Can Cancer Cause Diarrhea?

Yes, but in dogs with cancer, diarrhea is far more often caused by the treatment than by the tumor itself.

Chemotherapy is the most common culprit. Diarrhea is a recognized and expected side effect, usually appearing in the days following a treatment, and it is generally mild and self-limiting. Targeted therapies such as toceranib (Palladia) cause diarrhea even more frequently — it is the most commonly reported side effect on the label.

Tumors of the digestive tract can cause diarrhea directly, but these are uncommon, making up only a small fraction of cancers in dogs. Cancer can also cause diarrhea indirectly through mast cell tumor histamine release, radiation to the abdomen, antibiotics given during treatment, or infection during a period of low white blood cell counts.

And it bears repeating: most diarrhea in dogs has nothing to do with cancer. Dietary indiscretion, parasites, infections, stress, and abrupt food changes are all vastly more common.

One thing worth knowing before you call your veterinarian: the character of the diarrhea — whether it is large-volume and infrequent, or small-volume and urgent — narrows the possible causes considerably. It is the most useful thing you can describe accurately.

Key points

• Chemotherapy-related diarrhea is usually mild and short-lived, typically lasting around three days.
• The standard treatment has shifted. Current guidelines move away from routinely reaching for antibiotics such as metronidazole, in favour of options that do not disrupt the gut microbiome.
• Diarrhea is the most common side effect of toceranib (Palladia), and there are specific thresholds at which the drug should be paused — worth knowing before you need them.
• Small bowel and large bowel diarrhea look different and mean different things. Volume, frequency, straining, mucus, and the appearance of any blood are the distinguishing features.
• Black, tarry stool is digested blood from higher up. Fresh red blood or streaking usually comes from the lower bowel. These are different findings.
• Diarrhea plus fever within roughly two weeks of chemotherapy is an emergency, not something to manage at home.
• Dehydration is the main short-term danger, particularly in small dogs, puppies, and older dogs.
• Do not give human anti-diarrheal medication without veterinary direction — some are genuinely dangerous in certain breeds.

Detailed explanation

First: what kind of diarrhea?

This distinction guides everything that follows, and owners are well placed to observe it.

Small bowel diarrhea — originating in the small intestine:

• Large volumes per episode
• Only mildly increased frequency
• Watery or very loose
• Weight loss is common
• No straining
• Mucus is rare
• If blood is present, it appears as black, tarry stool (melena) — digested blood from further up

Large bowel diarrhea — originating in the colon:

• Small volumes per episode
• Markedly increased frequency, often with urgency and accidents indoors
• Semi-formed or gelatinous rather than watery
• Straining is common
• Mucus is frequently visible
• If blood is present, it is fresh and red, sometimes just streaks on the surface

Some dogs have features of both. But describing which pattern fits — "large amounts three times a day" versus "a small amount eight times with straining" — genuinely changes the list of likely causes and the tests that follow.

Photograph the stool if you can. It is unglamorous and it is useful.

Chemotherapy

This is the most common cause of diarrhea in a dog with cancer.

The 2026 AAHA Oncology Guidelines note that dogs generally tolerate chemotherapy well, with gastrointestinal effects that are usually self-limiting and last around three days. Diarrhea typically appears within a few days of treatment and resolves on its own.

What has changed is the treatment approach, and it is worth knowing because practice has not caught up everywhere. The guidelines describe a deliberate shift away from empirical antibiotic use for acute diarrhea after chemotherapy — partly for antibiotic stewardship, and partly to avoid disrupting the normal gut microbiome. Metronidazole in particular has been widely used for this and the supporting evidence is weak.

The alternatives the guidelines describe:

• Crofelemer, a plant-derived delayed-release tablet that reduces fluid secretion into the gut. It was conditionally approved by the FDA specifically for chemotherapy-induced diarrhea in dogs. It is not an antibiotic.
• Probiotics — one small study found a high-potency probiotic reduced the incidence of chemotherapy-induced diarrhea in dogs on multi-agent protocols.
• Smectite, a medical clay, which in one study resolved signs faster than metronidazole in dogs with chemotherapy-induced diarrhea.
• Dietary support and fiber.

If your dog develops diarrhea after each cycle, that pattern is worth reporting — it can often be anticipated and managed before the next treatment rather than chased afterward.

Targeted therapy — toceranib (Palladia)

If your dog is on toceranib, this section matters more than any other.

Diarrhea is listed among the most common side effects on the product label, alongside reduced appetite, lameness, weight loss, and blood in the stool. The drug is also known to be ulcerogenic, and serious gastrointestinal complications including perforation have been reported, though rarely.

There are specific thresholds at which the drug should be paused. Veterinary guidance advises temporarily stopping toceranib if a dog has more than two episodes of vomiting or diarrhea in 24 hours, loses its appetite, has fresh blood in the stool for more than two days, or develops unexplained bruising. Report any of these, and expect a dose reduction once your dog has returned to normal.

