Our muttly dog had fallen down the stairs once when he was younger so we figured it had just scared him too much to try again. He never went up to the second floor. However, when we took him to a new vet, they found that he had a trick knee that would pop out of joint if he bent it too far.
He also had developed arthritis in it. The vet recommended Glyco-Flex to see if it would help with the arthritis.
It seems to have worked too good. About three weeks later I had a huge scare when I was cleaning upstairs and felt something brush my leg. It was our dog come upstairs on his own to see what I was up to. Now that it doesn’t hurt so much, he has come visiting upstairs more often.