The most common cause of lethargy and loss of appetite in dogs is an infection in the body. Other reasons a dog would be lethargic and uninterested in eating are pain, medications, liver disease, heart disease and diabetes mellitus, according to WebMD. There are many conditions that would cause a dog to become weak, lethargic and have a loss of appetite, so it is important to bring a dog to see a veterinarian for a proper diagnosis.
Certain medications for humans, including ibuprofen, are toxic to dogs, which could cause lethargy, loss of appetite and other symptoms. Any time a dog has accidentally consumed human medication, it is important to contact animal poison control or a veterinarian immediately for proper treatment.