. If your dog’s gums are pale, if they seem weak or disoriented, or if the diarrhea is frequent and voluminous, treat it as an emergency and go to the nearest veterinary clinic right away.