Why Is My Dog Sluggish. That’s why when your dog becomes severely lethargic or the lethargy persists for more than a day or two, you do need to see a veterinarian. A lethargic dog is one who's more than just sleepy or worn out from a big day of hiking or swimming.
Lethargy, weakness and sudden tiredness are common signs of illness in dogs. There may be several reasons why your dog’s energy level has plunged. Why is my dog lethargic, and not eating or drinking?