Hi there. An anti-inflammatory medication prescribed by your vet (Deramaxx, Rimadyl, or Previcox) in conjunction with a medication called Tramadol that is a pain medication is what I use for arthritis in older dogs. The anti-inflammatory medications are expensive, though, especially for a dog that big. If money is an issue, I would have your vet prescribe Tramadol 50mg tablets, 4 tablets every 12 hours as needed for pain management. If you are looking for an over the counter medication, then I would use human adult buffered aspirin at a dose of 2 and 1/2 tablets every 8-12 hours as needed for pain management (about 800mg every 8-12 hours). If your dog has any history of bleeding disorders, do not give the aspirin. As always, consult your veterinarian before changing or giving any medications. Hope this helps... best of luck.