Black, tarry stool in dogs is almost always a sign of bleeding somewhere in the upper digestive tract, and it needs veterinary attention. The dark color comes from blood that has been digested as it passes through the stomach and intestines, a condition vets call melena. Unlike bright red blood in stool (which points to the lower colon or rectum), black tarry stool signals a problem higher up: the stomach, small intestine, or even swallowed blood from the mouth, nose, or throat.
Treatment depends entirely on what’s causing the bleeding, so the first and most important step is getting your dog to a vet. Here’s what you need to know about the causes, what the vet will look for, and what treatment typically involves.
Why Black Stool Is Different From Bloody Stool
Dogs need a surprisingly large amount of blood in their upper GI tract before their stool turns visibly black. That means if you can see it, there’s significant bleeding happening internally, even if your dog seems fine otherwise. The tarry texture and dark color result from blood being broken down by stomach acid and gut bacteria as it moves through the digestive system. This is fundamentally different from seeing streaks of fresh red blood on the outside of stool, which usually comes from the rectum or lower colon and has a different set of causes.
Common Causes of Black Stool in Dogs
Stomach Ulcers From Pain Medications
One of the most frequent causes is GI ulceration from nonsteroidal anti-inflammatory drugs (NSAIDs) like carprofen, meloxicam, or especially ibuprofen (which is particularly damaging to dogs because of how they metabolize it). These drugs strip away the stomach’s protective lining by blocking the production of compounds that normally shield the gut wall. The risk jumps significantly when an NSAID is combined with a steroid, or when a dog is given two different NSAIDs at the same time.
What makes this tricky is that most dogs with NSAID-related stomach damage show no symptoms at all. Endoscopy studies have found visible erosions, hemorrhages, or ulcers in many dogs taking these medications, yet many of those dogs appeared perfectly healthy. Black stool may be the first and only clue that something is wrong. If your dog is on any pain medication and you notice a change in stool color, contact your vet promptly.
Rat Poison Ingestion
Anticoagulant rodenticides (rat and mouse poisons) work by blocking the body’s ability to recycle vitamin K, which is essential for blood clotting. Without functional clotting, dogs begin bleeding internally. The dangerous part is the delay: clotting problems don’t show up for 2 to 5 days after ingestion, and visible bleeding, including black stool, typically appears 3 to 7 days later. If you suspect your dog got into rodenticide, don’t wait for symptoms. Early treatment before bleeding starts is far more effective.
Hookworms and Other Parasites
Hookworms (particularly Ancylostoma caninum) attach to the intestinal wall and feed on blood. A heavy infestation can cause anemia, weakness, weight loss, low protein levels, and black stool. Puppies and dogs that spend time outdoors in warm, humid climates are at highest risk. Standard deworming medications are effective against hookworms, and many monthly heartworm preventives also protect against them.
Other Possible Causes
Black stool can also result from tumors in the stomach or small intestine, blood-clotting disorders, liver disease, or even swallowed blood from a wound in the mouth or nose. In rare cases, eating large amounts of certain foods or supplements (like those high in iron) can darken stool without actual bleeding, but this produces a different appearance than the distinctive sticky, tar-like texture of true melena.
What Your Vet Will Do
Expect your vet to start with a physical exam and blood work. A complete blood count reveals whether your dog is anemic from blood loss and gives clues about infection or inflammation. Clotting tests are critical if poisoning is suspected. Your vet will likely ask about any medications your dog takes, any chance of toxin exposure, and how long the black stool has been present.
Depending on the initial findings, additional testing might include abdominal X-rays or ultrasound to look for masses or foreign objects, fecal testing for parasites, and in some cases endoscopy, where a tiny camera is passed into the stomach to directly visualize ulcers or other lesions.
How Black Stool Is Treated
Treatment targets the underlying cause, not just the symptom. There is no safe home remedy for melena. The black color itself will resolve once the bleeding stops, but stopping the bleeding requires identifying and addressing its source.
Stopping the Source of Bleeding
If an NSAID is the culprit, the first step is discontinuing the medication immediately. For rodenticide poisoning, treatment centers on replacing vitamin K through supplementation, often for several weeks. Hookworm infections are treated with deworming medications. Tumors or severe ulcers that don’t respond to medical management may require surgery.
Protecting and Healing the Gut
For ulcer-related bleeding, vets typically prescribe two types of medication working together. Acid-reducing drugs lower the amount of stomach acid attacking the damaged tissue. Separately, a mucosal protectant coats the ulcer like a bandage, shielding it from acid while it heals. These medications are given on a schedule throughout the day and may be needed for several weeks.
Dogs with significant blood loss may need intravenous fluids to maintain blood pressure, and in severe cases, a blood transfusion.
Feeding During Recovery
Your vet will likely recommend a therapeutic gastrointestinal diet during recovery. The old advice of boiled chicken and white rice has fallen out of favor. According to VCA Animal Hospitals, that combination is deficient in more than 10 essential nutrients for dogs, and the fat content varies wildly depending on which part of the chicken you use (thigh meat has twice the fat of breast meat). Commercially formulated veterinary GI diets are a better option because they’re nutritionally complete and specifically designed for dogs with digestive problems. Your vet can recommend a specific brand.
Signs That Indicate an Emergency
Any black, tarry stool warrants a vet visit, but certain combinations of symptoms mean you should go immediately rather than waiting for a regular appointment. Pale gums indicate significant blood loss. Weakness, collapse, or a distended abdomen suggest internal bleeding that may be progressing rapidly. Vomiting that looks like coffee grounds is another form of digested blood and confirms upper GI bleeding. A dog that is lethargic, refusing food, and passing black stool needs same-day veterinary care.
What Recovery Looks Like
Once treatment begins and the source of bleeding is controlled, stool color gradually returns to normal as the remaining digested blood clears the system. This can take a few days even after bleeding has stopped, simply because blood already in the intestines still needs to pass through. The overall recovery timeline varies by cause. Simple NSAID-related ulcers that are caught early may heal within a couple of weeks with proper medication and diet changes. Rodenticide poisoning requires weeks of vitamin K supplementation, and your vet will recheck clotting values before stopping treatment. Hookworm infections respond quickly to deworming, though anemic dogs may need supportive care while their red blood cell counts rebuild.
During recovery, keep track of your dog’s stool color and consistency and report any return of dark or tarry stool to your vet. Recurrence can indicate that the original problem hasn’t fully resolved or that a new issue has developed.