Antibiotic Drug Interactions
Antibiotics interact with drugs through CYP inhibition, chelation, gut flora disruption, and QT prolongation. These interactions affect nearly every medical specialty.
## Scope of the Problem
Antibiotics are prescribed to nearly every hospitalized patient and are among the most common outpatient medications. Their drug interactions span CYP enzyme inhibition, chelation, microbiome disruption, QT prolongation, and direct pharmacodynamic effects.
## Macrolides
Clarithromycin and erythromycin are potent CYP3A4 inhibitors (mechanism-based, irreversible). Major interactions:
- **Statins** — simvastatin and lovastatin AUC increases 5-10 fold; rhabdomyolysis risk
- **Carbamazepine** — toxicity with ataxia, diplopia, nausea
- **Colchicine** — fatal colchicine toxicity from CYP3A4 + P-gp inhibition; contraindicated in renal/hepatic impairment
- **QT-prolonging drugs** — additive QT prolongation
Azithromycin does NOT inhibit CYP enzymes but has mild QT-prolonging potential.
## Fluoroquinolones
### Chelation
Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) form insoluble chelates with divalent and trivalent cations (Ca2+, Mg2+, Al3+, Fe2+/3+, Zn2+). Antacids, sucralfate, iron supplements, and calcium-fortified foods reduce absorption by 50-90%. Separate administration by 2 hours before or 6 hours after the cation-containing product.
### CYP1A2 Inhibition
Ciprofloxacin strongly inhibits CYP1A2, increasing levels of:
- **Theophylline** — seizures and cardiac arrhythmias at toxic levels
- **Tizanidine** — severe hypotension and sedation (contraindicated)
- **Caffeine** — clinically noticeable but rarely dangerous
Levofloxacin and moxifloxacin do not significantly inhibit CYP1A2.
### QT Prolongation
Moxifloxacin has the greatest QT effect; levofloxacin is intermediate; ciprofloxacin has the least. Avoid combining with other QT-prolonging agents.
## Rifamycins
Rifampin is the most potent known CYP inducer, affecting CYP3A4, 2C9, 2C19, 1A2, and 2B6 plus P-gp and OATP1B1. It dramatically reduces levels of:
- Oral contraceptives (contraceptive failure)
- Warfarin, DOACs (thromboembolism)
- HIV antiretrovirals (treatment failure)
- Immunosuppressants (transplant rejection)
- Methadone (withdrawal symptoms)
Rifabutin is a weaker inducer and preferred in HIV patients on protease inhibitors.
## Metronidazole and TMP-SMX
- **Metronidazole** — inhibits CYP2C9; increases warfarin effect. Also causes disulfiram-like reaction with alcohol (debated but widely taught).
- **TMP-SMX** — trimethoprim inhibits renal creatinine secretion (raises serum creatinine without true GFR change) and inhibits ENaC (hyperkalemia risk with ACE inhibitors/ARBs). Sulfamethoxazole inhibits CYP2C9 (warfarin interaction).
## Tetracyclines
Chelation with divalent cations (similar to fluoroquinolones). Doxycycline is less affected than tetracycline. Tetracyclines may reduce oral contraceptive efficacy, though evidence is weak.
## Key Takeaways
- Clarithromycin and erythromycin are potent CYP3A4 inhibitors; azithromycin is not
- Fluoroquinolone absorption is reduced 50-90% by antacids, iron, and calcium
- Ciprofloxacin + tizanidine is contraindicated due to severe hypotension
- Rifampin induces nearly all CYP enzymes and transporters; expect reduced efficacy of most co-administered drugs
- TMP-SMX raises creatinine without true renal injury and causes hyperkalemia with RAAS inhibitors