About doxycyclinetetracycline derivative, antibiotic.mechanism of action of doxycyclinedoxycycline is a broad spectrum antibiotic which acts against both gram positive and gram negative organisms. it exerts its bacteriostatic action by accumulating inside the bacteria through specific transporter protein and by passive diffusion & inhibits bacterial protein synthesis by attaching to 30 s subunit of bacterial ribosome (which are absent in mammals).doxycycline interfering the attachment of aminoacyl-trna to the mrna-ribosome complex and peptide chain fails to grow. it chelates cations like ca &mg and inhibits functioning of various enzymes & ribosome. indications for doxycycline1. syphilis 2. lympho granuloma veneram 3. typhus fever 4. respiratory tract infections 5. prophylaxis of malaria 6. urinogenital tract infections 7. trachoma 8. pelvic inflammatory disease9. acne 10. diarrhoea 11. gonorrhea in penicillin allergic individuals 12. pneumonia