Biaxin, an antibiotic chemically related to erythromycin, is used to treat certain bacterial infections of the respiratory tract, including: Biaxin, an antibiotic chemically related to erythromycin, is used to treat certain bacterial infections of the respiratory tract, including:Strep throat , Pneumonia . Biaxin is also prescribed to treat infections of the skin. Combined with other medications, it is used to treat ulcers near the exit from the stomach (duodenal ulcers) caused by H. pylori bacteria.