Effects of Antacids on the Clinical Pharmacokinetics of Drugs

Since a previous review by Hurwitz was published in 1977 a large number of reports on drug interactions with antacids have appeared, few of which are of clinical relevance. Tetracyclines form insoluble complex molecules by metal ion chelation with various antacids; tetracycline absorption may be decreased by more than 90% by this interaction. Of the new class of quinolone antibiotics, the absorption of ciprofloxacin and ofloxacin is reduced by 50 to 90% in the presence of aluminium- and magnesium hydroxide-containing antacids. In contrast to early work showing inhibition of the absorption of beta-adrenergic blocking drugs by antacids, subsequent studies did not confirm a reduction in the bioavailability of either atenolol or propranolol during antacid treatment; indeed, they showed an increase in the plasma concentrations of metoprolol when the drug was coadministered with an antacid. The bioavailability of captopril was significantly reduced in the presence of an antacid, and lower plasma concentrations of this angiotensin-converting enzyme inhibitor were accompanied by a reduction of its effect on the systolic blood pressure of the patients. The absorption of the cardiac glycosides digoxin and digitoxin is not inhibited by antacids to a significant degree, although earlier studies had shown a positive effect when the dissolution of the glycoside preparations was relatively poor. Antacids reduce the bioavailability of the H2-receptor antagonists cimetidine and ranitidine only when high antacid doses are used and when the drugs are administered simultaneously. The bioavailability of famotidine was not significantly altered by a potent antacid preparation, although a trend towards reduced absorption was observed. Iron absorption is significantly decreased in the presence of sodium bicarbonate and calcium carbonate, but is nearly complete when coadministered with aluminium-magnesium hydroxide. Nonsteroidal anti-inflammatory drugs such as naproxen, tenoxicam, ketoprofen, ibuprofen and piroxicam are not affected in their absorption by antacid treatment. Theophylline bioavailability is unchanged when the drug is given together with antacids, although its rate of absorption may be altered, leading to a reduction or an increase in the time of the occurrence of peak plasma drug concentrations.