Might want to ask the vet about Tramadol.

That's what I had Dusty on the last couple month of her life when the Metacam quit working. It's not approved for use in cats either, but with 1/8 of a tablet once a day or every other day as needed, it certainly helped Dusty's arthritis pain. Tramadol has it's side effects too, but in Dusty's case it was more like kitty hospice care pain relief anyway. Higher doses made her drowsy and a bit 'high' but the 1/8 tablet dose seemed to work for her, she was 11 lbs as I recall.