In the past few weeks, he became lethargic. He wouldn't clean himself, didn't eat or drink much, and if he had to go sometimes he doesn't get to the litter box in time. Also he was rather nasty toward my other cats. In all, it seemed like he pretty much gave up living. After 12 years as a wonderful friend and companion, I had come to a hard decision that it was best to have him euthanized.
RIP Marshmellow, 12 years old.