The causes of lethargy in cats vary widely. Almost any condition that disrupts your cat’s sense of well-being can cause lethargy. Some of the common causes of lethargy in cats include: Disease. Both chronic and short-term diseases have a negative impact on overall behavior and can make a cat lethargic. Common cat diseases are liver disease, kidney disease, and diabetes mellitus. Mobility diseases. Diseases that hinder your cat’s ability to move can cause lethargy as well. Such diseases include hip dysplasia and arthritis. Infections. Infections caused by bacteria, parasites, and viruses can greatly weaken the body and make the cat lethargic. Poisoning. If your cat eats a slow-acting poison, she may display signs of lethargy as the poison takes its toll. Cancers. Cancers of different organs like the skin and mammary glands can be painful and make the cat lethargic. Anemia. Anemia is a condition that affects the health of red blood cells. Since red blood cells are responsible for oxygen delivery which is needed for energy production, anemia causes body weaknesses. Physical trauma. Pain from physical injuries like car accidents and falls hinders movement as well. Excessive weight gain. Heavier cats are usually less willing to move or participate in activities due to the extra weight.