It's an NSAID and does carry risk. I used it for my GSD for back issue, but tuck with alternatives mainly. I would use liver support with it, like milk thistle, or Country Life's "Liver Support Factors".
For the knee, you might want to start thinking about a consult with an ortho doc, just for an evalutation. I *think* the longer those knees are popping in and out, more wear and tear and arthritis, but I could be wrong.