Raw research findings for this medication. Mostly single studies, not curated or confirmed robust.
Tamoxifen is a selective oestrogen receptor modulator (SERM): in breast tissue it blocks oestrogen from driving cell growth. It is used to treat hormone receptor-positive breast cancer, to lower the chance of it coming back after surgery, and to reduce the chance of breast cancer in some people at high risk. It is sold as Nolvadex, among other names.
Tamoxifen itself is fairly weak; the liver enzyme CYP2D6 turns it into endoxifen, a much stronger form. People with little or no CYP2D6 activity have markedly lower endoxifen levels. Whether that changes how often the cancer returns has been debated, with studies pointing different ways, but CPIC and DPWG have published guidelines on CYP2D6 and tamoxifen, and the US FDA lists an effect on drug levels. Studies have also looked at CYP2C19, CYP3A5 and CYP19A1, with weaker evidence.
Clinical guidelines that name a gene for Tamoxifen. They sort people into groups by the gene copies they carry.
CPIC guideline: CYP2D6. CYP2D6 activity score 0 to 6 or more. Strong. 2018.
DPWG guideline: CYP2D6. CYP2D6 activity score 0 to 6 or more. 2011.
Every published finding on gene variants and Tamoxifen. Upload your DNA file to see which of these rows apply to you.
PharmGKB weighs all studies of a variant and rates how strong the evidence is, from 1A (strongest) to 4.
CYP2D6 CYP2D6*1, CYP2D6*2, CYP2D6*3, CYP2D6*4, CYP2D6*5, CYP2D6*6, CYP2D6*7, CYP2D6*9, CYP2D6*10, CYP2D6*10x2, CYP2D6*11, CYP2D6*17, CYP2D6*21, CYP2D6*36, CYP2D6*41: Level 1A · replicated. Efficacy: Breast Neoplasms. 56.
CYP2D6 CYP2D6*1, CYP2D6*2, CYP2D6*3, CYP2D6*4, CYP2D6*5, CYP2D6*6, CYP2D6*10, CYP2D6*17, CYP2D6*29, CYP2D6*35, CYP2D6*41: Level 1A · replicated. Metabolism/PK: Breast Neoplasms. 13.
CYP2C19 CYP2C19*1, CYP2C19*2, CYP2C19*17: Level 3. Efficacy, Metabolism/PK: Breast Neoplasms. 8.
ABCC2 rs3740065: Level 3. Efficacy: Breast Neoplasms. 2.
Studies of how quickly the body processes this medication.
Studies of how well it worked in people carrying a variant.
Studies of how much of the drug people needed or tolerated.
Study findings and evidence levels: PharmGKB (CC BY-SA 4.0). Guidelines: CPIC and DPWG, as bundled in PharmCAT. Most studies were done mainly in people of European ancestry.
More drugs from the same category
Not a prescribing recommendation. This page is educational information about pharmacogenomic research and clinical guidelines - it is not medical or dosing advice, and not a prediction of how any medication will work for you. Never start, stop, or change a medication or dose based on it; talk to your doctor or pharmacist, who can order clinical-grade testing if appropriate.
Content updated September 2026.
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