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Aims: Tamoxifen is widely used in the treatment of breast cancer. It is a pro-drug metabolized to the more active endoxifen through CYP2D6. Concomitant intake of CYP2D6 inhibitors results in lower endoxifen levels and could influence efficacy. The objective of this study was to evaluate the evolution of co-prescription of tamoxifen and CYP2D6 inhibitors in Belgium., Methods: Data were retrieved from the Pharmanet database of the National Institute for Health and Disability Insurance for the period January 2006-December 2009. For the analysis of the evolution of the co-prescription, the period was divided in subperiods of 2 months. The category tamoxifen+CYP2D6 inhibitor was defined as women who were delivered tamoxifen and a CYP2D6 inhibitor in that subperiod. The results were validated on the period December 2011-May 2012., Results: The percentage of co-prescription decreased over time for the strong CYP2D6 inhibitors and increased for the weak CYP2D6 inhibitor, with these trends persisting in 2012. Tamoxifen and CYP2D6 inhibitors were mostly prescribed by general practitioners and gynaecologists and by general practitioners and psychiatrists, respectively., Discussion: This study shows that a proportion of women taking tamoxifen in Belgium are prescribed a strong CYP2D6 inhibitor, which could affect tamoxifen efficacy. Over time, the concomitant intake decreased. Paroxetine was the most prescribed strong CYP2D6 inhibitor. Venlafaxine, a weak CYP2D6 inhibitor, was prescribed more often. This study also shows that tamoxifen and the CYP2D6 inhibitors are not only prescribed by physicians specialized in breast cancer; therefore, all physicians should be aware of this interaction.