Doxycycline is a highly active broad-spectrum antibiotic of tetracycline group. It is effective against a wide range of Gram-positive and Gram-negative bacteria, spirochete, mycoplasma, rickettsia and mycobacteria. Doxycycline is used as an alternate to penicillin in the treatment of gonorrhoea and syphilis. Doxycycline has its main mechanism of action on protein synthesis. Inside the bacterial cell, it inhibits protein synthesis.