You can start ARTHRO-IONX now and give concurrently while on Rimadyl. You just to need to give it at time away from the the drug so the body has time to focus on it. Once you see improvement ask your veterinarian about decreasing the Rimadyl. You might want to Google the side effects of Rimadyl and so you might be encouraged to use the lowest dose possible. Initially I like to split the ARTHRO-IONX dose and give half with breakfast and half with supper. When symptoms are much better you can decrease to a whole dose once a day. After more time you can try decreasing the frequency further. Longterm use does not result in negative side effects as with conventional NSAIDS.
I also recommend PROBIOTICS and OMEGA 3,6,9 to support the body when dealing with arthritis. Another thing that can be helpful is veterinary chiropractic to improve better movement.
Please let us know how you do.