Diarrhoea means stools are looser, more watery, more frequent or greater in volume than normal. A single soft stool in an otherwise bright adult pet may settle, but repeated watery stool, blood, black tarry stool, vomiting, weakness or dehydration can signal a problem requiring veterinary care.

Quick triage: Seek urgent in-person veterinary care if your pet has black tarry stool, a large amount of blood, repeated vomiting, severe lethargy, collapse, pale gums, marked abdominal pain, suspected poisoning, trouble keeping water down or signs of dehydration. Puppies, kittens, seniors, pregnant pets and animals with chronic disease should be assessed sooner because they have less reserve.

This guide can help you observe and report symptoms, but stool appearance alone cannot diagnose the cause.

What counts as diarrhoea?

Healthy stool is normally formed, firm but not hard, segmented or log-shaped, and easy to pick up. A commonly used seven-point faecal chart places ideal stool near score 2; scores 5–7 range from very moist stool that loses its shape to completely watery puddles.

Note changes in:

• Consistency and wateriness
• Frequency and urgency
• Volume passed each time
• Straining or pain
• Mucus, fresh blood or black tarry material
• Appetite, thirst, vomiting, energy and body weight

One abnormal bowel movement does not reveal the diagnosis. Duration, age, vaccination and deworming status, recent diet, possible toxin or foreign-body exposure, medication, travel, boarding and other symptoms all matter.

Emergency warning signs

Go to a clinic now

Do not wait for an online appointment if your dog or cat has:

• Collapse, profound weakness or reduced responsiveness
• Pale, white or blue-tinged gums
• Black, sticky or tar-like stool
• Profuse bloody diarrhoea or rapidly repeated watery stools
• Repeated vomiting or inability to keep water down
• A swollen or markedly painful abdomen
• Suspected toxin, spoiled-food or foreign-object ingestion
• Severe dehydration or signs of shock
• Diarrhoea with breathing difficulty, seizures or other major systemic signs

Black tarry faeces can represent digested blood from the stomach or small intestine. Acute haemorrhagic diarrhoea syndrome in dogs can cause sudden bloody diarrhoea, vomiting, dehydration and shock, and prompt fluid treatment is central to care.

Read more on pet emergency warning signs

Read more on dehydration signs in dogs 

Call a veterinarian promptly

Arrange same-day or early advice if:

• Blood or mucus appears more than once
• Diarrhoea is accompanied by vomiting, poor appetite, lethargy, fever, pain or weight loss
• The pet is a puppy, kitten, senior, pregnant, chronically ill or taking medication
• Vaccination or deworming is incomplete
• Diarrhoea follows a new medicine, scavenging event, raw food or abrupt food change
• The pet strains repeatedly or cannot pass a normal stool
• Loose stool keeps recurring

For an otherwise bright adult pet, diarrhoea lasting beyond one or two days should be discussed with a veterinarian. A cat with persistent diarrhoea plus poor appetite, vomiting or lethargy should be assessed as soon as possible.

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Small- and large-bowel signs

Veterinarians may use stool patterns to estimate which part of the intestine is most affected. These patterns overlap and do not identify the underlying disease by themselves.

Sign	More typical of small bowel	More typical of large bowel
Frequency	Normal or mildly increased	Markedly increased
Amount each time	Normal to large	Small
Urgency	Usually absent	Common
Straining	Usually absent	Common
Mucus	Less common	Common
Blood	Black/tarry if bleeding is higher in the tract	Fresh red blood may appear
Weight loss	Can occur when prolonged	Less typical unless disease is chronic or mixed

These distinctions align with standard veterinary descriptions, but mixed patterns occur.

What stool colour means

Colour provides a clue, not a diagnosis. Food, treats, medicines and supplements can change stool colour, while blood, altered bile flow and intestinal disease can produce similar-looking changes.

Brown

A range of brown shades can be normal when the stool is formed and the pet otherwise appears well. Focus on an unexplained change from that pet’s usual colour and consistency.

Bright red

Fresh red blood is called haematochezia and often comes from the colon, rectum or anus. It may appear with colitis, mucus and straining. A tiny streak can occur after straining, but repeated blood, a larger quantity or any accompanying illness needs prompt veterinary advice.

Black and tarry

Black, sticky or tar-like stool is called melena and can indicate digested blood from higher in the gastrointestinal tract. Treat this as urgent, especially with pale gums, weakness, vomiting or poor appetite.

Yellow or orange

Diet and rapid intestinal transit can influence yellow or orange stool, but the colour is nonspecific. Do not assume it is harmless bile or simply the result of chicken and rice if it persists or the pet is unwell.

Green

Green stool can follow grass, food colouring, rapid transit or other gastrointestinal changes. Contact a veterinarian if it is repeated, watery or accompanied by other symptoms.

Grey, pale or greasy

Pale, bulky or greasy-looking stool can be associated with impaired digestion or absorption. Prolonged small-bowel disease may also cause weight loss and altered appetite, so recurring pale or greasy stools merit veterinary investigation.

Common causes

Diarrhoea is a symptom with many possible causes:

• Abrupt diet change, overeating, fatty scraps or dietary indiscretion
• Food intolerance or adverse food reaction
• Parasites such as Giardia or intestinal worms
• Viral, bacterial or protozoal infection
• Stress, travel, boarding or household change
• Medication adverse effects
• Toxins or spoiled food
• Foreign material or partial obstruction
• Pancreatic, liver, kidney, hormonal or other systemic disease
• Chronic inflammatory enteropathy or intestinal cancer

A veterinarian may start with history and examination, then recommend faecal testing, blood tests, urinalysis, imaging or other investigations according to age, duration and clinical severity.

