Medicine & USMLE


Antibiotics / Antiparasitics
  1. Penicillin Overview
  2. Penicillinase-Sensitive vs. Penicillinase-Resistant Penicillins
  3. Anti-Pseudomonal Penicillins
  4. Cephalosporins Overview
  5. 1st Generation Cephalosporins
  6. 2nd Generation Cephalosporins
  7. 3rd Generation Cephalosporins
  8. 4th Generation Cephalosporins
  9. 5th Generation Cephalosporins
  10. Carbapenems
  11. Monobactams (Aztreonam)
  12. Vancomycin
  13. Aminoglycosides
  14. Tetracyclines
  15. Tigecycline
  16. Chloramphenicol
  17. Clindamycin
  18. Linezolid
  19. Macrolides
  20. Polymyxins
  21. Sulfonamides
  22. Dapsone
  23. Trimethoprim
  24. Fluoroquinolones
  25. Daptomycin
  26. Metronidazole
  27. Rifamycins (Rifampin, Rifabutin)
  28. Isoniazid
  29. Pyrazinamide
  30. Ethambutol
  31. Chloroquine


Tetracyclines are a class of antibiotics that end in -cycline, including doxycycline, minocycline, and a drug also called tetracycline. The tetracyclines as a class work by inhibiting the 30S subunit of bacterial ribosomes. This blocks bacterial protein synthesis, preventing bacterial growth. Tetracyclines are used for a wide variety of infections, but are especially potent for intracellular organisms like rickettsial diseases, chlamydia, as well as mycoplasma and borrelia. Importantly, tetracyclines should NOT be taken with milk or antacids, since doing so can reduce drug levels to subtherapeutic levels.  Side effects of taking tetracyclines include GI distress, discoloration of teeth, inhibition of bone growth, and photosensitivity. Tetracyclines are also teratogenic and should be avoided during pregnancy. Resistance to the tetracycline antibiotics develops by way of reduced uptake or increased efflux out of bacterial cells.

Key Points

  • Tetracyclines
    • Drug Names
      • Tetracycline
      • Doxycline
      • Minocycline
    • Mechanism
      • Binds to 30S subunit of bacterial ribosomes
        • Prevents attachment of aminoacyl-tRNA, preventing bacterial protein synthesis
      • Bacteriostatic
      • Do not take with milk, antacids, or iron-containing preparations
        • Calcium, magnesium, and iron ions (divalent ions) bind to drug, inhibiting drug’s absorption in gut
      • Accumulates inside cells
        • Good against intracellular agents
      • Limited CNS penetration
        • Not used for CNS infections (ceftriaxone used instead)
      • Doxycycline is fecally eliminated
        • Can be used in patients with renal failure
    • Clinical Use
      • Rickettsia
        • e.g. Rocky Mountain Spotted Fever
      • Borrelia burgdorferi
      • Chlamydia spp.
      • M. pneumoniae
      • Acne
      • Community-acquired MRSA
    • Adverse Effects
      • GI distress
        • Most common side effect
        • Presents with abdominal pain, nausea, vomiting
      • Discoloration of teeth and inhibition of bone growth in children
        • Tetracycline can bind to bone and teeth before mineralization is complete
      • Photosensitive rash
      • Teratogen
        • Can cause discoloration of fetal teeth
        • However, drug may still be administered in rare cases where benefit outweighs risk (e.g. Rocky Mountain Spotted Fever, Lyme)
      • Pill-induced esophagitis
        • Due to low pH of drug when dissolved in saliva
        • Should be upright and take with liquids
      • Rare hepatotoxicity
    • Resistance
      • Reduced uptake or increased efflux out of bacterial cells
        • By plasmid-encoded active transport pumps
      • Synthesis of protein that enables translation despite presence of tetracycline drug