Omeprazole has the potential to interfere with the cytochrome P-450 enzyme system and then to induce or inhibit the metabolism of drugs eg, diazepam, warfarin and phenytoin. Because of its profound and long-lasting inhibition of gastric acid secretion, omeprazole may interfere with absorption of drugs where gastric pH is important in the determination of their bioavailability (eg, ketoconazole, ampicillin, esters and iron salts).