Lots of things can cause diarrhea in dogs and a single episode is probably nothing to worry about. You may not even need to see a veterinarian if it happens only one time. The first thing that you should do is make sure that your dog did not eat anything poisonous. Avoid treats and table scraps for a day or two. Limit his food intake to regular dog food. Make sure that you keep him well hydrated. Michael Leed, Professor of Small Animal Internal Medicine at Virginia Maryland Regional College of Veterinary Medicine says that usuually nothing specific has to be done. He says that most cases of diarrhea in dogs get better on their own without any specific treatment or pet medication. However if the diarrhea becomes more frequent or if blood and vomiting develop, your dog then needs to see a veterinarian.