Disagree. The animal cannot tell you when it's in pain and/or suffering. Assuming the dose of medication you're administering takes care of the issue is irresponsible. If a pet contracts a terminal disease it is better to end their suffering ASAP, not anthropomorphize them and subject them to a slow agonizing death like we do to humans with terminal illnesses.
When his limp disappears and he acts young and energetic, the meds are working. When he limps and struggles to lay down, the meds have worn off. It's pretty simple so far. But if you think I'm going to kill my dog right before Christmas out of some sense of moralistic responsibility, you're kidding yourself.