How did vet discover the arthritis? X-ray, physical response to exam?
Rimadyl is one of several NSAIDs (nonsteroidal anti-inflammatories) used for arthritis in dogs. They lessen pain by reducing inflammation, similar to Prednisone (and as someone already said, you can’t give pred to a dog who’s taking an NSAID). Even if your dog isn’t showing pain, it may be worth starting an NSAID now, as the reduction of inflammation also delays the progression of the disease. When my dog was just starting to show symptoms (slipping going up steps sometimes, for example), the vet said we could use the NSAID on an as-needed basis.
The NSAIDs are scary because they’re strong, and can have negative effects as well as positive ones. It’s recommended that you watch for signs of stomach upset - vomiting, diarhea, etc - while they’re on it, and that you do blood work after an initial period, to make sure it’s not causing trouble. Giving an NSAID does carry some risk, as with any medication. The risk is generally considered acceptable when the reason for giving it is a disease as relentless and crippling as arthritis.
The three most common NSAIDs are probably Rimadyl (aka carprofen), Metacam (aka meloxicam) and Deramax (deracoxib). They work the same way, but a dog can respond differently to each, so if one doesn’t seem to work, or seems to produce a negative response, you can try another. My dog’s been on all three - Deramax worked beautifully, then the maker had a production shortage and we switched to Metacam. She reacted poorly to that (saw sudden uncontrolled urination) so we switched to Rimadyl. She’s done well on that.
Things like Amantadine and Gabapentin can be added to the NSAID as the arthritis progresses; they seem to both enhance the NSAID’s effectiveness and provide some pain relief themselves. Painkillers like Tramadol are also added at that point.
I don’t know much about Adequan. We tried one course of it, and it did seem to have a positive effect. It seems to be something you do early, before the symptoms become severe, so it might be nice for your dog.
Other ideas would be a high-quality fish oil (omega-3 fatty acid), and massage.