The effect of antacids on the systemic absorption of oral norfloxacin was evaluated in 12 healthy volunteers. Subjects were given each treatment in a balanced sequence at 7-day intervals. Treatments included 400 mg of norfloxacin alone, 400 mg of norfloxacin 5 min after aluminum-magnesium hydroxide (Maalox), Maalox 2 h after 400 mg of norfloxacin, and 400 mg of norfloxacin 5 min after calcium carbonate (Titralac). Blood and urine samples were collected at predetermined time intervals for 24 and 48 h, respectively. Norfloxacin concentrations in plasma and urine were determined by high-pressure liquid chromatography. The area under the plasma concentration-versus-time curve from time zero to infinity and urinary recovery were used to compare the relative bioavailability of norfloxacin with antacids with that of norfloxacin alone. Norfloxacin bioavailability was markedly reduced when subjects received antacid pretreatment. When norfiloxacin was given 5 min after Maalox and Titralac, the bioavailabilities were 9.02 and 37.5%, respectively, relative to that for 400 mg of norfloxacin alone. When Maalox was given 2 h after norfloxacin, maximal concentrations of norfloxacin in plasma occurred between 1 and 1.5 h postdose, and absorption was reduced to a lesser extent, with a relative bioavailability of 81.31%. Norfloxacin concentrations in urine were also reduced as a result of antacid administration. Antacids containing aluminum and magnesium salts and calcium carbonate should be avoided by patients taking norfloxacin.Norfloxacin is a fluoroquinolone antimicrobial agent that is indicated for treating both complicated and uncomplicated urinary tract infections in patients who are candidates for oral therapy. A prerequisite to using oral antimicrobial agents is assurance that the drug will be absorbed. Antacids containing aluminum and magnesium salts may reduce absorption of the fluoroquinolones (3-6). Chelation between these metal ions and the 3-carboxyl and 4-oxo substituents on the quinolone nucleus results in a complex that is more polar and unable to be absorbed (4). Because this interaction may cause markedly reduced bioavailability, its occurrence may lead to therapeutic failures.In this study, the effects of antacid administration on the systemic bioavailability of oral norfloxacin were evaluated. MATERIALS AND METHODSThe study included 12 males between the ages of 18 and 40 years. Written informed consent was obtained. The subjects were determined to be healthy by physical examination, medical history, electrocardiogram, and laboratory tests. They were free of significant physical or psychological abnormalities, drug abuse, and allergies.A balanced, four-period, crossover design was employed. Subjects received each treatment once, and each treatment period was separated by a 7-day washout period. Treatments included 400 mg of norfloxacin, 400 mg of norfloxacin 5 min after 30 ml of aluminum and magnesium hydroxide (Maalox), 400 mg of norfloxacin followed by 30 ml of Maalox 2 h later, and 400 mg of norfloxacin 5 m...

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