Ask your veterinarian to write these thresholds down when the drug is first prescribed. They are exactly the kind of instruction that is hard to recall at 9pm on a Sunday.

One further caution: anti-inflammatories should be used carefully alongside toceranib, because the combination increases the risk of gastrointestinal ulceration or perforation. If another practice prescribes an NSAID for something unrelated — a limp, dental work — make sure they know your dog is on toceranib.

Tumors in the digestive tract

These cause diarrhea directly, but they are uncommon; digestive tract tumors account for a small percentage of cancers in dogs and cats.

Lymphoma is the most frequent, followed by adenocarcinoma, leiomyosarcoma, and gastrointestinal stromal tumors. Signs depend on location: small intestinal tumors more often produce vomiting and large-volume diarrhea with weight loss, while large intestinal and rectal tumors more often cause frequent small-volume stools, straining, mucus, and fresh blood.

These tend to declare themselves gradually — weeks to months of intermittent signs with slow weight loss — rather than as sudden severe diarrhea. Diagnosis usually requires ultrasound, radiographs, endoscopy, or surgical biopsy.

Mast cell tumors

Histamine released by mast cell tumors affects the whole digestive tract. It is best known for causing stomach and duodenal ulceration, but it also increases gut motility, and diarrhea can accompany the vomiting, appetite loss, and lethargy that more commonly get attention.

If your dog has a mast cell tumor and develops diarrhea — particularly with black stool or vomiting — say so specifically when you call.

Radiation

Radiation directed at the abdomen or pelvis irritates the intestinal lining and can cause diarrhea during treatment and for a period afterward. Your radiation oncology team should tell you what to expect and when; if they have not, ask.

Antibiotics and other medications

Dogs undergoing cancer treatment often receive antibiotics — sometimes preventively around the expected low point in white cell counts. Antibiotics disrupt gut bacteria and commonly cause diarrhea in their own right. This is one reason the current guidelines discourage reaching for them unnecessarily.

Anti-inflammatories, some pain medications, and oral chemotherapy agents can also cause loose stools. If diarrhea began when a medication started, that timing is the most useful clue available.

Infection during low white cell counts

This one is the reason diarrhea is not always a wait-and-see problem.

Chemotherapy suppresses the bone marrow, and neutrophil counts typically reach their lowest point around seven days after treatment, though this varies by drug. During that window, the gut lining is also compromised, and bacteria can cross into the bloodstream.

Diarrhea plus fever, or diarrhea plus marked lethargy, in the week or two following chemotherapy requires urgent assessment. Febrile neutropenia is an oncologic emergency requiring hospitalization. A normal temperature does not fully exclude it — if your dog is unusually flat with diarrhea in that window, call.

What the stool can tell you

• Black, tarry, sticky stool (melena) — digested blood from the stomach or small intestine. Report the same day.
• Fresh red blood or red streaking (hematochezia) — bleeding lower down, in the colon or rectum. Common with large bowel problems and usually less alarming than melena, but still worth reporting.
• Mucus — a jelly-like coating, pointing toward large bowel involvement.
• Very pale or greasy stool — possible fat malabsorption.
• Watery, large-volume stool — small bowel origin, with a higher dehydration risk.

Dehydration — the practical danger

Diarrhea's most immediate risk is fluid loss, and it escalates faster than owners expect in small dogs, puppies, and elderly dogs.

Two rough home checks:

• Gums should feel moist. Tacky or dry gums suggest dehydration.
• Skin tent: gently lift the skin between the shoulder blades and release. It should fall back promptly. Skin that stays tented suggests dehydration. This is less reliable in very thin or elderly dogs.

Fresh water should always be available. If your dog cannot keep water down, or is drinking but losing more than it takes in, that needs veterinary attention rather than home management.

What not to do

Do not give human anti-diarrheal medication unless your veterinarian has specifically directed it. Some over-the-counter products are dangerous in dogs, and one commonly used anti-diarrheal is particularly risky in herding breeds — Collies, Australian Shepherds, Shelties and related breeds — which frequently carry a genetic variant affecting how their brains handle certain drugs. Ask before giving anything.

Do not withhold food for long periods. Brief rest of the gut is sometimes advised, but prolonged fasting is counterproductive in a dog already at risk of weight loss. Ask what and when to feed.

Do not start supplements or probiotics without telling your oncology team. Some interact with treatment, and if a probiotic is worth using, they will have a view on which one.

Do not assume it is "just the chemo" if it is severe, bloody, or accompanied by fever or lethargy.

