Question
Doxycycline
Answer
Doxycycline is a long-acting tetracycline.
MOA: doxycycline → reversibly binds the bacterial 30S ribosomal subunit → blocks the A-site, preventing aminoacyl-tRNA binding → protein synthesis stalls (bacteriostatic).
Uses: broad-spectrum, particularly effective against atypical/intracellular organisms lacking a conventional cell wall — first-line for atypical pneumonia (Mycoplasma, Chlamydia), Rickettsial infections, Lyme disease; also used for malaria prophylaxis and acne (lower-dose, anti-inflammatory action against Cutibacterium acnes).
Adverse effects: tooth discoloration and impaired bone growth in children (calcium chelation in mineralizing tissue — contraindicated under age 8 and in pregnancy), photosensitivity, GI irritation, chelation with divalent/trivalent cations (antacids, dairy, iron) reducing oral absorption.