According to the search results, the best antibiotic for feline upper respiratory infection depends on the type of infection and the infectious agent suspected. Doxycycline is recommended as a first-line drug option for feline acute bacterial upper respiratory infection, as it has a broad spectrum of activity against common feline nasal pathogens and is well-tolerated by cats. Amoxicillin is another option for feline acute bacterial upper respiratory infection. For feline chronic bacterial upper respiratory infection, the choice of antibiotic should be based on culture and antimicrobial susceptibility testing if available, but doxycycline and amoxicillin are also recommended. Primary bacterial upper respiratory infections caused by Bordetella or Chlamydophila will be treated with specific antibiotics that are effective against these diseases. It is important to note that antibiotics do not treat viral infections, which are responsible for most upper respiratory infections in cats. Therefore, it is recommended to consult with a veterinarian to determine if antibiotics are needed or likely to be helpful. Additionally, the optimal duration of therapy is unknown, but treatment should be continued for a minimum of seven days and for at least one week past clinical resolution or disease plateau. If treatment is ineffective after 48 hours of therapy, a switch to an antimicrobial in a different drug class should be considered, and bacterial culture and antimicrobial sensitivity testing is recommended if therapy is still ineffective.