My cat is drooling, low-energy, and lying down and seems much thinner overnight

There are many reasons a cat drools and acts listless. Common ones are mouth disease or calicivirus causing stomatitis, such as ulcers, tartar, oral tumors, or foreign objects that make chewing hard and trigger drooling; upper respiratory infection, painful disease, and rabies also cause it.

Also, licking toxic substances, heat stroke, or stress can cause drooling and lethargy. The owner should judge based on care history and an oral exam, but because causes are many, a veterinary clinic check and prompt targeted treatment are best to avoid delay.