Hi Brittany,
Pets, just like people, have different tolerance levels for pain. The fact that he is limping indicates something is definitely not right. How long has the limping been going on? It is sometimes difficult to know if something is a small or a large problem without going to the vet. Finding an answer is definitely worth it :). I hope he gets better as I can tell you care about him very much!
Diedra
Likely if your vet prescribed it he/she has considered the side effects of long term use and decided that your dog needs the medication. This is really something u should talk to your vet about.