If your dog is sick or ate poorly, he may stop eating. Megan E. McClosky, DVM, lecturer in internal medicine at the University of Pennsylvania School of Veterinary Medicine, says if your dog recovers quickly from "why is my dog not eating," it's not a significant issue. Call the vet even if your dog has a bellyache and isn't eating.
Teller adds infectious disorders like parvovirus or pancreatitis can also prevent dogs from eating. Your dog may have gastric dilatation and volvulus—a stomach twist that prevents food from passing—or cancer. Your dog may be lethargic, throw up, have diarrhea or bloody stools, cough, and sneeze while unwell.
Your dog may not eat because they ate something wrong. Dogs are voracious feeders and often devour toys, tennis balls, even tampons. It's especially important to avoid these backyard dog dangers if your dog is young and energetic.
"Dogs do a great job of trying to mask pain but that could be another reason why a dog is not eating," Teller says. "Maybe it just hurts to have to get up and walk to the bowl, or to stand or even lean down to eat." Dachshunds and Dobermans can have neck difficulties, osteoarthritis, or cancer.
"Dental problems can cause eating problems," McClosky says. "Dogs with dental problems may drop kibble, chew on one side, or drool excessively." She says older dogs, especially smaller breeds, are more susceptible to toothaches, but any dog can shatter a tooth.
While dogs aren't picky eaters, Teller says they won't consume rotten food. The dog's kibble may be bad or the chicken you boiled wasn't done. Did your dog stop eating because of that? "Your dog will eat a treat, but won't eat the food," he explains. Toss it and serve something different.
Teller says immunizations can cause moderate sleepiness or pain, lowering your dog's appetite, but this is rare. "This may last 24 hours. If your dog doesn't eat right after a vaccination, offer the next regular meal."
You must play detective to help your dog restore an appetite and start eating vet-approved dog food.