Diarrhea: Worry or wait?

If loose stool lasts more than two days, call the vet

Diarrhea is no fun for you or your dog, but the good news is that most cases resolve on their own. Mild cases are usually uncomplicated, and the dog is otherwise happy and normal.

“Many cases of diarrhea in dogs and cats are mild and self-limiting,” says Dr. Meredith Miller, associate clinical professor of small animal medicine. “If a pet stops eating, is lethargic, the diarrhea is black or tarry in quality, there is associated vomiting, or the diarrhea doesn’t resolve in 48-72 hours then veterinary care should be sought.” 

Under normal conditions, water is absorbed through your dog’s gastrointestinal (GI) tract to be used within the body. When they have diarrhea, everything passes through too quickly, and excess water is expelled along with the waste products. Over time, this can cause dehydration. Vomiting, in addition to diarrhea, further accelerates dehydration. Dehydration causes electrolyte imbalances in the body, puts extra strain on the kidneys and can lead to organ failure and death.

Anorexia and lethargy indicate that your dog does not feel good. This may just be due to gastrointestinal discomfort (no one wants to eat when their guts are twisting), but these symptoms can also accompany a variety of illnesses. If your dog does not bounce back to normal quickly, veterinary attention should be sought.

Black stool, officially called melena, is caused by partially digested blood coming out in the feces. The source of the blood could be in the upper GI tract, the mouth or even the respiratory tract. These bleeds can be caused by a foreign body, infections, inflammation, trauma, tumors or ulcers.

Bland diet

“Mild cases of diarrhea in both cats and dogs can be treated at home by feeding a bland diet such as boiled chicken or low-fat hamburger, and white rice,” says Miller. Cooked pasta is another option. These foods are easy to digest, so they give your dog’s GI tract a break.

The exception to this rule would be if your dog has an allergy or intolerance to one of these foods — if that is the case, avoid the problematic ingredient to prevent further upset. Another tip is to start by withholding all food for 12-24 hours, then introduce the bland diet. Note: These bland diets are not appropriate for long-term feeding as they are not nutritionally balanced.

If your dog is experiencing diarrhea after switching to a new food, it may be that you made the switch too quickly. Go back to feeding the original diet until their stools have firmed up again, then restart the transition process but go more gradually. For example, instead of feeding 75% the original diet and 25% the new diet on the first day, 50% of each on the second day, and then 25% original and 75% new on the third day, spread the transition over a week or two so that your dog’s GI tract has more time to adjust.

“Owners should avoid excessive treats or rich food until the diarrhea resolves,” says Miller.

If diarrhea continues for more than a day or two, schedule an appointment with your veterinarian to help your dog get some relief.


Transient causes of diarrhea include:

  • Dietary indiscretion
  • Switching to a new food too quickly
  • A stressful event, such as going to the veterinarian or being boarded
  • Internal parasites

Some more serious potential causes of diarrhea include:

  • Infection or inflammation in the GI tract
  • Viruses, such as parvovirus
  • Foreign body
  • Allergies
  • Cancer
  • Toxicity
  • Pancreatic disorders
  • Addison’s disease, liver or heart disease
  • Immune disorders


Treatment will depend on the exact cause of the diarrhea. Bring a stool sample when you head to the veterinarian. This allows the veterinarian to see what the diarrhea looks like, and a fecal exam can check for parasites.

Radiographs (x-rays) will be done if a blockage is suspected, and blood-work will be ordered if your veterinarian is concerned about a systemic illness. If your dog is dehydrated, they may receive fluids.

The most popular medication for diarrhea is metronidazole (brand name Flagyl). While metronidazole isn’t a miracle drug, it’s pretty close, and it targets microbes, protozoa and inflammation. Metronidazole can cross the blood-brain barrier, so at high doses it can sometimes cause neurological symptoms. This side effect will go away once treatment is discontinued.

Another commonly prescribed medication is tylosin (brand name Tylan). Your veterinarian may also send you home with a probiotic to help restore balance to your dog’s gastro-intestinal (GI) tract.

What you should do

You should see a veterinarian when:

  • Withholding food or trying a bland diet doesn’t work after 24-48 hours
  • Stool is black or tarry
  • Vomiting also occurs
  • Continued lack of appetite

Never try human medications such as Pepto Bismol (bismuth subsalicylate) or Imodium (loperamide) unless directed by a veterinarian, as they can be harmful to certain dogs and may not pair well with other medications.

This article has been reprinted with permission from the Cornell University College of Veterinary Medicine’s DogWatch newsletter, published by Belvoir Media Group. When you become a member of the Riney Canine Health Center, you will receive a free subscription to DogWatch.