Tamoxifen is an estrogen receptor antagonist used extensively for the therapy of women with breast cancer. In considering possible drug interactions, it’s crucial to appreciate that tamoxifen’s effect is in part determined by its active metabolites, endoxifen. Thus, the anti-tumour effects of tamoxifen are in fact due in part to endoxifen, which is produced by metabolism of tamoxifen mediated by the cytochrome CYP2D6. Thus, strong inhibitors of CYP2D6 such as bupropion, fluoxetine or paroxetine or moderate inhibitors such as sertraline can reduce the efficacy of tamoxifen in patients with cancer. In contrast, weak CYP2D6 inhibitors such as enlafaxine, citalopram or escitalopram appear to have minimal effects on the efficacy of tamoxifen. It should be emphasized that this isn’t a class effect related to SSRIs and that the basis of this effect is a change in CYP2D6 activity. Thus, any drug — for example, quinidine — that is a strong inhibitor of CYP2D6 can produce changes in the efficacy of tamoxifen.

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