A patient on ciprofloxacin for a urinary tract infection also takes an antacid containing aluminium hydroxide. Which pharmacokinetic interaction is most likely?

• A Antacids accelerate renal tubular secretion of ciprofloxacin
• B Aluminium induces hepatic CYP1A2, increasing ciprofloxacin metabolism
• C Alkalization of urine enhances ciprofloxacin tubular reabsorption
• D Chelation reduces ciprofloxacin absorption by up to 90% ✓

Explanation

Fluoroquinolones form insoluble chelates with divalent and trivalent metal cations (Al3+, Mg2+, Ca2+, Fe2+, Zn2+), dramatically reducing oral bioavailability—administration with aluminium-containing antacids can reduce ciprofloxacin AUC by up to 85–90%. Patients should be advised to separate quinolone dosing from antacids, iron, or dairy by at least 2 hours. Aluminium does not induce CYP1A2 (ciprofloxacin itself inhibits it); and the interaction is absorptive, not renal.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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