Doxycycline is an antibiotic related to tetracycline. It is a broad-spectrum antibiotic and can be used against a wide variety of bacterial infections. One of its advantages is that in its pill form, it only has to be taken once per day to have the appropriate effect. It is strongly indicated if the patient has anthrax, plague or Lyme disease. It is one of the few antibiotics that is effective against Rocky Mountain spotted fever.