I Consider My Pets Family

Anyone have experience with giving CBD to a pet?
I have a 17yo golden retriever that had been having a lot of pain getting around, hate the idea of having him euthanized, decided to try it. It does seem to be helping a little, after about a week, but does it build up in the system, where it would help him more?
I hope to let him live out the rest of his life without too much pain.