Published: 6 December 2019

Publications

Reminder: Avoid using CYP2D6 inhibitors with tamoxifen

Prescriber Update 40(4): 86
December 2019

Tamoxifen is a prodrug metabolised by CYP2D6 into its active metabolite, endoxifen¹. Endoxifen concentrations are about 75 percent lower in patients who lack CYP2D6 compared to those with normal CYP2D6 activity². Co-administration of strong CYP2D6 inhibitors reduces endoxifen levels to a similar extent².

Avoid using CYP2D6 inhibitors such as paroxetine and fluoxetine with tamoxifen². Prescribe an alternative antidepressant with little or no inhibition of CYP2D6 for patients who require antidepressant treatment while taking tamoxifen³. These may include citalopram, escitalopram, sertraline, mirtazapine and venlafaxine³.

The Centre for Adverse Reactions Monitoring (CARM) has received a report describing an interaction between tamoxifen and fluoxetine (CARM ID 115253). This co-administration resulted in reduced therapeutic response and progression of disease.

References

1. Hansten P. 2018. The underrated risks of tamoxifen drug interactions. Eur J Drug Metab Pharmacokinet 43: 495–508. DOI: https://doi.org/10.1007/s13318-018-0475-9 (accessed 25 September 2019).
2. Mylan New Zealand Ltd. 2018. Genox New Zealand Data Sheet 23 May 2018. URL: www.medsafe.govt.nz/profs/Datasheet/g/genoxtab.pdf (accessed 25 September 2019).
3. Medsafe. 2011. Tamoxifen: CYP2D6 interactions and variable clinical response. Prescriber Update 32(2): 10. URL: www.medsafe.govt.nz/profs/PUArticles/TamoxifenJune2011.htm (accessed 25 September 2019).