There is a website put out by the Food Animal Residue Appearance and Depletion Species Pages (FARAD) discussing the rules and regulations with regards to the use of medications with backyard poultry. The discussion is with Federal regulations.
The question becomes are chickens pets or food animals. The Food and Drug Administration (FDA), however, considers all chickens to be food animals regardless of how the owner feels about the chicken. As a result, the regulations require that all regulations related to the treatment of food animals will be applicable to backyard poultry.
If you have a particular question regarding the use of a particular medication you can submit a request for more information.