Some cats can handle a change to a new food and never have a single problem. Others have an upset stomach after eating a few bites of a different type of food. Cat owners ask their vets, “How long will my cat have diarrhea after changing food?” Here’s what I tell my clients…

The goal is to change food gradually enough that your cat doesn’t get diarrhea at all. If the cat’s stool is soft, you need to change your approach. Either stop the new food until the stool firms up or go much more slowly with the transition. If diarrhea persists for more than 48 hours, your cat should see a veterinarian. 

I’m going to discuss exactly what I tell clients to do when their cat is reacting to a food change. It can get confusing trying this on your own, so read on for my veterinary perspective.


  • Diarrhea after a food change is likely due to the cat’s digestive system and microbiome reacting to unfamiliar nutrients.
  • Gradual food transition, a temporary return to old food, and diagnostic testing can help resolve food-related feline diarrhea.
  • If diarrhea persists more than 48 hours, consult a veterinarian.


When we use the term diarrhea, we mean that a cat’s fecal matter has more moisture than normal. Cat diarrhea happens when there is a problem with the GI tract or with the food they’re eating (or both).

Symptoms of Diarrhea in Cats

  • Soft or liquid feces
  • Mucus or blood in feces
  • Larger or smaller amount of stool than normal
  • Increased frequency of bowel movements
  • Straining to pass stool
  • Passing more gas/gurgly tummy

Causes of Cat Diarrhea

Diarrhea is one of the most common diseases we veterinarians treat in cats. It’s a general symptom that has many possible causes. Changing cat food is one common cause. Here are a few more triggers of diarrhea: 

  • Internal parasites (worms, Tritrichomonas foetus, Giardia lamblia)
  • Toxin ingestion (plants, chemicals, mycotoxins)
  • Diet change or dietary indiscretion (eating garbage, rotten dead animals)
  • Viral infection (coronavirus, FeLV)
  • Stress, including surgery
  • Kidney and liver disease
  • Hyperthyroidism

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Why Does a Food Change Cause Diarrhea?

We don’t have much scientific data to explain exactly why it happens. Most experts theorize it has to do with the cat’s microbiome and digestive enzymes reacting to unfamiliar nutrients. 

A cat’s stomach and intestinal tract host friendly bacteria and yeast that help them digest food. These tiny helpers like different types of food. 

If a cat eats one type of food for a long time, their gut microbes become good at breaking down that specific food. If the cat suddenly starts eating a different type of food, the organisms may not know what to do with it. 

Acute diarrhea is a symptom of chaos in a cat’s gut. It takes time for a new population of adapted organisms to grow and take their place.

Food Factors That May Cause Diarrhea

You’d be surprised how different two cat foods can be even when they appear similar at first glance. Some differences that often cause gut problems include

  • Protein source (chicken to fish, for example)
  • Moisture level (dry food to moist food)
  • Fat content (low to higher fat)
  • Fiber content (low to high fiber)

There is always the possibility that the new food is not really to blame for the digestive upset. The food change could have been a coincidence or it could have unmasked an underlying disease. 

Sad yellow and white cat. (How long will my cat have diarrhea after changing food?)
Most cats should never get diarrhea from changing food if you do it gradually!

Treating Diarrhea in Cats After Changing Food

So what should you do if you’ve changed your cat’s food and now they have diarrhea? First, if the cat has had diarrhea for more than 48 hours, they need to see a veterinarian. 

If your kitty has soft stool intermittently and has no other symptoms, consider the following…

Make a More Gradual Transition to New Food

I tell my clients to change from the original food to the new one over the course of 7 days. But that may not be gradual enough for cats with a sensitive stomach. It could take several weeks to complete the process while avoiding a digestive upset. 

Consider how important it is that the cat eat the new food. If it was done on a whim or to increase convenience, it might not even be worth the hassle of changing! If it’s a prescription diet or something recommended by your vet, ask them if there is a different option to try. 

Temporary Return to Old Food 

If you slow down your food transition process, you should notice your cat’s poop firming up within a day or two. If they still have diarrhea after that, you can try returning to feeding 100% of the last food the cat did well with. 

Feed the original food for 2-4 weeks until stools stabilize. Then you can restart the transition using a more gradual method. 

Consultation With a Veterinarian

Food intolerance can be temporary or long-term. If your cat’s GI tract seems unable to adapt to a new type of cat food despite your best efforts, it’s time to consult your veterinarian again. 

Your vet will do a physical examination first. They may recommend diagnostic testing for parasites, viral infection, pancreatitis, IBD, liver, kidney, thyroid and other systemic diseases.

Your vet may recommend a therapeutic diet trial if they suspect a food allergy. This requires feeding cat food that is highly digestible and contains a novel protein or hydrolyzed protein source. The special diet needs to be fed exclusively for a period of time. Meanwhile, you must avoid feeding all other foods to see if the cat’s digestion improves. 

Sometimes vets recommend probiotic supplements during a food transition. But not all pet probiotics are created equally. Ask your vet for a recommended product and exactly how you should use it. 

Frequently Asked Questions (FAQs)

Cats should not get diarrhea after changing food if it is done gradually. If your cat’s stool is loose after a food change, return to the previous food and make the change more gradually.

You may have changed food too quickly or your cat could have a food intolerance.

Veterinarians recommend changing food gradually over a period of 7-14 days to allow a cat’s stomach and intestines to adjust. Changing a cat’s food should not cause diarrhea.

Change back to the previous food the cat tolerated. Next time you change food, try doing it more gradually over a longer period of time. If the diarrhea persists longer than 48 hours, your cat should see a veterinarian.


If your cat is going through this, don’t panic! Diarrhea can sometimes happen when you change your their food. It takes time for their digestive system to adapt to new combinations of nutrients.

Remember to switch foods gradually to avoid stomach upset. But if the diarrhea lasts more than 48 hours, it’s important to consult a veterinarian. They can provide guidance and rule out any other health issues. provides content for informational and entertainment purposes. You should always seek care from a veterinarian to diagnose and treat your unique pet. Visit the Privacy Policy and Terms of Use section of this site to learn more.

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  1. Bybee, S. N., Scorza, A. V., & Lappin, M. R. (2011). Effect of the probiotic Enterococcus faecium SF68 on presence of diarrhea in cats and dogs housed in an animal shelter. Journal of Veterinary Internal Medicine, 25(4), 856-860.
  2. Grandi, M., Vecchiato, C. G., Biagi, G., Zironi, E., Tondo, M. T., Pagliuca, G., … & Gazzotti, T. (2019). Occurrence of mycotoxins in extruded commercial cat food. ACS omega, 4(9), 14004-14012.
  3. Guilford, W. G., Jones, B. R., Markwell, P. J., Arthur, D. G., Collett, M. G., & Harte, J. G. (2001). Food sensitivity in cats with chronic idiopathic gastrointestinal problems. Journal of Veterinary Internal Medicine, 15(1), 7-13.
  4. Sabshin, S. J., Levy, J. K., Tupler, T., Tucker, S. J., Greiner, E. C., & Leutenegger, C. M. (2012). Enteropathogens identified in cats entering a Florida animal shelter with normal feces or diarrhea. Journal of the American Veterinary Medical Association, 241(3), 331-337.Suchodolski, J. S., Foster, M. L., Sohail, M. U., Leutenegger, C., Queen, E. V., Steiner, J. M., & Marks, S. L. (2015). The fecal microbiome in cats with diarrhea. PloS one, 10(5), e0127378.