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, fouler 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 liver disease or general gut inflammation; ulcers are an uncommon result of kidney disease in dogs and are not a recognised feature of it in cats.
Diet, Supplements or Medication
Iron supplements, bismuth-containing stomach remedies (one bismuth carbonate tablet is authorised in Great Britain for dogs and cats; human bismuth subsalicylate remedies are not authorised for pets, and in cats their salicylate can itself cause stomach ulceration), and some strongly pigmented foods or treats can darken stool colour without any underlying bleeding, mimicking the appearance of melaena (black, tarry stool caused by digested blood). Stool darkened in this way is usually not sticky or tar-like, although appearance alone does not reliably separate it from digested blood.
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 is 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 plasma protein from the gut lining rather than whole blood, so it typically causes gradual protein loss rather than the kind of blood loss that darkens stool. Melaena 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 and are seen here mainly in dogs imported from abroad. 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 small intestine wall can ulcerate its surface and bleed slowly into the gut, producing melaena 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
Black, tarry stool appears only once a substantial amount of blood has entered the stomach or small intestine, so a single episode can reflect significant bleeding, even in an animal that otherwise seems well. Stomach ulcers linked to anti-inflammatory medicines can bleed with only vague outward signs before they progress.
Later presentation
Black, tarry stool alongside pale gums, weakness, collapse or fast breathing reflects blood loss large enough to affect circulation, which can become life-threatening within hours. Known access to rodenticide or anti-inflammatory medicines carries the same weight, because clotting failure and ulcer perforation can both progress quickly. Stools that stay black across several bowel movements point towards bleeding that is continuing.
Because blood takes time to pass through and be digested within the gut, melaena appears after bleeding has started, and sudden heavy bleeding can cause weakness or collapse before any black stool is passed. A normal stool afterwards does not show that bleeding has stopped.
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 melaena alongside other bleeding signs.
Intestinal Parasites
The common UK hookworm, Uncinaria stenocephala, feeds mainly on plasma protein and rarely causes blood loss. Blood-feeding Ancylostoma hookworms, seen in the UK mainly in dogs imported from warmer countries, can cause enough chronic blood loss to darken the stool, particularly in puppies.
Gastric Adenocarcinoma
A tumour within the stomach wall can ulcerate and bleed slowly into the gut, producing intermittent melaena.
Chronic Kidney Disease in Cats
In cats, the stomach changes seen with kidney disease are mainly scarring and mineral deposits rather than ulcers, so melaena is an uncommon feature of feline kidney disease.
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.
Intestinal Adenocarcinoma
Intestinal adenocarcinoma may produce black or tarry stools when tumours in the small intestine bleed slowly into the gut, allowing the blood to be partially digested before it is passed. Tumours in the large intestine more often produce fresh red blood in or on the stool.
Endocarditis
Endocarditis can lead to black or tarry stools when bacteria-induced damage to heart valves triggers systemic inflammation and occasionally secondary gastrointestinal bleeding through effects on blood clotting and vessel integrity. Melaena in this context tends to appear alongside other signs of systemic illness.
Immune-Mediated Haemolytic Anaemia (IMHA)
Immune-mediated haemolytic anaemia may be accompanied by black, tarry stools, particularly when a low platelet count allows bleeding from the gut lining, or when certain treatments affect the gastrointestinal lining. A low platelet count in this condition has been linked to a more serious course, and melaena in this setting often accompanies other signs of severe anaemia.
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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.
A photograph of the stool, the time it was passed, and a note of gum colour, energy level, any medicines or supplements, possible rodenticide access and recent diet changes give a veterinary team the details that characterise how significant the change is.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS