Medicine & USMLE


Anti-Infective Drugs
  1. Vancomycin
  2. Metronidazole
  3. Penicillins
  4. Cephalosporins
  5. Macrolides
  6. Fluoroquinolones
  7. Aminoglycosides
  8. Tetracyclines
  9. Sulfonamides
  10. Rifampin
  11. Isoniazid
  12. Ethambutol
  13. Chloroquine
  14. Acyclovir
  15. Oseltamivir
  16. Azoles
  17. Nystatin
  18. Amphotericin B


Sulfonamides, also called sulfa drugs, are easily recognized because they start with “sulfa-”. Sulfonamides are antibiotics that treat a wide variety of bacterial infections. Sulfa allergies are quite common, and it’s important to remember that there is a cross-sensitivity between other sulfa-derivative drugs like sulfonylureas and thiazide diuretics. One side effect is crystalluria, so it’s important that the patient stays well-hydrated to avoid kidney damage. Sulfonamides can also cause photosensitivity, so advise the patient to wear sunscreen and avoid direct sunlight. Sulfonamides may cause a folate deficiency, so there’s a chance that the patient will also be prescribed a folic acid supplementation. Sulfonamides are teratogenic and should not be taken by pregnant women.

Key Points

  • Sulfonamides
    • Key Drugs (Sulfa-)
      • Do not confuse with sulfonylureas - drug class used to treat type II diabetes (e.g. glipizide, glyburide). They are also a sulfa- derivative, but are used for a different purpose.
      • Sulfamethoxazole (SMX)
        • Often combined with another antibacterial agent, trimethoprim (TMP) into TMP-SMX, commonly known as Bactrim
      • Sulfadiazine
      • Sulfasalazine (SSZ) - used as an auto-immune drug
    • Mechanism
      • Antibiotic
        • Inhibit synthesis of folic acid, an essential component of bacterial growth
        • Bacteriostatic
    • Clinical Use
      • Bacterial Infections
        • Urinary tract infections
        • Ear infections
        • Meningococcal meningitis
        • E. coli
        • MRSA
        • Used as an alternative for patients allergic to penicillin
    • Side Effects and Adverse Reactions
      • Hypersensitivity/Allergy
        • Cross-allergenicity with other sulfa drugs (e.g. sulfonylureas and thiazide diuretics)
      • Crystalluria
        • Crystallization in the urine
        • Can cause kidney injury
        • Increase fluid intake to maintain adequate urine output
          • Ideally, patient should be consuming at least 2,000 mL/day
          • Increased hydration reduces the risk of crystal formation
      • Photosensitivity
        • Avoid direct sunlight, wear sunscreen and protective clothing during sun exposure
        • May escalate into Stevens-Johnson Syndrome / Toxic Epidermal Necrolysis
          • If rash presents, patient should stop taking medication and it should be reported immediately to the provider
      • Folate deficiency
        • Folic acid supplementation may be needed
      • Teratogenic
        • Should be avoided in pregnancy
        • Can cause neural tube defects and infantile kernicterus, especially if taken in the 3rd trimester
      • Red-orange discoloration of body fluids
      • Anemia, agranulocytosis, low platelet count
        • Due to a reduction in folic acid
        • Report bruising, bleeding, petechial rash
      • Hemolysis is especially common in G6PD deficiency
      • Superinfection
        • e.g. C. difficile overgrowth
      • GI disturbances
        • Nausea, vomiting, anorexia