Why is My Dog or Cat Losing Weight? What Causes Cats and Dogs to be Underweight

Why is My Dog or Cat Losing Weight?
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Dogs and cats can be underweight for various reasons. Here are the most common causes.

Pets can lose weight for a wide range of reasons, including disease, parasites, and anxiety, to name just a few. Regardless of the cause, take any weight loss seriously. Being underweight can impact a pet’s quality of life and result in health problems. If your pet has been losing weight, or if a newly adopted cat or dog seems a bit too svelte, read on for some possible causes.


There are several illnesses that are known to have weight loss as a symptom:

  • Diabetes: While strongly associated with obesity as a cause, a common early sign of diabetes is weight loss.

  • Diseases including certain cancers, such as lymphoma and feline leukemia, liver disease, kidney disease, and heart disease.  

  • Inflammatory bowel disease: A loss of appetite can point to IBD, an illness that interferes with a pet’s digestion.

As well as these diseases, dental problems can also result in cats and dogs losing their appetites, since chewing and swallowing can become painful.


Your pet doesn’t have an easy way to let you know about stress, depression, and anxiety, but a loss of appetite can be a big clue. Changes–from another new pet in the household to travel to a new home–can easily throw a pet off and lead to a decreased appetite.

Picky Eaters

Some pets are very particular about their food. For finicky pets, be careful to slowly transition between brands and varieties of food, and to make sure that food is available at consistent times.


Hookworms, whipworms, giardia, and other parasites can all interfere with either a pet’s appetite or ability to absorb and utilize the nutritional value of food. If this is the cause of your cat or dog’s decreased appetite or weight loss, getting rid of the worms is the first step to restoring weight and appetite, and is an essential step in preventing further harm to your pet and possibly to yourself. Some of these parasites can spread to people through contaminated water or through oral routes, so see a veterinarian right away.


Pet food that lacks nutritional value can result in a loss of weight, as pets eat their regular amount, but do not receive all the nutrients and fats they require. And of course, if pets are not fed a sufficient amount of food, weight loss is a natural result.

Vaccines and Medications

One common side effect of vaccinations is a temporary a loss of appetite. This often only occurs for a few hours or a day, and amounts to a missed meal or so. In some cases, the diminished appetite lasts for several days, which may be cause for concern. If your cat or dog doesn’t seem interested in food for several days following a vaccine, and particularly if your pet has other symptoms like a fever or irritation at the site of the injection, it’s a good idea to consult your veterinarian.

Some medications also tend to have a loss of appetite as a side effect. Talk to your vet about whether to expect this when your pet starts a new medication, and how to prepare for it.

More on Keeping Your Pet a Healthy Weight

The Visual Check Chart
How to Tell if Your Dog is a Healthy Weight

This information is for informational purposes only and is not meant as a substitute for the professional advice of, or diagnosis or treatment by, your veterinarian with respect to your pet. It has, however, been verified by a licensed veterinarian for accuracy.

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