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It's probably time to get the vet on the phone.

By Brendan Howard
December 03, 2020
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Making a lifestyle change like getting more active outdoors or changing food or treats can cause your dog's weight to fluctuate. While this is normal, sudden and rapid weight loss in dogs can be a serious sign of illness and health problems.

At first it can be difficult to determine if your dog is losing weight in a healthy way or an unhealthy way, but keeping an eye on her eating habits can give you your first clue. If you notice your dog isn't eating or drinking as much as she used to, it's time for a trip to the veterinarian for a checkup. A dog who loses weight but still seems to be eating and drinking as much as before might need a visit to the veterinarian, too.

"They're not taking in enough calories to support themselves," says Shawn Finch, DVM, with Gentle Doctor Animal Hospitals in Omaha, Neb. "Either they're a healthy dog being underfed, or they're unwell and taking in less than they need, or they're unwell and needing more [food] than usual because of an issue with absorption or digestion."

At the checkup, the veterinarian will check your dog's weight against those of previous visits and may also assess your dog's body condition score and muscle condition score, which are crucial elements to determine a dog’s overall health outside of what it says on the scale.

brown boxer looking up at camera
Credit: Elva Etienne / Getty

Reasons Why Your Dog May Be Losing Weight

Whether your dog's appetite has suddenly dwindled or she’s eating and drinking just as much or more than usual, a veterinary visit is important to figure out whether your dog's weight loss calls for an easy change of diet or requires more serious medical treatment.

"Anything that causes decreased appetite, like dental disease or gastrointestinal issues, or increased nutrient use, like cancer, is a possibility," Finch says.

Common conditions that can cause weight loss in dogs vary widely, so it can sometimes be difficult to distinguish the exact cause (or causes) on your own. Some possibilities include:

  • Dental disease. If it hurts your dog to eat, she may eat less or fail to properly chew food. Because dental problems can have serious repercussions for your dog's health, it's important to prevent them with regular brushing and teeth cleanings.
  • Diet changes. Is your dog eating less or differently? Did you change the frequency, type, or amount of food you offer? Your dog may be taking in fewer calories for some reason. If your senior dog is getting older, there's also a chance that your current food isn't working as well for your aging pet, according to Lori Prantil, MPS, DVM, who counsels pet owners on nutrition at VCA South Shore Weymouth. Prantil says digestibility studies have shown some foods are less digestible to senior dogs. 
  • Gastrointestinal disorders. Any condition that makes it difficult for your dog's digestive system to break down food and absorb nutrients can cause weight loss. "A lot of patients come to me for issues like inflammatory bowel disease," Prantil says. 
  • Heart disease.
  • Kidney disease. Whether sudden and acute, or chronic and long-term, kidney disease can come with weight loss in dogs. Is your dog drinking or urinating a lot and losing weight? "Excess drinking of water with excess urination can be a sign of kidney disease," Finch says.
  • Liver disease.
  • Metabolic disorders. Unexplained weight loss can be a sign for conditions like diabetes mellitus and hypoadrenocorticism a.k.a. Addison's disease.
  • Parasites. Pests like Giardia and various intestinal worms like hookworms can cause weight loss and require help to diagnose and treat.
  • Stress and anxiety. A worried dog may eat less or develop digestive issues because of new or ongoing stressors around the home.
  • Cancer.

What to Do If Your Dog Is Losing Weight Fast

Unexplained weight loss in a dog, when it's not part of a plan developed with your veterinarian to help your dog lose weight, is serious.

"I aim for 1 to 2 percent of a dog's body weight per week for a weight loss plan," Prantil says. Anything more is cause for concern.

To properly diagnose your dog's weight loss, a veterinarian will need to get your dog's medical history and do a complete physical exam. The vet may also need to conduct further testing to figure out what the issue is.

Finch says you should closely watch for other signs along with weight loss, such as lack of energy, decreased appetite, excessive drinking or urination, decreased mobility or pain, vomiting, head shaking, and anything else out of the ordinary, which would warrant at least a call to the vet and usually a visit.