7 Sep 2026

Cat Diarrhea: What Stool Changes Can Tell You and When to See a Vet

Dr. Katrina Rezunova

Finding loose stool in the litter box may seem like a minor digestive problem, particularly if your cat is still eating and behaving normally. But diarrhea in cats has a wide range of possible causes—from a brief dietary disturbance to intestinal parasites, chronic inflammatory disease, endocrine disorders, or gastrointestinal cancer.

For veterinarians, the stool itself is only one part of the diagnostic picture.

How long the diarrhea has been occurring, how frequently your cat defecates, whether there is blood or mucus, whether weight is being lost, and whether appetite or behavior has changed can provide important clues about what is happening.

Age matters too. Diarrhea in a recently adopted kitten raises a different set of concerns than chronic loose stool and weight loss in a 14-year-old cat.

What Counts as Diarrhea in a Cat?

Normal feline stool should generally be formed enough to maintain its shape.

Diarrhea occurs when feces contain excessive water, resulting in stool that may range from unusually soft and poorly formed to completely liquid.

Some cats with diarrhea also:

1. defecate more frequently;
2. urgently run to the litter box;
3. strain during defecation;
4. pass unusually large or small amounts;
5. have mucus or blood in the stool;
6. defecate outside the litter box.

An occasional soft stool and persistent watery diarrhea are not clinically equivalent.

The pattern matters.

Acute vs. Chronic Diarrhea in Cats

Veterinarians commonly distinguish diarrhea based partly on how long it has been occurring.

Acute diarrhea develops suddenly and may be associated with dietary changes, parasites, infection, medications, toxins, stress, or other short-term gastrointestinal disturbances.

Chronic diarrhea persists or repeatedly returns and raises greater concern for an ongoing gastrointestinal or systemic disorder.

Chronic diarrhea is particularly important when accompanied by:

1. weight loss;
2. changes in appetite;
3. vomiting;
4. poor body condition;
5. dull or abnormal coat quality;
6. increased thirst;
7. changes in activity;
8. persistent blood or mucus.

A cat that has had “soft poop for months” should not necessarily be considered normal simply because the cat has adapted to it.

The Litter Box Can Provide Diagnostic Clues

One advantage cat owners have is that stool can often be observed closely in the litter box.

Before cleaning it away, consider:

1. Is the stool completely liquid or just softer than usual?
2. Is there a large amount or only a small amount?
3. How many times has your cat defecated?
4. Is there mucus?
5. Is there fresh red blood?
6. Is the stool black and tar-like?
7. Has the color changed?
8. Is your cat straining?
9. Is your cat still urinating normally?

The last question is particularly important in male cats.

A cat repeatedly straining in the litter box may appear constipated or have diarrhea when the actual problem is difficulty urinating. Urethral obstruction is a life-threatening emergency.

If you cannot confirm that your cat is passing urine normally, seek veterinary care promptly.

Small-Intestinal vs. Large-Intestinal Diarrhea

One way veterinarians approach diarrhea is by determining whether clinical signs suggest the small intestine, large intestine, or both are primarily involved.

This distinction can help narrow the diagnostic possibilities.

Signs More Consistent With Small-Intestinal Diarrhea

Cats with predominantly small-intestinal disease may have:

1. larger amounts of stool;
2. moderately increased defecation frequency;
3. weight loss when disease is chronic;
4. dark or black stool if gastrointestinal bleeding is present;
5. vomiting in some cases.

Fresh mucus and straining are generally less characteristic.

Signs More Consistent With Large-Intestinal Diarrhea

Large-intestinal diarrhea, often referred to clinically as colitis, more commonly causes:

1. frequent trips to the litter box;
2. small amounts of stool at a time;
3. urgency;
4. straining;
5. mucus;
6. fresh red blood.

Some diseases affect multiple areas of the gastrointestinal tract, so this classification is not a diagnosis by itself.

It is a clinical clue.

Why Does My Cat Have Diarrhea?

Diarrhea occurs when normal digestion, absorption, intestinal movement, secretion, or the intestinal lining is disrupted.

Many conditions can cause this.

A Sudden Change in Diet

Cats can develop diarrhea after an abrupt change in food.

Even when the new diet is high quality, rapidly switching from one formulation to another can affect gastrointestinal function.

