Doxycycline is an antibiotic that is used in the treatment of a number of types of infections caused by bacteria and protozoa. It is useful for bacterial pneumonia, acne, chlamydia infections, early Lyme disease, cholera and syphilis. It is also useful for the treatment of malaria when used with quinine and for the prevention of malaria. Doxycycline can be used either by mouth or intravenously.