Chapter 35  ·  Module 12

Drug Interactions, Adverse Effects, and Special Populations

Key interactions, class toxicities, renal/hepatic adjustment, and antibiotic safety in pregnancy and pediatrics

Abbreviations: CYP = cytochrome P450  |  QTc = corrected QT interval  |  MAO = monoamine oxidase  |  CrCl = creatinine clearance  |  RID = relative infant dose  |  G6PD = glucose-6-phosphate dehydrogenase  |  TMP-SMX = trimethoprim-sulfamethoxazole  |  TDM = therapeutic drug monitoring

Drug Interactions

CYP Interactions

Macrolides and Rifampin

Macrolides — CYP3A4 Inhibitors
  • Erythromycin, clarithromycin: potent CYP3A4 inhibition
  • ↑ statins (simvastatin, lovastatin) → rhabdomyolysis risk — contraindicated
  • ↑ calcineurin inhibitors, benzodiazepines, warfarin
  • Azithromycin: much weaker CYP inhibitor; lower interaction burden
Rifampin — Potent CYP Inducer
  • Induces CYP3A4, CYP2C9, CYP2C19
  • ↓ oral contraceptives, warfarin, antiretrovirals, calcineurin inhibitors
  • Induction onset and offset: 1–2 weeks each

Other Key Interactions

QTc, Serotonin, and Additive Toxicity

QTc Prolongation
  • Moxifloxacin > levofloxacin > ciprofloxacin; also azithromycin, clarithromycin
  • Check baseline QTc, electrolytes, concurrent QTc drugs
  • Avoid if QTc >500 ms
Serotonin Syndrome (Linezolid)
  • Linezolid = reversible nonselective MAO inhibitor
  • Avoid with SSRIs, SNRIs, TCAs, tramadol, meperidine
  • Triad: agitation, hyperthermia, neuromuscular signs
Additive Toxicity
  • Aminoglycosides + loop diuretics → synergistic ototoxicity
  • Vancomycin + pip-tazo → increased acute kidney injury risk
  • Tetracyclines + penicillins → bactericidal antagonism

Major Adverse Effects by Class

Class Key Toxicity Clinical Points
Beta-lactams Hypersensitivity (IgE-mediated) True anaphylaxis rare; cross-reactivity penicillin-cephalosporin ~1–2% (not 10%); skin test-negative patients can receive penicillins normally
Aminoglycosides Nephrotoxicity; ototoxicity (cumulative, irreversible) Extended-interval dosing preferred; monitor TDM; audiometric monitoring for courses >14 days
Fluoroquinolones Tendinopathy/rupture; peripheral neuropathy; CNS effects; QTc prolongation FDA black box warning (5 categories); contraindicated in myasthenia gravis; restrict to serious infections
Tetracyclines Photosensitivity; esophageal ulceration; dental/bone effects in children Doxycycline: remain upright 30 min after dosing; avoid in children <8 and pregnancy
Vancomycin Nephrotoxicity; red man syndrome (rate-dependent) AUC-guided TDM; infuse over ≥60 min; red man syndrome is NOT a true allergy
Daptomycin Myopathy (CPK elevation) Weekly CPK monitoring; suspend statins; NEVER use for pneumonia (surfactant inactivation)

Renal and Hepatic Dose Adjustment

Renal Impairment

Key Adjustments

Use Cockcroft-Gault for dosing
  • Most antibiotics require dose reduction as CrCl falls
  • Ceftriaxone: NO renal adjustment (biliary elimination) — exception
  • Imipenem: reduce when CrCl <70 (seizure risk)
  • Aminoglycosides: switch from once-daily to TDM-guided at low CrCl
  • Augmented renal clearance (CrCl >130): standard doses may be subtherapeutic

Hepatic Impairment

Key Adjustments

Adjust in severe hepatic impairment
  • Metronidazole: reduce dose; avoid prolonged courses
  • Clindamycin: reduce in severe impairment
  • Chloramphenicol: accumulates → gray baby syndrome in neonates
  • Rifampin: hepatotoxic; monitor liver function tests
  • Macrolides: caution in significant liver disease

Pregnancy and Pediatrics

Pregnancy Safety

Safe, Avoid, Contraindicated

Safe Throughout
  • Penicillins, cephalosporins, carbapenems, aztreonam
  • Erythromycin base, azithromycin, clindamycin
Avoid First Trimester
  • TMP-SMX (folate antagonism → neural tube risk)
Avoid at Term
  • TMP-SMX (kernicterus), nitrofurantoin (hemolytic anemia in G6PD-deficient neonates)
Contraindicated Throughout
  • Tetracyclines (bone/tooth dysplasia)
  • Fluoroquinolones (arthropathy)
  • Aminoglycosides (ototoxicity; especially streptomycin)

Pediatrics and Lactation

Special Considerations

Neonatal Pharmacokinetics
  • Immature CYP3A4 (~30% adult activity at birth)
  • Immature renal filtration and tubular secretion
  • Aminoglycosides: extended intervals in neonates
  • Chloramphenicol: immature glucuronidation → gray baby syndrome
  • Fluoroquinolones: restrict; arthropathy concern
Breastfeeding (RID <10% = generally safe)
  • Beta-lactams: RID <1% — compatible
  • Tetracyclines: chelated by milk calcium; short courses acceptable
  • Metronidazole: RID <10%; generally compatible

Chapter 35 Complete

Modules 8–12 complete the Antibacterial Agents chapter. Key cross-module themes: mechanisms determine toxicity (mitochondrial inhibition → bone marrow suppression in chloramphenicol and linezolid); pharmacokinetics determine dose adjustment (renal elimination → dose reduction; immature neonatal enzymes → accumulation); resistance mechanisms predict which agents fail and why (ESBL → use carbapenems; MRSA → use agents that bind PBP2a or bypass the cell wall).