SYMPTOM
Black or tarry stools
Owners typically notice the stool has turned unusually dark, almost black, with a glossy, sticky, tar-like texture rather than its usual form, and sometimes a stronger, more metallic odour than normal.
Upper Gastrointestinal Ulceration
Blood from an ulcer in the stomach or upper small intestine is partially digested as it passes through the gut, turning the stool black and tarry rather than red. This is a recognised effect of non-steroidal anti-inflammatory use, and can also develop alongside chronic kidney disease, liver disease, or general gut inflammation, particularly in older dogs and cats.
Diet, Supplements or Medication
Iron supplements, bismuth-containing products, and some strongly pigmented foods or treats can darken stool colour without any underlying bleeding, mimicking the appearance of melena. This benign cause is worth distinguishing from true blood loss, since stool texture and smell often differ from genuine tarry stool.
Swallowed Blood From Elsewhere
Blood swallowed from a nosebleed, a mouth or dental injury, or licking at a wound can pass through the gut and appear as dark stool, even though the bleeding itself is not gastrointestinal in origin. This pathway is more likely when there's a visible source of bleeding elsewhere on the body or face.
Clotting or Bleeding Disorders
Anticoagulant rodenticide exposure or reduced clotting factor production from liver disease can allow spontaneous bleeding into the gut. Because clotting is affected throughout the body, bruising, nosebleeds, or blood in urine sometimes accompany the dark stool.
Hookworm Infection
The hookworm species found in the UK, Uncinaria stenocephala, attaches to the intestinal lining but feeds mainly on tissue fluid and mucus rather than whole blood, so it typically causes gradual protein loss rather than the kind of blood loss that darkens stool. Melena from hookworm infection is accordingly uncommon in UK dogs, unlike the more avidly blood-feeding hookworm species found in warmer climates that are not established in Great Britain. Where a heavy burden does contribute to visible blood loss, this tends to develop alongside weight loss or a dull coat in puppies or dogs with high environmental exposure, rather than appearing in isolation.
Gastrointestinal Neoplasia
A tumour within the stomach or intestinal wall can ulcerate its surface and bleed slowly into the gut, producing melena as an early or intermittent sign. This is less common overall and tends to occur in older animals, sometimes alongside a gradual change in appetite or body condition.
Why timing matters
Early observation
A single episode of darker stool, especially in an animal that otherwise seems well, often reflects a small, self-limiting amount of blood or a dietary cause rather than ongoing bleeding.
Later presentation
Stools that stay black and tarry across several bowel movements, or that appear alongside paleness or lethargy, point toward blood loss that has been occurring for some time and in a greater volume.
Because blood takes time to travel through and be digested within the gut, melena is itself already a slightly delayed sign relative to when bleeding started; a pattern that continues or worsens over days reflects an ongoing rather than a resolved process.
Conditions commonly associated
Gastric Ulceration
Erosion of the stomach lining allows blood to enter the gut, where it is digested and passes as dark, tarry stool.
Anticoagulant Rodenticide Toxicity
Interference with normal clotting factor production can allow spontaneous bleeding into the gastrointestinal tract, producing melena alongside other bleeding signs.
Intestinal Parasites
Hookworms attach to and feed from the intestinal lining, and a heavy burden can cause enough chronic blood loss to darken the stool.
Gastric Adenocarcinoma
A tumour within the stomach wall can ulcerate and bleed slowly into the gut, producing intermittent melena.
Chronic Kidney Disease in Cats
Reduced kidney function allows uraemic toxins to build up, which can damage the stomach lining and lead to gastric ulceration and associated melena.
Foreign Body Ingestion
A swallowed object can mechanically irritate or erode the lining of the stomach or intestine as it passes through, causing localised bleeding.
When to explore further
Whether the dark, tarry stool is a single occurrence or is recurring across multiple bowel movements.
Whether pale gums, unusual tiredness, or reduced interest in normal activity accompany the stool change, which can reflect the degree of blood loss involved.
Known recent access to anti-inflammatory medication, rodenticide, or other substances not typically part of the diet.
Whether vomiting, abdominal discomfort, or a reduced appetite is also present alongside the stool change.
Any known chronic condition, such as kidney or liver disease, in an older dog or cat showing this sign.
Photographing the stool's colour and texture at each occurrence, and noting whether it is a one-off or repeating, helps build a picture of the pattern over time. It's also useful to log any recent diet changes, supplements, or medications given, along with gum colour and general energy level, since these details characterise how significant the change is.
Last reviewed: 13 September 2026 · Dr Alastair Greenway MRCVS