CLARITHROMYCIN (Biaxin® or Biaxin® XL) is an antibiotic. Clarithromycin kills certain bacteria or stops their growth. It treats ear, lung, sinus, throat and skin infections. This medicine is also is used to treat a certain bacteria that can cause stomach ulcers. Clarithromycin will not work for colds, flu, or other virus infections. Biaxin® XL is an extended-release form of clarithromycin that only needs to be taken once daily.