When changing diets, a gradual transition is often preferable unless a veterinarian recommends an immediate change for medical reasons.

Dietary Intolerance and Food-Responsive Disease

Some cats develop chronic gastrointestinal signs related to ingredients or characteristics of their diet.

These cats may experience:

1. chronic or intermittent diarrhea;
2. vomiting;
3. weight loss;
4. changes in appetite.

Determining whether food is involved generally requires a properly controlled dietary trial, often using a hydrolyzed or novel-protein veterinary diet selected for the individual cat.

Simply changing among several commercial foods every few days does not provide the same diagnostic information.

Intestinal Parasites in Cats

Parasites deserve particular attention in kittens, newly adopted cats, cats from shelters or catteries, outdoor cats, and multi-cat households.

Potential causes include:

1. roundworms;
2. hookworms;
3. Giardia;
4. coccidia;
5. Tritrichomonas foetus.

Not all parasites are equally likely in every cat, and not all are detected with the same test.

Giardia

Giardia is a protozoal parasite that can cause soft, poorly formed, or watery stool. Some infected cats may have intermittent signs, while others may carry the organism without obvious diarrhea.

Diagnosis may involve fecal antigen testing, microscopy, or molecular testing depending on the circumstances.

Tritrichomonas Foetus

Tritrichomonas foetus is an especially important differential diagnosis for chronic large-bowel diarrhea in cats, particularly young cats from multi-cat environments.

Affected cats may have:

1. frequent loose stool;
2. mucus;
3. fresh blood;
4. straining;
5. fecal urgency;
6. irritation around the anus.

Despite persistent diarrhea, some cats remain bright and maintain a relatively good appetite and body condition.

This can lead owners to underestimate the problem.

PCR testing of an appropriate fecal sample may be used when Tritrichomonas infection is suspected.

Because treatment decisions and drug safety require veterinary supervision, owners should not attempt to treat suspected Tritrichomonas infection using medications obtained without a diagnosis.

Diarrhea in Kittens

Diarrhea can become more significant in kittens than in healthy adult cats.

Young kittens have limited reserves and may develop dehydration, weakness, and metabolic abnormalities more quickly.

Potential causes include parasites, infectious disease, dietary problems, sudden food changes, and other gastrointestinal disorders.

Prompt veterinary assessment is particularly important when a kitten has diarrhea accompanied by:

1. vomiting;
2. poor appetite;
3. lethargy;
4. weakness;
5. blood in the stool;
6. dehydration;
7. weight loss or failure to gain weight.

A kitten with persistent diarrhea should not simply receive repeated empiric deworming without considering whether additional diagnostic testing is needed.

Can Stress Cause Diarrhea in Cats?

Stress can affect gastrointestinal function.

Moving to a new home, boarding, introducing another animal, household conflict, changes in routine, or environmental disruption may contribute to gastrointestinal signs in some cats.

However, stress is a diagnosis of context, not an explanation for every episode of diarrhea.

Persistent diarrhea, weight loss, vomiting, blood in the stool, or other abnormalities should not be attributed to anxiety without considering medical causes.

Can Antibiotics or Other Medications Cause Diarrhea?

Yes.

Some medications can alter gastrointestinal function or affect the intestinal microbial community.

If diarrhea develops shortly after starting a medication, contact the veterinarian who prescribed it.

Do not stop an important medication or alter the dose without veterinary guidance unless you have been specifically instructed to do so.

It is also important to tell your veterinarian about supplements, over-the-counter products, and any human medications your cat may have accidentally accessed.

Why Is There Mucus in My Cat's Stool?

The colon produces mucus naturally.

When the large intestine becomes irritated or inflamed, more mucus may become visible in the stool.

Mucus is therefore often associated with large-bowel diarrhea.

It can occur with:

1. colitis;
2. parasites;
3. dietary disease;
4. infectious organisms;
5. chronic intestinal disorders.

Mucus alone cannot identify the cause.

Persistent mucus, particularly when accompanied by blood, straining, weight loss, or frequent diarrhea, warrants veterinary investigation.

Blood in Cat Diarrhea: Bright Red vs. Black

The appearance of blood can provide clues about where bleeding may be occurring.

Bright Red Blood

Fresh red blood in the stool is called hematochezia.

