Ulsen can prolong the elimination of diazepam, warfarin and phenytoin, drugs that are metabolized by
oxidation in the liver. Although in normal subjects no interaction with theophylline or propranolol was found, there
have been clinical reports of interaction with other drugs metabolized via the cytochrome P-450 system (e.g.,
cyclosporine, disulfiram, benzodiazepines). Patients should be monitored to determine if it is necessary to adjust
the dosage of these drugs when taken concomitantly with omeprazole.
Because of its profound and long lasting inhibition of gastric acid secretion, it is theoretically possible that
omeprazole may interfere with absorption of drugs where gastric pH is an important determinant of their
bioavailability (e.g., ketoconazole, ampicillin esters, and iron salts). In the clinical trials, antacids were
used concomitantly with the administration of omeprazole.
Combination Therapy with Clarithromycin
Co-administration of omeprazole and clarithromycin may result in increases in plasma levels of ompeprazole,
clarithromycin, and 14-hydroxy-clarithromycin.
Concomitant administration of clarithromycin with cisapride, pimozide, or terfenadine is contraindicated.
There have been reports of an intereaction between erythromycin and astemizole resulting in QT prolongation and
torsades de points. Concomitant administration of erythromycin and astemizole is contraindicated. Because
clarithromycin is also metabolized by cytochrome P450, concomitant administration of clarithromycin with astemizole
is not recommended.