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


Macrolides are a class of antibiotics that end in the suffix “-thromycin”: including azithromycin, clarithromycin, and erythromycin. Macrolides work by inhibiting the 50S subunit of bacterial ribosomes, stopping protein production to inhibit bacterial growth. Clinically, macrolides are used for a wide variety of bacterial infections, including the treatment of atypical pneumonias from organisms like Legionella or Mycoplasma, as well as treatment of sexually-transmitted infections like chlamydia and gonorrhoea. Important adverse effects to know include hepatitis, GI upset, and QT prolongation, with a risk of causing Torsades de Pointes. It’s also extremely important to remember that macrolides can cause drug interactions by inhibiting CYP enzymes in the liver.

Key Points

  • Macrolides
    • Drug Names (-thromycin ending)
      • Azithromycin
      • Clarithromycin
      • Erythromycin
    • Mechanism
      • Bind to 50S subunit of bacterial ribosomes
        • Bind to the 23S rRNA of the 50S ribosomal subunit, to blocking transpeptidation, translocation, and chain elongation
        • Overall effect is a blocking of bacterial protein synthesis
      • Bacteriostatic
    • Clinical Use
      • Atypical pneumonias (Mycoplasma, Chlamydia, Legionella)
      • STIs (Chlamydia, N. gonorrhoae)
      • Bordetella pertussis
      • Gram-positive cocci (streptococcal) in patients allergic to penicillin
    • Adverse Effects
      • Inhibits CYP450 Enzymes
        • Leads to drug interactions (e.g. warfarin, theophylline)
      • GI Upset
        • All macrolides act as agonists at motilin receptors in the GI tract, increasing GI motility
      • Prolonged QT interval (Torsades de Pointes risk)
      • Acute cholestatic hepatitis
      • Rash
      • Eosinophilia
    • Mechanism of Resistance
      • Methylation of 23S rRNA-binding site prevents binding of drug