It is generally associated with bleeding from the lower gastrointestinal tract, particularly the colon or rectum.

Small streaks can occur with colonic inflammation, but repeated or substantial bleeding requires veterinary attention.

Black, Tarry Stool

Black, sticky, tar-like stool is called melena.

Melena represents digested blood and can indicate bleeding higher in the gastrointestinal tract.

True melena is different from stool that is simply dark brown.

A cat producing black, tarry stool should be evaluated promptly, especially if there is weakness, pale gums, vomiting, poor appetite, or other signs of illness.

What Can Stool Color Tell You?

Owners often search for specific explanations based on stool color.

Color can provide useful context, but it rarely establishes a diagnosis by itself.

Yellow Cat Diarrhea

Yellow stool can occur when intestinal contents move rapidly through the digestive tract or when digestion is altered.

Persistent yellow diarrhea should be interpreted alongside appetite, weight, vomiting, medications, diet, and other clinical signs.

Green Cat Diarrhea

Green coloration may reflect rapid intestinal transit, something the cat consumed, or other gastrointestinal changes.

If toxin exposure is possible, contact a veterinarian immediately.

Very Pale or Gray Stool

Persistently pale or unusually colored stool may warrant investigation of digestion and hepatobiliary or pancreatic function, depending on the clinical context.

Rather than diagnosing disease from color alone, photograph unusual stool and show it to your veterinarian.

Chronic Diarrhea and Weight Loss Are Particularly Important

A cat that continues eating but gradually loses weight while having chronic diarrhea deserves investigation.

This combination can occur when nutrients are not being properly digested or absorbed or when systemic disease increases metabolic demands.

Potential causes include:

1. chronic enteropathy;
2. food-responsive gastrointestinal disease;
3. intestinal lymphoma;
4. hyperthyroidism;
5. pancreatic disease;
6. intestinal parasites;
7. other systemic disorders.

The diagnostic approach depends heavily on the cat's age, physical examination, laboratory findings, and pattern of clinical signs.

Hyperthyroidism Can Cause Diarrhea in Older Cats

Hyperthyroidism is an important disease of middle-aged and older cats.

Excess thyroid hormone increases metabolic activity throughout the body.

Common signs can include:

1. weight loss despite a normal or increased appetite;
2. increased activity or restlessness;
3. increased thirst;
4. increased urination;
5. vomiting;
6. diarrhea or increased stool volume;
7. rapid heart rate;
8. poor coat condition.

Not every hyperthyroid cat has diarrhea, but hyperthyroidism should be considered when an older cat develops gastrointestinal signs together with unexplained weight loss.

Blood testing is used as part of the diagnostic evaluation.

Pancreatic Disease and Diarrhea

The pancreas plays essential roles in digestion and metabolism.

Pancreatitis

Cats with pancreatitis can have vague and nonspecific symptoms.

Unlike the classic presentation sometimes seen in dogs, affected cats may simply become quiet, stop eating, lose weight, vomit, or develop diarrhea.

Because signs can be subtle, diagnosis may require a combination of physical examination, laboratory testing, pancreatic-specific testing, and imaging.

Exocrine Pancreatic Insufficiency

Exocrine pancreatic insufficiency (EPI) occurs when the pancreas does not produce enough digestive enzymes.

Although less commonly recognized in cats than in dogs, feline EPI is an important cause of chronic gastrointestinal disease.

Affected cats may develop:

1. weight loss;
2. chronic loose stool or diarrhea;
3. increased or altered appetite;
4. poor body condition.

Cats with EPI can also have cobalamin (vitamin B12) deficiency, which may require specific supplementation.

Diagnosis typically involves measurement of feline trypsin-like immunoreactivity (fTLI), along with other testing as indicated.

Why Vitamin B12 Matters in Cats With Chronic Diarrhea

Cobalamin deserves special attention in feline chronic gastrointestinal disease.

Vitamin B12 is absorbed in the distal small intestine, and cats with chronic intestinal or pancreatic disease may develop low cobalamin concentrations.

Deficiency can contribute to gastrointestinal dysfunction and poor response to treatment.

For cats with chronic diarrhea, weight loss, suspected enteropathy, or pancreatic disease, veterinarians may evaluate serum cobalamin and folate concentrations as part of the diagnostic workup.

