Hormonal therapy

Tamoxifen

Taken for years after early breast cancer, and it too is a prodrug. Whether the enzyme that activates it should change prescribing is one of the longest unsettled arguments in pharmacogenomics.

What this drug connects to

The genes involved

CYP2D6

Breast Cancer →

A genuine finding that has never settled into a clear instruction, and the page says so rather than choosing a side. Tamoxifen is a prodrug: CYP2D6 turns it into endoxifen, which does most of the antioestrogen work. Patients with reduced-function CYP2D6 genotypes, and patients taking strong CYP2D6 inhibitors, have lower endoxifen levels and — in the guideline's own careful wording — a higher risk of recurrence "in some studies". Trials have disagreed, which is why CPIC issues recommendations while the underlying evidence stays contested. The practical part that is not contested: some antidepressants are strong CYP2D6 inhibitors, and a prescriber choosing one for a woman on tamoxifen will take that into account. Do not stop tamoxifen over this.

CPIC Guideline for CYP2D6 and Tamoxifen Therapy, Clinical Pharmacology & Therapeutics (PMID 29385237)

This page is educational and contains no dosing information. Where a clinical guideline covers one of these gene-drug pairs it is written for prescribers and works through validated algorithms alongside clinical monitoring. Nothing here is a reason to start, stop, or change a medication — that conversation belongs with the clinician or pharmacist managing your treatment.