skip to main content

Tamoxifen CYP2D6 — SCE Medical Oncology MCQ

Instant feedback + full explanation. One question, done properly.

ModerateBreast CancerTamoxifen CYP2D6SCE Medical Oncology

A 55-year-old woman with ER+/HER2- breast cancer on adjuvant tamoxifen is co-prescribed fluoxetine for depression by her GP. Her oncologist raises a concern about this combination. What is the issue?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — Fluoxetine is a potent CYP2D6 inhibitor that blocks conversion of tamoxifen to its active metabolite endoxifen, potentially reducing tamoxifen efficacy

Tamoxifen is a prodrug requiring CYP2D6 metabolism to its active metabolite endoxifen. Strong CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion) significantly reduce endoxifen levels, potentially compromising tamoxifen efficacy. SSRIs with minimal CYP2D6 inhibition (citalopram, escitalopram, sertraline, venlafaxine) are preferred alternatives. This pharmacogenomic interaction is clinically important and should be flagged during prescribing. CYP2D6 genotyping can also identify poor metabolisers.

Reference: BNF Tamoxifen/Fluoxetine interaction; NICE NG101; ESMO 2024 Early Breast Cancer