Black, tarry stool in dogs is called melena, and it signals that digested blood is present in the feces. This almost always points to bleeding somewhere in the upper digestive tract, typically the stomach or small intestine. The dark color and sticky, tar-like texture come from blood that has been broken down by stomach acid, which transforms the red pigment in blood into a black compound called hematin. If your dog’s stool looks like this, it’s not something to wait on. Melena warrants a same-day call to your veterinarian.
Why the Blood Looks Black, Not Red
The color of blood in your dog’s stool tells you roughly where the bleeding is happening. Black, tarry stool means the blood has traveled through the stomach and been chemically altered by digestive acids before reaching the intestines. By the time it exits, it’s dark, sticky, and often has a glistening sheen. It also tends to have a particularly foul smell compared to normal stool.
Bright red blood in stool is a different condition called hematochezia. That indicates fresher bleeding from the lower digestive tract, such as the colon or rectum. Both are concerning, but they point to different problems and different locations in the body. When speaking with your vet, describing the color and texture of the stool accurately helps narrow the diagnosis faster.
Most Common Causes of Melena in Dogs
Pain Medications and Anti-Inflammatories
One of the leading causes of upper GI bleeding in dogs is the use of nonsteroidal anti-inflammatory drugs (NSAIDs). These medications work by reducing compounds called prostaglandins, which control inflammation but also protect the stomach lining. When that protection drops, the digestive tract becomes vulnerable to ulcers and even perforations. The FDA lists bloody or black stool as a known side effect of NSAIDs in dogs. Giving two NSAIDs at the same time, or combining an NSAID with a steroid like prednisone, significantly increases the risk. If your dog is on any pain or anti-inflammatory medication and develops dark stool, contact your vet immediately.
Stomach and Intestinal Ulcers
Ulcers in the stomach or upper intestine can develop from medications, stress, liver disease, kidney disease, or other underlying conditions. These open sores bleed into the digestive tract, producing the characteristic black stool. In some cases, the bleeding is slow enough that dogs show no obvious pain but gradually become lethargic or lose their appetite.
Hookworms and Other Parasites
Hookworms are a well-documented cause of melena, especially in puppies. These parasites latch onto the intestinal wall to feed on blood, then detach and reattach at new sites, leaving small bleeding ulcers behind. A heavy hookworm burden can cause significant blood loss. Treatment involves deworming medication repeated every two to three weeks to catch new adults that were still in larval form during the first round. Severely affected dogs may need fluid therapy or even blood transfusions. Monthly heartworm preventatives that include a deworming component help prevent reinfection.
Tumors in the Digestive Tract
GI tumors are a more serious potential cause, particularly in older dogs. The most common stomach tumor in dogs is adenocarcinoma, while lymphoma is more frequently found in the small intestine. These growths can erode into blood vessels and bleed into the digestive tract. Common signs include vomiting (sometimes with blood), weight loss, decreased appetite, and lethargy, often developing gradually over weeks. Diagnosis typically requires imaging and tissue biopsy. Early detection improves the range of treatment options available.
Swallowed Blood
Not all melena originates in the stomach or intestines. Dogs can swallow blood from dental disease, a mouth injury, or bleeding in the nasal passages, and that blood gets digested just the same. If your dog has recently had dental work, a nosebleed, or an oral wound, this is worth mentioning to your vet as a possible explanation.
Foods and Supplements That Mimic Melena
Before assuming the worst, consider whether your dog has recently eaten something that could darken the stool without any bleeding involved. Charcoal treats, blueberries, and other dark-colored foods can produce uniformly dark stool that looks alarming but is harmless. Iron supplements and bismuth subsalicylate (the active ingredient in Pepto-Bismol) also turn stool black. The key difference: these tend to produce a uniform dark color without the sticky, tar-like consistency or the distinctive sheen of true melena. If you’re unsure, your vet can run a fecal occult blood test to check for hidden blood. These tests can detect blood at levels 20 to 50 times lower than what’s needed to produce visible melena, making them a reliable way to settle the question.
Warning Signs That Need Urgent Attention
Melena on its own is reason enough to see a vet, but certain accompanying signs suggest your dog may be losing blood fast enough to become a medical emergency. Check your dog’s gums. Healthy gums are pink and moist. Pale, white, or grayish gums indicate poor blood circulation and possible shock. Other red flags to watch for alongside black stool:
- Weakness or collapse: difficulty standing, wobbling, or reluctance to move
- Lethargy: unusual tiredness that goes beyond normal rest
- Abdominal swelling: a belly that looks distended or feels tight
- Vomiting, especially if it contains blood or looks like coffee grounds
- Difficulty breathing
- Bruising on the skin or inside the ears
Any combination of these signs with melena suggests significant blood loss. Shock from internal bleeding is a life-threatening emergency where minutes matter.
How Vets Find the Source of Bleeding
Your vet will likely start with a physical exam, bloodwork, and a fecal test. The fecal occult blood test confirms whether blood is genuinely present, which is especially useful when the stool’s appearance is ambiguous. Blood panels reveal anemia, low protein levels, and organ function issues that help narrow the list of possible causes. A fecal parasite exam checks for hookworms and other intestinal parasites.
If the initial tests don’t pinpoint the problem, imaging comes next. Abdominal ultrasound can identify masses, thickened intestinal walls, or free fluid in the abdomen. In some cases, endoscopy (a camera passed into the stomach and upper intestine) allows the vet to directly visualize ulcers or tumors and take tissue samples. CT scans can help distinguish between tumor types before surgery, though a biopsy remains necessary for a definitive diagnosis.
What Recovery Looks Like
Treatment depends entirely on the underlying cause. A dog with NSAID-related ulcers may need the medication stopped and a course of stomach-protecting drugs to allow healing. A parasitic infection calls for targeted deworming. Tumors may require surgery, and in some cases additional therapy afterward.
Regardless of the cause, dogs recovering from GI bleeding typically do best on a carefully managed diet. Veterinary therapeutic GI diets are formulated to be highly digestible, meaning your dog absorbs more nutrients from less food, reducing the workload on a healing gut. Your vet may recommend feeding smaller meals more frequently, such as four or more small portions throughout the day instead of the usual two. Keeping food low in fat helps dogs with concurrent pancreatitis or chronic digestive issues.
Current veterinary guidance discourages withholding food during GI recovery. The intestinal lining needs nutrients to heal, and fasting can actually delay recovery and cause additional damage. Canned therapeutic diets provide extra hydration, but fresh water should always be available. Avoid treats and table scraps during recovery, as even small dietary disruptions can set things back. Keeping a food diary and weighing your dog regularly helps your vet track whether your dog is maintaining adequate nutrition while healing.