Doxycycline, hydrochloride: Doxycycline is a relative of tetracycline reported to be more effective against sensitive organisms.3 Crossresistance is common although some tetracyclineresistant S. aureus respond to doxycycline. It is most often used as a broad spectrum antibiotic and bacteriostatic agent against bacteria. Doxycycline has also been reported to have antiprotozoal properties. It is used in the treatment of chlamydia, rickettsia, mycoplasma, and some spirochete infections, in addition to the standard treatment of Gram positive/negative infections.