When to contact a veterinarian

Seek emergency care immediately if your dog:

• Has diarrhea and a fever, particularly within about two weeks of chemotherapy
• Has diarrhea and is collapsed, very weak, or has pale gums
• Is passing large amounts of blood, or black tarry stool, and seems unwell
• Cannot keep water down alongside diarrhea
• Has severe abdominal pain, or a tense, painful abdomen
• Is severely dehydrated — dry gums, skin that stays tented, marked lethargy

Contact your veterinarian within a day if your dog:

• Has had diarrhea for more than 24–48 hours
• Has diarrhea with any blood or mucus
• Is on toceranib and has had more than two episodes of diarrhea in 24 hours, has lost appetite, or has had fresh blood in the stool for more than two days
• Has a mast cell tumor and has developed diarrhea
• Has diarrhea alongside vomiting, weight loss, or reduced appetite
• Is a small, young, or elderly dog with diarrhea — dehydration develops faster
• Started a new medication shortly before the diarrhea began
• Develops diarrhea after every chemotherapy cycle, so it can be pre-empted next time

When you call, be ready to describe: how many times a day, roughly how much each time, whether there is straining, whether you see mucus or blood and what colour, whether your dog is eating and drinking, and any recent treatment or medication changes.

Questions to ask your veterinarian

• Does this look like small bowel or large bowel diarrhea, and what does that suggest?
• Is this likely related to the cancer, the treatment, or something separate?
• Do we need to test for parasites or infection before assuming it is treatment-related?
• Are antibiotics necessary here, or is there a better option that spares the gut microbiome?
• Is crofelemer appropriate for my dog?
• Would a probiotic help, and which one would you recommend?
• Is my dog dehydrated? Do they need fluids?
• What should I feed while this settles, and when do I go back to normal food?
• My dog is on toceranib — at exactly what point should I stop giving it and call you?
• Can we prevent this after the next chemotherapy treatment rather than treating it after it starts?
• What would make you want to see my dog urgently rather than tomorrow?

Sources

Christensen, J., Johnson, K., Ettinger, S., Garrett, L., Gordon, I., Ireifej, S., Love, A., & Wisecup, M. (2026). AAHA oncology guidelines for dogs and cats. Journal of the American Animal Hospital Association, 62(1), 1–37. https://doi.org/10.5326/JAAHA-MS-7549

American Animal Hospital Association. (2026). 2026 AAHA oncology guidelines for dogs and cats: Section 7. Supportive and symptomatic care. https://www.aaha.org/resources/2026-aaha-oncology-guidelines-for-dogs-and-cats/section-7-supportive-and-symptomatic-care/

Ellis, C., Odunayo, A., & Tolbert, M. K. (2023). The use of metronidazole in acute diarrhea in dogs: A narrative review. Topics in Companion Animal Medicine, 56–57, 100824.

Fournier, Q., Serra, J. C., Williams, C., et al. (2021). Chemotherapy-induced diarrhoea in dogs and its management with smectite: Results of a monocentric open-label randomized clinical trial. Veterinary and Comparative Oncology, 19(1), 25–33.

Jugan, M. C., Wouda, R. M., & Higginbotham, M. L. (2021). Preliminary evaluation of probiotic effects on gastrointestinal signs in dogs with multicentric lymphoma undergoing multi-agent chemotherapy: A randomised, placebo-controlled study. Veterinary Record Open, 8(1), e2.

Merck Veterinary Manual. (2025). Gastrointestinal neoplasia in dogs and cats. https://www.merckvetmanual.com/digestive-system/neoplasia-of-the-gastrointestinal-tract-in-small-animals/gastrointestinal-neoplasia-in-dogs-and-cats

Purina Institute. (n.d.). Small bowel versus large bowel diarrhea. https://www.purinainstitute.com/centresquare/therapeutic-nutrition/why-distinguish-between-small-and-large-bowel-diarrhea

U.S. Food and Drug Administration. (2021). FDA conditionally approves first oral tablet to treat chemotherapy-induced diarrhea in dogs. https://www.fda.gov/news-events/press-announcements/fda-conditionally-approves-first-oral-tablet-treat-chemotherapy-induced-diarrhea-dogs

University of Pennsylvania School of Veterinary Medicine. (2025). Intestinal tumors in dogs and cats. https://www.vet.upenn.edu/wp-content/uploads/2025/05/intestinal-tumors.pdf

VCA Animal Hospitals. (n.d.). Toceranib phosphate. https://vcahospitals.com/know-your-pet/toceranib-phosphate

Veterinary Information Network. (n.d.). Toceranib phosphate (Palladia). Veterinary Partner. https://veterinarypartner.vin.com/default.aspx?pid=19239&catId=102894&id=4952879

Zoetis. (n.d.). Palladia (toceranib phosphate) tablets — client information sheet. DailyMed, U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/medguide.cfm?setid=ae09ae83-0812-4ddc-92e3-730f7a1ae449

This article is for educational purposes only and is not a substitute for veterinary care. It cannot diagnose any condition in your dog, and no medication mentioned here should be given without your veterinarian's direction. Most diarrhea in dogs is not caused by cancer, but diarrhea in a dog undergoing cancer treatment should always be reported. If your dog has diarrhea with a fever following chemotherapy, is passing black or bloody stool, cannot keep water down, or is collapsed or very weak, seek emergency veterinary care immediately.