Isoniazid is used with other medications to treat active tuberculosis (TB) infections. It is also used alone to prevent active TB infections in people who may be infected with the bacteria (people with positive TB skin test). Isoniazid is an antibiotic that works by stopping the growth of bacteria. This antibiotic treats only bacterial infections. It will not work for viral infections (such as common cold, or flu). Using any antibiotic when it is not needed can cause it to not work for future infections.