Azithromycin is a broad-spectrum antibiotic used to treat a wide variety of bacterial, rickettsial and parasitic infections in animals. Commonly known under the brand name Zithromax, it is only FDA-approved for use in humans, but veterinarians prescribe it for a wide variety of animal species. This flavored oral suspension is long acting and can be given once daily. While you may not see any outward effects of the medicine right away, it should start taking effect within one to two hours, and your furry friend should be feeling better in one or two days. Consult with your veterinarian about the recommended dosage.