There are many different ailments and medications that may cause a lethargic dog. Diagnosis is difficult because your dog can't alert you to the problem and a wide variety of symptoms can cause it. However, considering recent changes to your dog's lifestyle can often help you determine a cause.
Causes of Lethargy
Lethargy is a general disinterest in stimulus that used to excite your dog, such as toys, food and walks. A lethargic dog will have a decreased energy level and will be less social with members of the family, often seeking quiet rooms to sleep rather than following you around.
There can be many causes to this, including many medical disorders and depression. If you have recently made a dramatic life change, such as changing your job, moving to a new city, adding a new baby or significant other to your life or losing a pet, your dog's lethargy may be caused by depression.
If not, it may be caused by a variety of medical disorders, such as anemia or other blood disorders, heart or respiratory disorders, hormonal disorders, urinary tract infections, most types of cancer, immune diseases or neurologic disorders. It could also be caused by a simple cold or infection that will go away on its own in a few days.
Nutritional disorders can also cause depression. If you have recently changed foods, this may be the culprit. Check the date of the food to make sure it hasn't spoiled and check the news for food recalls. Your dog also may have gotten into the trash or a toxin that is causing the lethargy.
Medication Causing Lethargy
If your dog is on any medication, you are certainly already aware of your dog's medical issues. If he was already lethargic before the medication, the illness may not have resolved itself yet. Many symptoms of illness are caused by the body's changes in response to fighting the infection. Your dog may be using a lot of its energy resources toward fighting the illness and has little energy for other things.
However, the medication itself may also have caused the lethargy. If your dog wasn't lethargic before taking the medicine, that could very well be the culprit, especially if the medication is a long-term medication that your dog is taking for a serious illness. Many medications work by killing off harmful viruses or bacteria in the body. In doing so, they also kill good bacteria and other necessary cells. At first, this may make your dog feel worse, not better.
Thyroid medication is a good example. A dog suffering from hypothyroidism may actually see decreased energy levels for up to six months after beginning the medication as the body adjusts to the new hormone levels in the body.
If you are concerned, talk to your veterinarian about the dosage and alternative medications. However, once your dog has adjusted to the dosage and the body begins to heal, he will often rebound to his normal energy level.