Signalling possible drug-drug interactions in a spontaneous reporting system: delay of withdrawal bleeding during concomitant use of oral contraceptives and itraconazole

Aims In spontaneous adverse drug reaction reporting systems, there is a gro wing need for methods facilitating the automated detection of signals conce rning possible adverse drug reactions. In addition, special attention is ne eded for the detection of adverse drug reactions resulting from possible dr ug-drug interactions. We describe a method for detecting possible drug-drug interactions using logistic regression analysis to calculate ADR reporting odds ratios. Methods To illustrate this method, we analysed the adverse drug reaction 'd elayed withdrawal bleeding' resulting from a possible interaction between i traconazole and oral contraceptives in reports received by the Netherlands Pharmacovigilance Foundation LAREB between 1991 and 1998. Results In total 5,503 reports were included in the study. The odds ratio, adjusted for year of reporting, age and source of the reports, for a delaye d withdrawal bleeding in women who used both drugs concomitantly compared w ith women who used neither oral contraceptives, nor itraconazole, was 85 (9 5% CI: 32-230). Conclusions Since spontaneous reporting systems can only generate signals c oncerning possible relationships, this association needs to be analysed by other methods in more detail in order to determine the real strength of the relationship. This approach might be a promising tool for the development of procedures for automated detection of possible drug-drug interactions in spontaneous reporting systems.