What causes lethargy or weakness in dogs?

Lethargy in dogs can be caused by a variety of conditions that range from a one-day stint that resolves on its own, to more serious underlying conditions that need diagnostics and medical attention.

Generally speaking, older dogs tend to slow down as they age and potentially become less interested in food and exercise. It’s best to consult with your veterinarian to help your aging dog transition into a comfortable lifestyle.

Some of the common causes of lethargy include:

Physical exertion

Many dogs have a day of decreased energy after a day of exercise. Like humans, it’s normal to be tired after physical exertion. If your dog experiences tiredness for more than 24 hours, it’s time to start looking into other possibilities.

Heat stroke

If your dog has spent too much time in the hot sun, they may be suffering from heat stroke. In addition to lethargy, your dog may experience excessive panting, red gums, and hypersalivation.

If you think your dog might be experiencing heat stroke, it’s important to provide them with lots of fresh water and ice packs, and seek veterinary attention right away as heat stroke can be life threatening.

Dogs with heat stroke or a fever are usually feeling sick, and need veterinary attentiony. Fevers should never be diagnosed based purely on how a dog feels to touch.


Your dog might be feeling lethargic and reluctant to move if they are in pain. Other indications that your dog might be in pain are if they bark or yelp when you touch them in a certain area, or if they aren’t eating.

Pain is a broad term that can be caused by a pulled muscle, torn ligament, broken bone, arthritis, or gastrointestinal issue.

Mental and emotional issues

Just like with humans, your dog may become lethargic because of their mental and/or emotional state. It may be a sign that they are bored and lacking motivation, depressed or lacking something, such as exercise, nutrition, mental or social stimulation.

Being aware of your dog’s current mental state and any changes in their lifestyle can provide clues about the cause of your dog’s lethargy.


Anemia is caused when your dog’s body isn’t producing enough red blood cells. Anemia is not a specific disease, rather a cause of other underlying diseases and conditions. If your dog is anemic, they will feel lethargic and their gums will look pale pink to white in color.

Pale pink or white gums in dogs are a sign that something more serious is going on.

Low blood sugar

Also referred to as hypoglycemia, a low blood sugar is defined as a lack of glucose in the bloodstream and will cause your dog to feel lethargic.

Low blood sugar can be caused by a lack of glucose intake (if they aren’t eating, or if they’re eating foods that are not adequate), diabetes mellitus, or other gastrointestinal diseases.


If your dog is not eating, or if they are eating an imbalanced diet, they may not be receiving the proper nutrients they need to create energy and use blood sugar appropriately. 

Ensure your dog is receiving a canine formulated diet and check with your veterinarian if you are unsure about the quality of the food they are eating.


Exposure to toxic substances such as insect repellents, chemicals, or a bug/snake bite can have serious side effects that include lethargy.

If you think your dog may have gotten into something they shouldn’t have, or if you notice swelling that may indicate an insect bite, seek veterinary attention right away and call the ASPCA Animal Poison Control Center, available 24/7 and 365 days a year. 


Everyone’s least favorite word – cancer. Tumors can have a wide range of effects on your dog’s health and the way that they feel.

This can include lethargy, inappetence, pain and discomfort. Your veterinarian will be able to provide a physical exam and blood tests to determine if this is the cause.

If you witness your dog collapse or find them collapsed, the first thing to do is to assess if they are conscious and breathing. Collapse or fainting are usually emergency situations.

Emergency conditions

Emergency conditions such as bloat (gastric dilatation and volvulus) or a splenic rupture (hemangiosarcoma) will cause your dog to have an acute, sudden onset of lethargy and/or collapse.

Both of these conditions are emergent and need veterinary attention within minutes.

Pro Tips:

  • Any kind of sudden lethargy combined with other symptoms may be signs of an urgent issue.
  • Try to recall when you first noticed your dog’s lethargy, and make a mental note of their physical activity and diet or any food they could’ve gotten into. This will be helpful for your veterinarian to know.