Has anyone used this for arthritis? I have an 11 yr male lab who is showing signs of stiffness in the hind legs. Today we went for a walk and he could not get back in the car. This afternoon he clearly is in some discomfort. I'd like to keep him off of Rimadyl (which my vet recommends) because I think in long term use it causes more problems then its worth (we had another old lab who took that for over a year). I've read about Synflex but don't know anyone who uses it. Does it cause any stomach problems? Is it effective for pain?