What antibiotic are you looking for? What symptoms are you seeing besides sneezing? How is the dust or air circulation in your coop? Do they sneeze all of the time, or just occasionally, and is it more when they are eating?
Since viruses do not respond to antibiotics, I would try to determine what all the symptoms are. Bacterial respiratory diseases include mycoplasma (MG,) and coryza. Symptoms include cough or sneeze, nasal drainage, crackles when breathing, bubbles in one eye, or swelling of the eyelid. Coryza all has a bad odor, and may cause thick yellow nasal secretions. Tylan 50 injectable given orally or Tylosin powder in the water may treat MG symptoms. Coryza is better treated with sulfadimethoxine or other sulfa antibiotics.
Viruses, such as infectious bronchitis can cause sneezing, mild congestion, and the nostrils may be wet and collect dirt. ILT is a more serious virus, causing gaping, gasping, lots of secretions, and bloody mucus from the beak. There are a few other viruses.
