Doxycycline antibiotic molecule. Molecular model of a molecule of the antibiotic drug doxycycline. Atoms are represented as spheres and are colour- coded: carbon (green), hydrogen (white), oxygen (red) and nitrogen (blue). Doxycycline is a broad- spectrum antibiotic. It is used to treat all forms of anthrax infections, sexually transmitted diseases, such as chlamydia and gonorrhoea, and acne.