“My Senior Dog’s Poop Is Suddenly Very Dark”: what it could mean and what to do

Seeing a sudden change in your senior dog’s poop can be unsettling, especially when the stool looks very dark brown, nearly black, or even tar-like. In older dogs, this change matters because it can be linked to medication side effects, chronic disease, or bleeding somewhere in the digestive tract.

This article can help you sort through the most likely possibilities and decide what to do next, but it cannot diagnose your dog. Always check with your dog’s veterinarian, and if your dog seems weak, is vomiting, or the stool is truly black and sticky, treat it as urgent.

First: is it just “dark,” or is it truly black and tarry?

A lot of normal stools are “dark,” especially in dogs eating richer proteins or darker foods. What raises the alarm is stool that is:

• Black like asphalt or coal
• Sticky or shiny
• Smears like tar
• Often has a strong, unusual odor

That classic black, tarry stool is called melena, which typically means blood has been digested on its way through the upper gastrointestinal tract (stomach or small intestine). Veterinary GI experts also point out a common pitfall: “dark” is vague, and it is the pitch-black, tarry quality that’s more concerning than a normal darker brown.

If you are unsure, put a small amount of fresh stool on a white paper towel and look closely under good light. This does not replace a fecal test, but it can help you describe what you are seeing to your vet more accurately.

What can cause very dark poop in a senior dog?

1) Digested blood (melena) from the stomach or small intestine

Melena is most often associated with bleeding in the upper GI tract. In senior dogs, common reasons include stomach or intestinal ulcers, inflammation, tumors, and some systemic illnesses that predispose to ulceration or bleeding.

A key point from veterinary references: the overall incidence of GI ulceration in dogs is not well defined, and many dogs with ulcers do not have obvious bleeding. That uncertainty is one reason vets rely on history, exam, and lab work rather than stool color alone.

2) Medications that can injure the stomach lining (especially NSAIDs)

Many senior dogs take NSAIDs for arthritis pain. NSAIDs are a well-known contributor to gastroduodenal ulceration in dogs, and melena can be one of the signs when bleeding occurs.

Risk tends to rise when:

• An NSAID is combined with a corticosteroid
• Multiple pain meds are combined without veterinary guidance
• The dog has underlying disease (kidney, liver, endocrine disease) or is dehydrated

Recent veterinary literature continues to document clinically significant ulceration in dogs receiving NSAIDs and highlights the need for careful prescribing and monitoring.

Important: never start, stop, or switch pain meds without your veterinarian’s direction, especially in a senior dog.

3) “False alarm” dark stool from bismuth or other substances

Some substances can darken stool without bleeding. The big one is bismuth subsalicylate (the active ingredient in products like Pepto-Bismol). Veterinary pharmacology resources note that bismuth products can be involved in toxicity if overdosed and should only be used under veterinary guidance.

Also, activated charcoal (sometimes used for toxin cases under veterinary supervision) can make stool very dark. (If your dog was given charcoal by a clinic, they likely warned you.)

Do not give human GI medications unless your vet explicitly tells you to. Cornell’s veterinary guidance specifically warns against using products like bismuth subsalicylate unless directed, because they can be harmful to some dogs and may interact with other medications.

4) Iron supplements or iron-heavy foods

Iron can darken stool in people and is also discussed as a potential cause of darker stool in dogs in many pet resources. The bigger practical point is this: if your dog is taking iron, your veterinarian should already be monitoring why, and any sudden stool change is still worth a check-in because iron is often given when anemia is already a concern. Evidence quality on “iron causes black stool in dogs” is not as standardized in veterinary primary references as it is in human medicine, so treat this as a possible contributor, not a reassurance.

5) Bleeding disorders or systemic disease

Senior dogs can develop problems that affect clotting or cause bleeding indirectly (for example, liver disease, certain cancers, immune-mediated disease, and some toxins). GI ulceration references list hepatic disease and neoplasia among common reported causes associated with ulceration in dogs.

If your dog has unexplained bruising, nosebleeds, bleeding gums, or very pale gums along with dark stool, your vet will likely prioritize checking blood counts and clotting.

