See article Marijuana: If It’s Legal, Should You Give It to Your Pet?
The Federal Drug Enforcement Administration (DEA): The DEA regulates use of all controlled substances and classifies marijuana as a Schedule 1 drug, which means that according to Federal law it has no recognized medical use. All medical professionals, including veterinarians who dispense controlled substances must abide by DEA rules and regulations. In states where human use of medical marijuana is authorized, physicians are granted exemption to this prohibition, but veterinarians are not, and do not have clearance to dispense Schedule 1 drugs.
The Federal Food and Drug Administration (FDA): The FDA approves all food and drug products for veterinary use and to date has not approved marijuana for any use in animals. The agency, therefore, does not ensure the safety or effectiveness of any product currently being marketed and has warned that consumers should be wary of purchasing Cannabis containing products for their pets.
The California Veterinary Medical Board (VMB): The VMB issues veterinary licenses and ensures that veterinarians follow all state and federal laws and regulations. The VMB maintains that because the DEA has not authorized veterinarians to possess, dispense, or prescribe Cannabis or cannabinoid products, federal law prohibits vets from administering them. Furthermore, although California law has provided mechanisms for physicians to recommend marijuana to humans for medical use, there is nothing in state law that allows veterinarians to prescribe or even approve marijuana use in animals.