Most acute diarrhea in an adult cat clears within 24 to 48 hours and is caused by a sudden diet change, something unusual eaten, or stress. Kittens, senior cats, and cats with blood in the stool, vomiting, or lethargy are a different matter: those need a vet the same day, because dehydration and panleukopenia can kill a small kitten fast.

The common causes, roughly by frequency

Dietary upset is the most frequent trigger: a new food switched too quickly, too many treats, milk, or something scavenged. Parasites come next — roundworms, hookworms, Giardia, coccidia, and in young cats from catteries or shelters, Tritrichomonas foetus, which causes a waxing and waxy diarrhea that ordinary dewormers do not touch.

Stress matters more than owners expect: a move, a new pet, boarding, or building work can produce loose stools for days. Less common but important causes include bacterial overgrowth, food allergy, inflammatory bowel disease, hyperthyroidism, lymphoma in older cats, toxins and houseplants, and a foreign body or blockage. Sudden access to rich food, rubbish, or a new treat is a classic setup, and the fix is usually time rather than medicine. In an unvaccinated kitten, panleukopenia is the first thing to rule out.

Small bowel or large bowel: a useful clue

Small-bowel diarrhea means large volume, few episodes, weight loss, and no mucus. Large-bowel diarrhea means small amounts, frequent trips, straining, mucus, and fresh red blood — the cat goes often and produces little. The distinction helps a vet narrow the list, but it is not a diagnosis.

Colour and consistency matter too. Black, tarry stool suggests digested blood from higher up. Pale greasy stool suggests fat malabsorption. Simply soft pudding-like stool with no other signs is the most common and least alarming pattern.

What you can do at home

Keep water available and offer small, frequent meals of plain cooked chicken with the fat and bones removed, or a vet-approved bland diet, for two or three days while the gut settles. Do not fast a kitten at all, and do not fast an adult cat for more than 12 to 24 hours — cats that stop eating can develop liver complications within a few days.

A probiotic designed for cats may shorten mild episodes; ask the clinic which one they stock, because strains matter. Keep the litter box clean, wash bedding, and wash your hands after contact, especially if there are children or an immunocompromised person at home. If several cats are affected, assume something infectious and separate them.

What not to do

Do not give human anti-diarrheal medicine. Loperamide can cause neurological signs in cats and the margin is narrow; bismuth subsalicylate contains a salicylate that cats clear poorly. Do not use antibiotics from an old prescription, and do not give human painkillers of any kind.

Do not assume worms are the answer and deworm blindly for weeks. Do not switch foods repeatedly during an episode — the gut needs consistency, not novelty. And do not delay a vet visit because the cat "seems fine" while it is losing weight.

Red flags: vet today

  • Any blood in the stool, especially black tarry stool or large amounts of fresh blood.
  • Diarrhea plus vomiting, lethargy, or refusal to eat.
  • Any diarrhea in a kitten under 12 weeks, or in an unvaccinated cat of any age.
  • Signs of dehydration: tacky gums, skin that stays tented, sunken eyes, weakness.
  • A hard, painful, or bloated abdomen, or straining with nothing produced.
  • Fever, collapse, tremors, or a kitten that feels cold.
  • Diarrhea lasting more than 48 hours in an adult, or two weeks of intermittent soft stool.

What the vet will probably do

Expect a physical exam, weight, hydration check, and a faecal test — flotation for worm eggs, and PCR or a specific test if Giardia or Tritrichomonas is suspected. Bloodwork is reasonable if the cat is older, thin, or unwell. X-rays or ultrasound look for foreign bodies, thickening, or masses. Most cases need no more than that. Take a photo of the stool and note the date, because a picture is more useful than a description recalled a week later.

Chronic diarrhea — three weeks or more, or recurring — deserves a structured workup rather than repeated courses of the same dewormer. Diet trials, ultrasound, and sometimes biopsy are the usual next steps.

Preventing the next episode

Change food over seven to ten days, mixing increasing proportions of the new food. Keep treats to a small fraction of daily calories and skip dairy. Deworm on a schedule matched to lifestyle, and have a stool sample checked at the annual visit. Store food sealed and out of reach, and check houseplants against a toxicity list.

If a specific food or treat reliably triggers loose stool, write it down. Patterns that are obvious in a note are invisible in memory.

This is general information, not a diagnosis.

Frequently asked

Should I withhold food when my cat has diarrhea?

Not for kittens, and not for long in adults. Current thinking favours small, frequent bland meals rather than fasting, because the gut recovers better with gentle nutrition and cats that stop eating can develop liver problems within a few days. Offer water at all times.

Can I give my cat human anti-diarrheal tablets?

No. Loperamide can cause neurological signs in cats and bismuth subsalicylate contains a salicylate cats clear poorly. Both are easy to overdose. If the diarrhea is bad enough that you want medication, that is the point to call the clinic.

How long can I wait before seeing a vet?

For a bright adult cat with soft stool and no other signs, 48 hours of home support is reasonable. For a kitten, an unvaccinated cat, or any cat with blood, vomiting, lethargy, pain, or dehydration, go the same day — waiting is the main reason these cases become critical.