When dark stool is an emergency

Seek urgent veterinary care (same day, emergency clinic if needed) if any of these are true:

• Stool is black and tar-like (true melena), especially if new
• Vomiting (especially if it looks like coffee grounds or contains blood)
• Weakness, collapse, wobbliness, or unusually low energy
• Pale gums, rapid breathing, or a swollen belly
• Refusing food and water
• Known or possible ingestion of human medications (NSAIDs, aspirin, bismuth products) or toxins

If there’s any chance your dog got into medications or toxins, you can also contact an animal poison hotline while you head to care. The ASPCA Animal Poison Control Center number is listed on ASPCA’s site.

What to do at home right now (safe, practical steps)

1. Take a photo in good light. Your vet will thank you.
2. Write down what changed in the last 7 to 14 days:
   • New meds, dose changes, missed meals
   • Any NSAIDs or steroids, or any human OTC meds
   • New chews, treats, bones, or “counter surfing” incidents
   • Vomiting, appetite changes, weight loss, increased thirst
3. Check gum color and energy level. Pale gums, weakness, or fast breathing should push you toward urgent care.
4. Do not give Pepto-Bismol, aspirin, ibuprofen, naproxen, or “stomach coating” meds unless your veterinarian instructed you to.
5. If your dog is stable (normal energy, eating, no vomiting) but the stool is still very dark: call your vet promptly and describe the stool’s color and texture. “Dark brown” and “black tar-like” can lead to very different triage decisions.

What your vet may do (and why)

To find the cause, your veterinarian may recommend:

• Physical exam and history review (meds, diet, toxin exposure)
• Fecal testing to confirm blood or rule out parasites, depending on the case
• Bloodwork (CBC and chemistry) to look for anemia, inflammation, kidney and liver issues
• Clotting tests if there are signs of a bleeding disorder
• Imaging (x-rays or ultrasound) to look for masses, foreign material, or organ changes
• Endoscopy in some cases, especially when ulcers, tumors, or persistent bleeding is suspected. Veterinary GI ulcer references note that ultrasound sensitivity for non-perforated ulcers can be low, and endoscopy may be needed for a clearer diagnosis.

Treatment depends entirely on the cause. For ulceration, primary veterinary references emphasize addressing the underlying trigger (for example, discontinuing an offending NSAID under veterinary supervision) and using acid-suppressing therapy such as proton pump inhibitors as part of the plan.

Why this can be trickier in senior dogs

Older dogs are more likely to have:

• Chronic pain management plans (NSAIDs and other meds)
• Kidney or liver disease that changes how drugs are tolerated
• Higher risk of tumors and chronic GI inflammation
• Lower “physiologic reserve,” meaning blood loss can make them feel unwell faster

That is why a “wait and see” approach that might be reasonable for a young dog with one odd stool can be riskier in a senior dog, especially if the stool looks truly tarry or the dog seems off.

Research limits and what we do not know perfectly

Veterinary medicine has strong consensus that melena is a meaningful clinical sign and that NSAIDs, neoplasia, and hepatic disease are among commonly reported causes associated with gastroduodenal ulceration. At the same time, authoritative veterinary references note that the overall incidence of GI ulceration in dogs is not well established, and diagnostic tools like ultrasound may miss some non-perforated ulcers. So while we can list likely causes and red flags, a stool photo and a veterinary exam plus targeted testing are what turn “possible” into “probable.”

Bottom line

If your senior dog’s poop is suddenly very dark, do not panic, but do take it seriously. Dark food or certain substances can change stool color, but black, tar-like stool can signal internal bleeding and warrants prompt veterinary guidance. When in doubt, call your veterinarian, describe the stool carefully, and err on the side of getting your dog checked.

And again: always confirm next steps with your dog’s veterinarian, especially before giving any over-the-counter medications.

Sources

• Merck Veterinary Manual: Gastrointestinal Ulcers in Small Animals (Merck Veterinary Manual)
• VIN / WSAVA Proceedings: GI tract ulceration and GI bleeding, discussion of stool color vs true melena (Vin)
• dvm360 proceedings: notes on “dark stools” vs pitch-black tarry stool and bismuth confounding (DVM360)
• American College of Veterinary Pharmacists: Bismuth subsalicylate cautions (ACVP)
• Cornell University College of Veterinary Medicine: diarrhea guidance and warning about human meds like bismuth unless directed (Cornell Vet Medicine)
• JAVMA article on dogs with GI ulceration lesions receiving NSAIDs and/or corticosteroids (clinical context) (AVMA Journals)
• Mississippi State CVM PDF: NSAID toxicity signs including melena (MSU Vet Med)
• ASPCA Animal Poison Control Center contact page (ASPCA)