9 Reasons Why Your Dog Is Losing Weight

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Why Your Dog Is Losing Weight

Reviewed and updated for accuracy on December 10, 2019, by Dr. Natalie Stilwell, DVM, MS, PhD

Unplanned or rapid weight loss in dogs, while not uncommon, can be alarming for any pet owner.

If your dog is losing weight unexpectedly, it can be an indication of an underlying health or behavioral issue that requires veterinary attention.

Here are nine of the most common causes of unanticipated weight loss in dogs.


Dr. Cailin Heinze, MS, VMD, DACVN, assistant professor of nutrition at the Cummings School of Veterinary Medicine at Tufts University, says that weight loss due to parasitism is less common than it used to be, because many pets receive monthly protection against heartworm and intestinal parasites.

However, not all products are equally effective against all worms.

Whipworms, in particular, aren’t killed by a lot of products,” says Dr. Heinze. Pets contract these by ingesting the eggs directly from the environment, from contaminated soil or water.

Symptoms of intestinal parasite infection in dogs can include:

  • Weight loss

  • Vomiting (intermittent or persistent)

  • Soft stool

  • Diarrhea

  • Increased gas

  • Decreased appetite

Intestinal parasites are typically diagnosed through a fecal exam to identify the eggs or other parasite life stages (depending on the type of parasite present).

Treatment of intestinal parasites involves the use of a broad-spectrum dewormer, Dr. Heinze says, followed by a monthly preventative medication to prevent reinfection.


Various types of cancer can cause unexplained weight loss in dogs, says Dr. Heinze.

Cancer is generally more common in older dogs, and malignant forms can be life-threatening. Weight loss is associated with several malignant cancers in dogs, including hemangiosarcoma, lymphoma and osteosarcoma.

With these tumors, weight loss may occur due to the metabolic demands of the tumor itself, or because pain and discomfort is causing reduced appetite and activity.

Other canine cancers, such as oral melanoma, may make eating and swallowing more painful, causing the dog to avoid food.

Kidney Disease

Unlike some of the other conditions for which weight loss is a symptom, pets with kidney disease typically don’t drop pounds right away. Rather, weight loss is typically associated with chronic forms of the disease.

“If they’re only getting diagnosed after they’ve started losing weight, they’ve probably had the disease for a long time,” Dr. Heinze explains. As the illness progresses, dogs generally feel unwell and experience vomiting, loss of appetite and dehydration.

Weight loss generally affects both body fat and muscle, resulting in emaciation.

Advanced Heart Disease

Like kidney disease, pets with heart disease won’t start losing weight immediately. In fact, some dogs may appear to gain weight—despite a loss of appetite—due to fluid accumulation in the body.

Generally speaking, Dr. Heinze says loss of appetite is the easiest way to tell if your pet’s weight loss is concerning or not.

If you’ve noticed a small degree of unplanned weight loss, try adding calories to their diet, she says. “If they eat more and gain weight, they’re probably OK,” she says.

In addition, make sure you’ve isolated their food from the other pets in the house to reduce food sharing or stealing. The experiment must be controlled, which means it’s important to avoid feeding table scraps or too many treats.

Metabolic Disorders

Metabolic disorders, such as diabetes mellitus and hypoadrenocorticism (also known as Addison’s disease), can alter your pet’s body condition.

The clinical signs associated with metabolic conditions are complex, and advanced diagnostic testing may be required to identify these conditions.

Gastrointestinal Disorders

GI problems can lead to weight loss over time. Excessive vomiting and/or diarrhea decrease the amount of available nutrients for your pet.

Both signs can result from a variety of causes, so it’s best to talk to your veterinarian, who can help determine the underlying problem.

Also, malabsorption or maldigestion disorders may impair the intestine’s ability to break down and absorb valuable nutrients from the diet.

Pets who are suffering from these types of conditions will generally be significantly underweight, no matter what type or amount of diet is offered.

Dental Disease

Oral pain, says Dr. Lara Bartl, DVM, Diplomate DABVP (Canine/Feline Practice), assistant professor at the Virginia-Maryland College of Veterinary Medicine at Virginia Tech University, may lead to weight loss in pets, as dogs will have a hard time chewing hard kibble when they’re dealing with an abscess or other gum problem.

Also, dogs with advanced dental disease generally feel unwell and may lose body condition.

As dental disease progresses, oral bacteria can spread throughout the body, leading to painful infections in the heart, liver and kidneys.

Treating the underlying dental disease should help address your dog’s weight loss, but it’s important to monitor your pet’s appetite closely to make sure it’s back on track.


While not as common as other causes, Dr. Bartl says that stress and anxiety can significantly affect a pet’s appetite and should not be overlooked.

Dogs are generally adaptable to our lifestyles, but they are also highly perceptive and will notice even minor changes in their surroundings. Dr. Bartl explains that changes in the household or the addition of a new pet could be possible sources of stress-induced weight loss.

Chronic stress can also weaken the immune system and increase your dog’s risk of developing an infection. If your dog appears highly nervous or depressed, then she may be more interested in factors other than her food.

Changes in Diet

Not all cases of seemingly inexplicable weight loss are caused by disease. Dr. Heinze says one of the most common explanations for weight loss is changing your pet’s food.

“I have had people come to me because their dog is losing weight. They had a complete workup done and no one could find anything wrong,” she says. “It turns out the dog’s food was changed. The owners were feeding them the same amount every day, but the new food had 20 percent fewer calories.”

Dr. Heinze says it’s easy to miss this because foods are marketed in seemingly confusing ways when it comes to nutrition.

“Sometimes, a food that is marketed for overweight pets—it might say ‘healthy weight’ or ‘weight maintenance’ or ‘reduced calorie’—might have more calories than ‘regular’ food,” she says.

If you plan to change your pet’s food, check the ingredient labels of both brands and make the swap based on calories. Dr. Heinze says that if you’re looking for food that’s truly meant to help your dog lose weight, talk to your veterinarian.

By: Dr. Natalie Stilwell, DVM, PhD

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