Puppies with severe or bloody diarrhoea require particular caution because parvovirus is one possible cause, especially when vaccination is incomplete. Cats with chronic diarrhoea may need evaluation for intestinal, pancreatic, thyroid or systemic disease rather than repeated empirical diet changes.

Safe steps at home

Home monitoring is appropriate only for a bright, alert adult pet with a mild, short-lived episode, no blood or black stool, no repeated vomiting, no suspected toxin or foreign object, and normal drinking.

Keep water available

Diarrhoea can cause fluid and electrolyte loss. Offer access to fresh water and monitor drinking and urination. If the pet cannot keep water down, refuses water, becomes weak or shows dry/tacky gums or sunken eyes, seek veterinary help.

Do not force large volumes of water or give homemade salt-and-sugar mixtures without veterinary direction. Fluid requirements and electrolyte abnormalities cannot be safely estimated from an article.

Ask about a digestible diet

A veterinarian may advise small amounts of a highly digestible gastrointestinal diet. Early, appropriate enteral nutrition can support intestinal recovery, while prolonged fasting is no longer a universal recommendation.

Short-term chicken-and-rice mixtures are often described as “bland,” but they are not complete and balanced for long-term feeding and may be unsuitable for pets with allergies, pancreatitis or other medical conditions. Cats should not be routinely fasted; contact a veterinarian if a cat is not eating.

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Avoid sudden switches

If a food change is not part of the treatment plan, keep the diet consistent. When moving to a new everyday food after recovery, transition gradually to reduce the chance of further digestive upset.

Do not self-medicate

Do not give loperamide, bismuth products, antibiotics, painkillers, leftover dewormers or other human or veterinary medicine without instruction. Some human anti-diarrhoeal products can harm dogs or cats, and over-the-counter products can be particularly dangerous in cats.

Evidence-based guidance recommends against routine antimicrobials for dogs with mild or moderate acute diarrhoea, including many cases with fresh blood, when systemic illness is absent or responds to fluids. Antibiotics are reserved for selected severe cases or specific diagnosed infections.

Use supplements only with guidance

Probiotic and fibre products are not interchangeable, and evidence for shortening acute canine diarrhoea is inconsistent. Ask a veterinarian whether a specific product, strain and dose fits the pet’s condition rather than choosing a generic human supplement.

What to record

Before calling or visiting the veterinarian, record:

• When the diarrhoea began and how often it occurs
• A photo showing colour and consistency
• Whether blood, mucus, worms or foreign material is visible
• Appetite, water intake, urination, vomiting and energy
• Recent food, treats, table scraps and food changes
• Possible access to rubbish, toxins, medicines or foreign objects
• Vaccination, deworming and parasite-prevention history
• Current medicines and supplements
• Travel, boarding, daycare or contact with ill animals

If the clinic requests it, bring a fresh stool sample in a clean, leakproof container. Diagnostic testing may include faecal parasite or antigen tests, blood work, urinalysis, X-rays or ultrasound depending on the case.

Preventing recurrence

• Transition foods gradually.
• Prevent access to rubbish, spoiled food, bones, toys and household toxins.
• Limit fatty table scraps and excessive treats.
• Keep vaccinations and veterinarian-advised parasite control current.
• Store pet food correctly and discard spoiled wet food.
• Maintain clean bowls, litter trays and living areas.
• Separate sick animals where an infectious cause is suspected and wash hands after handling stool.
• Investigate recurring diarrhoea rather than repeatedly changing food or using over-the-counter remedies.

Good sanitation and prompt removal of faeces help limit transmission of gastrointestinal pathogens.

Frequently asked questions

When is diarrhoea an emergency in a dog or cat?

It is an emergency when diarrhoea occurs with collapse, severe weakness, pale gums, black tarry stool, profuse blood, repeated vomiting, marked pain, suspected poisoning, inability to keep water down or severe dehydration.

Can a pet be monitored at home for one loose stool?

A bright, alert adult pet that is eating, drinking and behaving normally may be observed closely after one mild episode. Contact a veterinarian sooner if the pet is very young, old, pregnant, chronically ill or develops another symptom.

What can be fed to a dog with mild diarrhoea?

Ask a veterinarian whether to offer small meals of a complete, highly digestible gastrointestinal diet. A homemade bland mixture may be used briefly only when recommended, but it is not a balanced long-term diet.

Should a cat with diarrhoea be fasted?

No routine fasting should be started without veterinary advice. Cats that stop eating can develop additional health problems, and diarrhoea with poor appetite warrants prompt assessment.

What does fresh red blood in stool mean?

Fresh red blood often comes from the lower bowel and may appear with mucus, urgency and straining. It still requires veterinary advice if repeated, substantial or accompanied by illness.

What does black stool mean?

Black, tarry stool may be melena—partly digested blood from higher in the gastrointestinal tract—and requires prompt veterinary assessment.

Can human anti-diarrhoea medicine be given to pets?

No medicine made for people should be given unless a veterinarian specifically prescribes it for that animal. Some products are toxic or unsuitable and may mask serious disease.

Do dogs with diarrhoea need antibiotics?

Most mild and moderate cases of acute canine diarrhoea do not need antibiotics. Evidence-based guidance reserves antimicrobial treatment for selected severe cases, systemic infection or a specific diagnosed pathogen.

How long is too long?

For an otherwise bright adult pet, diarrhoea that persists beyond one or two days should be discussed with a veterinarian. Do not wait that long when red flags are present or the animal is young, old or medically vulnerable.