I'm late to the game but wanted to throw in some info. Metacam is safe for rats, as are several narcotic pain relievers (which are good if the rat is on steroids, in which case Metacam isn't an option). Metacam isn't expensive and the dose is pretty small. I haven't had the need to use it with my rats yet so I'm not sure of the exact dose.

If your vet doesn't know how to treat pain, it's definitely time to find a new vet if at all possible. Google "Rat Guide" for proper dosing of Metacam (which is an anti-inflammatory, so would be great for arthritis) if you must go back to this vet.
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Kadee Sedtal

home of dogs Fancy (beagle) and Crusher (yorkie), rats Tchaikovsky, Pachebel, and Fillmore, mice Bartok, Ishka, Pel, Orange Blossom, Seven, Jhamel, Neela, Ardra, EC, Ba'el, Star, Orla, Leeta, Worf, Kahless, Ben Sisko, Geordi LaForge, and Glenda, dwarf hammie Birdie, spiny mice Gimme, Cage, Alf, Sonic, and Tails, and several humans whose names I can't recall
"I wish I had a dollar for every time I spent a dollar, because then, yahoo! I'd have all my money back." -Jack Handey