Diarrhea in cats stems from a wide range of causes, from something as simple as a sudden food switch to serious conditions like intestinal disease or organ dysfunction. Most cases are acute, resolving within a few days, but diarrhea lasting three to four weeks or longer is classified as chronic and typically signals an underlying problem that needs veterinary attention.
Sudden Diet Changes
One of the most common triggers is an abrupt switch in food. When you change your cat’s diet overnight, undigested nutrients sit in the intestinal tract and pull water into the gut through osmosis, producing loose, watery stool. This is called osmotic diarrhea, and it happens because your cat’s digestive enzymes haven’t had time to adapt to the new protein or fat source. The fix is straightforward: transition to any new food gradually over seven to ten days, mixing increasing amounts of the new food with the old.
Food intolerances and allergies work through a different mechanism. Instead of a temporary adjustment issue, certain ingredients trigger an immune or inflammatory response in the intestinal lining. This can cause secretory diarrhea, where the gut actively pumps extra fluid into the intestine rather than absorbing it normally. Common culprits include beef, fish, and dairy. If your cat gets diarrhea repeatedly on the same food, an intolerance is worth investigating with your vet through an elimination diet trial.
Parasites and Infections
Intestinal parasites are a frequent cause of diarrhea, especially in kittens and cats with weakened immune systems. Adult cats can carry parasites with no visible symptoms at all, which makes diagnosis tricky. Egg shedding in stool is often intermittent, so a single negative fecal test doesn’t rule parasites out.
The specific parasites produce different types of stool changes:
- Giardia causes fatty, foul-smelling diarrhea. Blood and mucus are uncommon, and you typically won’t see fever or vomiting alongside it.
- Coccidia (including Cryptosporidium) colonizes the small intestine and causes high-volume, infrequent stools along with weight loss. Cats with chronic infections can develop straining and signs of inflammation in the large intestine. This parasite is especially persistent because thin-walled cysts stay in the gut and cause reinfection from within.
- Roundworms are one of the most common intestinal worms in cats. Infection is often silent in healthy adults but can cause significant diarrhea and poor growth in kittens.
Bacterial infections from organisms like Salmonella and Campylobacter also cause diarrhea, usually through contaminated raw food, prey animals, or contact with infected feces. These infections tend to produce more acute, severe symptoms including vomiting and lethargy.
Inflammatory Bowel Disease
Inflammatory bowel disease (IBD) is one of the most common causes of chronic diarrhea in cats. It involves persistent inflammation of the intestinal lining that lasts longer than two to four weeks and keeps coming back. Unlike a simple stomach bug, IBD doesn’t have a single identifiable cause. The current thinking is that initial damage to the intestinal lining, possibly from a virus or dietary reaction, increases gut permeability. This lets bacteria and food proteins leak through the barrier, triggering an ongoing immune response that perpetuates the inflammation even after the original trigger is gone.
Cats with IBD may also vomit, lose weight, and eat less. Because the symptoms overlap with other serious conditions, including intestinal lymphoma, diagnosis usually requires intestinal biopsies. Treatment typically involves dietary management combined with medications that suppress the overactive immune response in the gut.
Hyperthyroidism and Organ Disease
In middle-aged and older cats, diarrhea can be a sign of systemic disease rather than a gut problem. Hyperthyroidism, one of the most common hormonal disorders in older cats, produces a classic cluster of symptoms: weight loss despite a big appetite, increased thirst and urination, vomiting, diarrhea, and an unkempt coat. The excess thyroid hormone speeds up metabolism throughout the body, including gut motility, which pushes food through the intestines too quickly for proper absorption.
Kidney disease and liver disease can also cause diarrhea as waste products build up in the bloodstream and irritate the digestive tract. Pancreatic insufficiency, where the pancreas doesn’t produce enough digestive enzymes, leads to malabsorption and greasy, voluminous stools. In any cat over seven or eight years old with persistent diarrhea, bloodwork to check organ function and thyroid levels is a standard first step.
Household Toxins and Medications
Cats are unusually sensitive to many common household substances because their livers lack certain detoxification enzymes that other species have. Several everyday items can cause diarrhea if ingested:
- Ibuprofen and other NSAIDs cause vomiting, depression, loss of appetite, and diarrhea. Even topical pain creams containing these drugs can poison a cat that grooms the residue off your skin.
- Fertilizers cause mild to moderate gastrointestinal irritation, including vomiting, diarrhea, drooling, and abdominal pain.
- Lead (found in old paint, fishing weights, and some imported items) causes vomiting, diarrhea, abdominal pain, and in severe cases, seizures and blindness.
- Insecticides containing organophosphates or carbamates trigger vomiting, diarrhea, drooling, muscle tremors, and seizures.
Many common houseplants, including lilies, pothos, and philodendrons, also irritate the GI tract. If you suspect your cat has ingested something toxic, the speed of treatment matters more than identifying the exact substance first.
Stress-Related Diarrhea
Cats are creatures of routine, and disruptions to their environment can directly affect their gut. Moving to a new home, introducing a new pet, construction noise, changes in your schedule, or even rearranging furniture can trigger stress colitis, inflammation of the large intestine that produces soft stool, sometimes with mucus or small amounts of blood. This type of diarrhea usually involves frequent, small-volume stools with straining, as opposed to the large, watery stools associated with small intestinal problems. It typically resolves once the cat adjusts, but recurring stress-related episodes suggest the cat needs environmental enrichment or anxiety management.
Small Intestine vs. Large Intestine Diarrhea
Paying attention to what your cat’s diarrhea actually looks like gives useful information about where the problem is happening. Small intestinal diarrhea produces large volumes of watery or fatty stool, passed at normal frequency, often accompanied by weight loss. Large intestinal diarrhea looks different: small, frequent stools, sometimes with visible mucus or fresh blood, and you may notice your cat straining in the litter box. Some conditions affect both, but this distinction helps your vet narrow down the cause faster.
Managing Mild Diarrhea at Home
For an otherwise healthy adult cat with no other symptoms, a brief episode of diarrhea can often be managed with a temporary bland diet. The standard approach is a mix of 75% boiled white rice and 25% boiled lean chicken (no skin or bones) or lean ground beef. This combination is low in fiber, fat, and protein but high in easily digestible carbohydrates, giving the gut a chance to recover without much digestive work.
Stools should be nearly back to normal within about seven days. The typical bland diet period lasts around ten days, after which you gradually reintroduce your cat’s regular food. Make sure fresh water is always available, since diarrhea causes fluid loss quickly in small animals.
Signs That Need Prompt Veterinary Care
Diarrhea paired with any of the following warrants a vet visit sooner rather than later: vomiting, blood in the stool, weakness, unusual drooling, loss of appetite, or signs of dehydration like sunken eyes, a dry nose, or sticky gums. Kittens, senior cats, and cats with known health conditions are at higher risk of dehydrating rapidly, so even a day or two of diarrhea in these groups is worth a call to your vet.