Why is my dog’s stool black or tarry (melena)?

Black, tarry stool (melena) usually means digested blood from the upper GI tract and can be an emergency. Learn causes, red flags, home steps, and what to expect at the vet.

What does black or tarry stool mean?

When a dog’s feces look black, shiny, or tar-like, veterinarians call it melena. Melena means blood has been digested as it passed through the upper gastrointestinal tract (esophagus, stomach, or small intestine). Digested blood turns dark because stomach acid and digestive enzymes change the blood’s appearance. That distinguishes melena from hematochezia, which is bright red blood passed fresh from the lower bowel (rectum or colon).

Melena is important because it often indicates bleeding higher in the GI tract — and that bleeding can be mild or life‑threatening depending on the cause and the amount lost.

When to See a Vet Immediately

Seek veterinary care right away (emergency) if your dog has any of the following along with black/tarry stool:

• Pale, white, or tacky gums
• Weakness, collapse, fainting, or lethargy
• Rapid breathing, fast heart rate, or difficulty breathing
• Vomiting blood or coffee‑ground‑like vomit
• Ongoing vomiting, especially if bloody
• Rapidly worsening clinical signs or collapse

Red Flags — Seek Emergency Care

• Repeated episodes of melena
• Pale mucous membranes (gums, eyelids)
• Persistent vomiting or retching
• Blood in vomit
• Collapse, unresponsiveness, or seizures
• Known exposure to rodenticide or toxic substances
• Recent high‑dose or long‑term NSAID use (aspirin, ibuprofen, naproxen) or other medications known to cause ulcers

Differential diagnosis — common causes (ranked by likelihood)

Note: likelihood depends on location, signalment (age, breed), history, and recent exposures. This list is a general ranking for dogs presenting with melena.

Could iron supplements or medicines be a false alarm?

Yes. Iron supplements commonly turn stool black without any bleeding. Bismuth‑containing products (e.g., Pepto‑Bismol), charcoal, and some diets can also darken stool. If your dog is taking any of these, tell your veterinarian — but don’t assume the stool is harmless. When in doubt, get veterinary advice, especially if you also see any of the red flags above.

Home steps you can take (safe, while arranging veterinary care)

• Keep your dog calm and confined; limit activity. Do not let them run or play vigorously.
• Collect a fresh stool sample in a clean container (plastic bag or container) and note the time and appearance. Photos can help your vet.
• Bring a list or packaging of recent medications, supplements (iron, Pepto, NSAIDs), and any possible toxin exposures (rodenticide, human medication, household chemicals).
• Do not give human NSAIDs or other over‑the‑counter medications to your dog.
• If your dog is vomiting heavily or appears unstable, do not attempt home treatments — seek emergency care immediately.

What the vet will ask and examine

Expect the clinician to take a focused history: onset and timing of dark stool, presence of vomiting (blood or coffee‑ground material), appetite, activity, medication and toxin exposure (rodenticides, NSAIDs, supplements), recent surgeries, preexisting disease (liver disease, cancer), and any trauma or access to foreign objects.

The physical exam will check hydration, capillary refill time, heart rate and rhythm, respiratory rate, mucous membrane color (gums), abdominal pain or masses, and rectal exam to look for fresh blood or foreign bodies.

What to expect in diagnostics at the clinic

Initial diagnostics are aimed at determining the severity of bleeding, cause, and whether immediate stabilization is needed:

• Packed cell volume (PCV) / hematocrit and total solids (TS) to assess blood loss and hydration
• Complete blood count (CBC) for anemia, white blood cells, and platelet count
• Blood chemistry (electrolytes, kidney and liver values)
• Coagulation profile (PT, aPTT) if bleeding disorder or rodenticide exposure suspected
• Blood type or crossmatch if transfusion may be required
• Fecal test for occult blood (may be positive with melena) and fecal parasite testing
• Abdominal imaging (X‑rays, ultrasound) to look for foreign bodies, masses, or evidence of GI disease
• Endoscopy (gastroscopy) to directly visualize and possibly treat upper GI bleeding (biopsy, cauterize lesions) when indicated
• Exploratory surgery may be necessary for perforation, foreign bodies, or uncontrolled bleeding

Typical treatments your dog may receive

Treatment depends on underlying cause and severity:

• Stabilization: intravenous fluids, oxygen, warming, and hospitalization for monitoring
• Blood products: packed red blood cells or whole blood if anemia or shock is severe
• Gastroprotectants: proton pump inhibitors (omeprazole) or H2 blockers to reduce stomach acid and help ulcers heal
• Antiemetics and stomach protectants (sucralfate) as directed by your vet
• Antibiotics only if infection or specific indications
• Vitamin K therapy for anticoagulant rodenticide poisoning
• Surgical or endoscopic intervention for foreign bodies, severe ulcers, or bleeding tumors
• Treatment of underlying diseases: hepatic therapy, immunosuppressive therapy for immune-mediated disease, parasite treatment

Preventing future episodes

• Use NSAIDs only under veterinary direction and at the correct dose; do not give human NSAIDs.
• Secure rodenticides and other toxins out of reach.
• Keep small/foreign items away from curious dogs that chew and swallow objects.
• Maintain regular parasite control and dental care to reduce bleeding sources.
• Monitor and report any new medications or supplements to your veterinarian.

Key Takeaways

• Black, tarry stool (melena) usually means digested blood from the upper GI tract and can be serious.
• Melena may originate from ulcers, NSAID toxicity, coagulopathies (e.g., rodenticide poisoning), liver disease, tumors, or swallowed blood — but some medications/supplements (iron, bismuth) can mimic melena.
• If your dog shows weakness, pale gums, vomiting blood, collapse, or other red flags, seek emergency veterinary care immediately.
• Do not try to diagnose or treat significant bleeding at home. Collect a stool sample, list medications/exposures, keep your dog calm, and get veterinary evaluation.
• At the clinic your dog may need blood work, imaging, endoscopy, hospitalization, transfusion, and targeted therapy depending on the cause.

Further reading and trusted sources

• Merck Veterinary Manual — Gastrointestinal Hemorrhage in Small Animals: https://www.merckvetmanual.com/digestive-system/gastrointestinal-hemorrhage/gastrointestinal-hemorrhage-in-small-animals

Frequently Asked Questions