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If the diarrhea happened within a day or so of changing cat foods or introducing a new food, it could be the new diet. Go back to the old food that was not causing problems and the stool may go back to normal. When the stool is firmed up, try changing the diet very gradually, a little new food at a time. If you suspect your cat is allergic to her food, try changing her diet. Food sensitivities (allergies) do occur in cats and may cause diarrhea. So, when changing the diet, make sure the new food has very different ingredients than the old food, otherwise, there may not be much difference to the cat at all. Your veterinarian can also discuss with you trying a high fiber diet for your cat. This may include the use of a prescription diet available only through veterinary channels, and not found on the pet store shelves. Royal Canin, Hill’s Prescription Diets and Purina pet food companies have a veterinary-only subset of diets created for special medical conditions. For some pets, you might slowly introduce a new food. A good ratio might be 90 percent current food and 10 percent new food. Gradually increase the amount until you reach all new food in about 10 days. Let your kitty tell you how fast you can switch to the new diet. Other cats may prefer adding 10% new food and remaining at that ratio for 3-5 days before adding in 10% more. There is no hard and fast rule and it is not a race. Adding half a teaspoon of unflavored Metamucil into your cat's food once or twice a day for 5-7 days  may help firm things up. Canned plain pumpkin may do the same thing. Both Metamucil and canned pumpkin are high in fiber. Probiotics add good intestinal “bugs” back into the gastrointestinal system to help rebalance a system that is out of sorts with the diarrhea. An easy and tasty option is FortiFlora for cats, an over the counter (OTC) Purina product. Water loss can be significant with ongoing diarrhea and your pet can get dehydrated. To detect dehydration, gently pinch the normally loose skin at the back of the neck. When a cat is normally hydrated, the skin snaps right back down. If the pinched skin smooths out slowly, or remains tented, the cat is dehydrated. If the cat appears to be dehydrated, take her to the vet right away.

Summary:
Consider any changes to the cat's food. Consider if your cat has food allergies. Gradually introduce a new diet. Use Metamucil. Add probiotics to your cat’s diet. Offer plenty of water.