Black, tarry stool in a dog is a veterinary emergency in most cases. The medical term is melena, and it typically signals bleeding somewhere in the upper digestive tract, often the stomach or small intestine. If your dog’s stool is truly black and sticky or tar-like, rather than just dark brown, the safest course of action is to contact a veterinarian right away, especially if your dog also seems lethargic, weak, or has lost appetite.
That said, not every dark stool means a crisis. A few harmless causes can temporarily darken poop. Knowing the difference can help you respond appropriately.
What Makes Stool Turn Black
When a dog bleeds from the stomach or upper small intestine, that blood gets partially digested as it moves through the rest of the digestive tract. The digestion process turns the blood dark, producing stool that looks black and has a distinct tarry, sticky texture. It takes a meaningful amount of blood to cause this color change. In humans, roughly 50 ml (about 3 tablespoons) is needed to turn stool black, and a similar principle applies in dogs.
This is different from bright red blood in stool, which comes from the lower digestive tract (the colon or rectum) and hasn’t been digested. Red blood can indicate problems too, but black tarry stool specifically points to bleeding higher up in the gut, which tends to be more dangerous because it can go unnoticed longer and involve larger blood loss before symptoms become obvious.
Common Causes of Black Stool
Gastrointestinal Ulcers
Ulcers are the most common serious cause. These are open sores on the lining of the stomach or small intestine that bleed when irritated. They can develop from long-term use of pain medications (particularly NSAIDs), severe stress, or underlying organ disease like kidney or liver failure. NSAIDs are the single most frequently cited drug class linked to stomach ulceration and even perforation in dogs, and this includes veterinary-prescribed versions used at normal therapeutic doses, not just accidental overdoses. If your dog takes any pain or anti-inflammatory medication and you notice black stool, that connection matters and your vet needs to know immediately.
Tumors or Growths
Bleeding tumors in the stomach or intestines can cause chronic, low-level melena. This is more common in older dogs and often appears alongside gradual weight loss, decreased appetite, or general decline. The bleeding may come and go, so the stool might not be black every time.
Parasites
Hookworms latch onto the intestinal wall and feed on blood. In severe infections, particularly in puppies or dogs that haven’t been on regular parasite prevention, this blood loss can be heavy enough to produce black stool. Most intestinal parasites cause red blood in stool instead, so black stool from parasites usually signals a more advanced infestation.
Blood-Clotting Disorders
Dogs with low platelet counts or other clotting problems can bleed from the lining of the digestive tract. Melena in these cases often shows up alongside other signs of abnormal bleeding, like small bruises on the skin, bleeding gums, or blood in the urine.
Ingested Blood
A dog that swallowed blood from a mouth injury, a nosebleed, or even from licking a wound can pass dark stool afterward. This is less dangerous, but it can look identical to melena from internal bleeding, so it still warrants attention if you’re not sure of the source.
When It’s Not Actually an Emergency
Several harmless things can make your dog’s stool look unusually dark. Before you panic, consider whether any of these apply:
- Iron supplements commonly turn stool very dark or black.
- Bismuth-containing medications (the active ingredient in Pepto-Bismol) are well known for darkening stool temporarily.
- Activated charcoal, sometimes given after a poisoning event, will produce very dark stool for a day or two.
- Organ meats, liver, beef blood, or blueberries can darken stool naturally, especially in raw-fed dogs.
- Dark-colored kibble or treats with artificial dyes can shift stool color toward dark brown, though usually not true black.
The key distinction is texture and consistency. Dietary causes tend to produce stool that’s dark but otherwise normal in form. True melena has a sticky, tarry quality and often a particularly strong, foul smell. If the stool is dark but firm and your dog is acting completely normal, a dietary cause is more likely. If it’s black, loose, and tar-like, or your dog seems unwell in any way, treat it as urgent.
Warning Signs That Increase Urgency
Black stool on its own warrants a vet call, but certain combinations of symptoms suggest more rapid blood loss or a more serious underlying problem. These include pale gums (check by lifting your dog’s lip and pressing the gum briefly; it should return to pink within two seconds), weakness or wobbliness, rapid breathing, vomiting (especially if the vomit looks like dark coffee grounds, which is also digested blood), collapse, or refusal to eat. A dog showing any of these signs alongside black stool needs emergency care, not a wait-and-see approach. Internal hemorrhage can range from mild and self-limiting to rapid and fatal, and the difference isn’t always obvious from the outside early on.
What Happens at the Vet
Your vet will start with a physical exam and likely run bloodwork to check for anemia (low red blood cell count from blood loss), signs of kidney or liver disease, and indicators of infection or inflammation. A fecal analysis can identify parasites or bacterial infections. If these initial tests don’t pinpoint the cause, imaging such as X-rays or ultrasound gives a better view of the stomach and intestines, helping identify tumors, foreign objects, or structural abnormalities. In some cases, a CT scan or endoscopy (a small camera passed into the stomach) may be needed.
Bringing a fresh stool sample with you saves time. Even a photo on your phone helps your vet assess the color and consistency if you can’t collect a sample.
NSAID Use Deserves Special Attention
Because anti-inflammatory medications are the most common drug-related cause of stomach ulcers in dogs, this connection is worth understanding. Dogs are frequently prescribed NSAIDs for arthritis, post-surgical pain, or other chronic conditions. A study published in the Journal of Veterinary Internal Medicine found gastrointestinal lesions in dogs on common veterinary NSAIDs including carprofen, meloxicam, and firocoxib, even at standard prescribed doses. One dog in the study developed problems while receiving a dose only slightly above normal because the owner adjusted it based on how the dog seemed to be feeling.
This doesn’t mean these medications are inherently dangerous. They’re effective and widely used. But if your dog is on any pain medication and you notice dark or black stool, don’t wait to see if it resolves. The combination is a well-established red flag. Never give your dog human NSAIDs like ibuprofen or aspirin, as these carry an even higher risk of GI bleeding in dogs than veterinary formulations.