This is one example of why chronic feline diarrhea often requires more than simply changing food or giving a general probiotic.

Chronic Enteropathy and Intestinal Lymphoma

Persistent gastrointestinal signs in cats can sometimes reflect chronic inflammatory disease or gastrointestinal lymphoma.

These conditions can produce similar clinical signs, including:

1. chronic diarrhea;
2. vomiting;
3. weight loss;
4. appetite changes;
5. loss of muscle mass.

Distinguishing between chronic inflammatory enteropathy and certain forms of intestinal lymphoma can be challenging.

Bloodwork and ultrasound may identify abnormalities but do not always provide a definitive diagnosis.

In selected patients, gastrointestinal biopsy and histopathology may be needed.

This diagnostic overlap is particularly relevant in older cats with persistent gastrointestinal signs and progressive weight loss.

When Is Cat Diarrhea an Emergency?

Diarrhea itself is not always an emergency.

The urgency depends on the severity of the diarrhea and the cat's overall condition.

Seek urgent veterinary care if diarrhea is accompanied by:

1. severe lethargy or weakness;
2. collapse;
3. repeated vomiting;
4. inability to keep water down;
5. significant or repeated blood loss;
6. black, tarry stool;
7. pale gums;
8. significant abdominal pain;
9. suspected toxin exposure;
10. suspected foreign-body ingestion;
11. severe dehydration;
12. major changes in responsiveness.

Kittens and cats with existing medical conditions may require veterinary attention sooner because they can have less physiologic reserve.

What If My Cat Has Diarrhea but Seems Fine?

An otherwise healthy adult cat with a single soft stool and completely normal behavior may not require emergency treatment.

Monitor:

1. appetite;
2. water intake;
3. activity;
4. urination;
5. frequency of bowel movements;
6. vomiting;
7. stool consistency;
8. blood or mucus.

If diarrhea continues, repeatedly returns, or another symptom develops, contact your veterinarian.

Cats are particularly good at hiding illness, so “still acting normal” should not be the only criterion used to decide whether a persistent problem needs investigation.

Should You Change Your Cat's Food Immediately?

Not necessarily.

Repeatedly switching foods whenever diarrhea appears can create additional gastrointestinal disruption and make food-responsive disease more difficult to investigate.

If the diarrhea is mild and recent, ask your veterinarian whether dietary modification is appropriate.

For chronic diarrhea, a veterinarian may recommend a structured diet trial rather than random food changes.

A diagnostic diet trial must be followed carefully. Feeding treats, flavored medications, table food, or other protein sources can interfere with interpretation in some cases.

Should You Give Your Cat Human Anti-Diarrhea Medication?

No human anti-diarrheal medication should be given to a cat unless specifically directed by a veterinarian.

Cats metabolize many medications differently from people, and some human drugs can be dangerous or inappropriate.

Do not give:

1. leftover antibiotics;
2. another pet's medication;
3. human gastrointestinal medication;
4. herbal or over-the-counter remedies

without veterinary guidance.

Stopping diarrhea is not the same as treating its cause.

Are Probiotics Useful for Cats With Diarrhea?

Probiotics may be used in selected feline gastrointestinal conditions, but “probiotic” describes a broad category of products rather than a single standardized treatment.

Different products contain different organisms, strains, concentrations, and formulations.

Evidence is stronger for some products and clinical situations than others.

A probiotic should not replace diagnostic investigation when a cat has persistent diarrhea, weight loss, vomiting, blood in the stool, or systemic illness.

Ask your veterinarian whether a specific veterinary probiotic is appropriate for your cat.

How Veterinarians Diagnose Chronic or Recurrent Diarrhea

The diagnostic approach should be individualized.

Your veterinarian will consider the cat's age, diet, lifestyle, parasite exposure, medications, duration of symptoms, appetite, body weight, and physical examination findings.

Testing may include:

1. fecal examination;
2. Giardia testing;
3. PCR testing for selected infectious organisms;
4. complete blood count;
5. serum chemistry;
6. urinalysis;
7. thyroid testing in appropriate cats;
8. feline pancreatic lipase testing;
9. fTLI testing for suspected EPI;
10. cobalamin and folate testing;
11. abdominal ultrasound;
12. dietary trials;
13. gastrointestinal biopsy in selected cases.

Not every cat requires every test.

