PATIENTS TAKING WARFARIN SHOULD BE MONITORED REGULARLY FOR CHANGES IN PROTHROMBIN TIME OR INR. SUBSTANCES THAT ARE INDUCERS OF CYP3A4 ACTIVITY LIKE RIFAMPICIN & PHENYTOIN INCREASE THE METABOLISM OF GEFITINIB AND DECREASE ITS PLASMA CONCENTRATIONS. SUBSTANCES THAT ARE POTENT INHIBITORS OF CYP3A4 ACTIVITY (EG, KETOCONAZOLE AND ITRACONAZOLE) DECREASE GEFITINIB METABOLISM AND INCREASE GEFITINIB PLASMA CONCENTRATIONS. DRUGS THAT CAUSE SIGNIFICANT SUSTAINED ELEVATION IN GASTRIC PH (HISTAMINE H2-RECEPTOR ANTAGONISTS SUCH AS RANITIDINE OR CIMETIDINE) MAY REDUCE PLASMA CONCENTRATIONS OF IRESSA AND THEREFORE POTENTIALLY MAY REDUCE EFFICACY. WHERE IRESSA AND VINORELBINE HAVE BEEN USED CONCOMITANTLY, DATA INDICATE THAT IRESSA MAY EXACERBATE THE NEUTROPENIC EFFECT OF VINORELBINE.