The objective is to develop a logical diagnostic plan based on the most likely causes for that particular patient.

Bring a Stool Sample to the Veterinary Appointment

A fresh fecal sample can be valuable.

If your veterinarian requests one, collect a recent sample into a clean, sealed container and follow the clinic's instructions for storage and transport.

For some organisms, sample quality and collection technique are particularly important.

A photograph of the stool can also help document:

1. color;
2. consistency;
3. mucus;
4. visible blood;
5. approximate volume.

If several cats share litter boxes, determining which cat has diarrhea may require temporarily monitoring them separately.

Multi-Cat Households Require Extra Investigation

Diarrhea can be particularly difficult to evaluate when several cats share litter boxes.

First, identify which cat is affected.

Then consider whether more than one cat has gastrointestinal signs.

When multiple cats develop diarrhea, veterinarians may place greater emphasis on shared exposures such as:

1. infectious organisms;
2. parasites;
3. diet;
4. contaminated food or water;
5. environmental factors.

This information can significantly change the diagnostic approach.

Frequently Asked Questions

Why does my cat have diarrhea but act normal?

Mild gastrointestinal disturbances and some parasites can cause diarrhea before a cat appears systemically ill. Some cats with chronic intestinal disease can also maintain relatively normal behavior for a period of time. Persistent or recurrent diarrhea should therefore be investigated even if your cat seems otherwise well.

When should I worry about diarrhea in my cat?

Contact a veterinarian if diarrhea persists, repeatedly returns, contains significant blood, is black and tarry, or occurs with vomiting, weight loss, poor appetite, lethargy, pain, or dehydration. Kittens and medically fragile cats may need attention sooner.

Why does my cat have diarrhea and vomiting?

Both symptoms can occur with dietary disease, parasites, infections, pancreatitis, chronic enteropathy, toxins, foreign bodies, and systemic illness. Repeated vomiting combined with diarrhea increases the risk of dehydration and warrants veterinary evaluation.

What does mucus in cat poop mean?

Visible mucus commonly indicates irritation or inflammation of the large intestine. It can occur with colitis, parasites, dietary disease, and other gastrointestinal conditions. Persistent mucus should be evaluated in context with the cat's other symptoms.

What causes chronic diarrhea in older cats?

Potential causes include chronic enteropathy, hyperthyroidism, intestinal lymphoma, pancreatic disease, dietary disorders, and other systemic conditions. Diagnostic testing is usually necessary because several diseases can produce similar symptoms.

Can parasites cause chronic diarrhea in an indoor cat?

Yes. Indoor status does not completely eliminate parasite risk. Exposure can occur before adoption, through other animals, contaminated environments, or other routes. Your veterinarian can determine which parasite tests are appropriate.

Why is my cat losing weight but still eating and having diarrhea?

Weight loss despite a good or increased appetite can occur with conditions such as hyperthyroidism, intestinal disease, malabsorption, pancreatic disease, and other disorders. This combination warrants veterinary evaluation.

Is bloody diarrhea in cats an emergency?

A small streak of fresh blood can occur with colonic inflammation, but repeated or substantial bleeding requires veterinary attention. Black, tarry stool can represent digested gastrointestinal blood and should be evaluated promptly.

Can I give my cat something at home to stop diarrhea?

Do not give human anti-diarrheal medications or leftover veterinary drugs without professional guidance. Treatment depends on the underlying cause, and some medications that are used in people or dogs may be unsafe or inappropriate for cats.

Persistent Diarrhea Deserves a Diagnosis

A single episode of soft stool in an otherwise healthy adult cat may resolve without becoming a major medical problem.

But chronic or recurrent diarrhea is different.

The pattern of stool, the cat's age, body weight, appetite, vomiting history, parasite exposure, and other clinical signs can point toward very different diseases—from protozoal infections in young cats to hyperthyroidism, pancreatic disease, chronic enteropathy, or intestinal lymphoma in older cats.

That is why the goal should not simply be to stop the diarrhea.

The goal is to determine why it is happening.

If your cat has persistent diarrhea, unexplained weight loss, recurrent vomiting, blood in the stool, appetite changes, or other signs of illness, schedule a veterinary evaluation. A structured diagnostic approach is much more useful than repeatedly changing foods or trying nonspecific treatments at home.