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PEBC: What You Need to Know: Tamoxifen

Updated: Jul 25, 2020


adjuvant for breast cancer treatment


Tamoxifen is a prodrug which is converted to the active metabolite endoxifen via CYP 2D6 metabolism. Endoxifen blocks the receptor site for estrogen to exert its effects on breast cancer tissue.

Usual Dosing:

20 mg once daily for 10 years (extended treatment) *use in premenopausal patients (if post menopausal, use aromatase inhibitors)

Adverse Effects: LOTS!

-GI upset

-hot flashes (related to anti-estrogen mechanism)

-increased risk of clots (stroke, deep vein thrombosis, pulmonary embolism)

-increased risk of endometrial cancer

-bone and muscle pain

-platelet reduction


-altered lipid profile

-vaginal bleeding

Drug Interactions:

CYP 2D6 inhibitors (terbinafine, fluoxetine, paroxetine, bupropion): reduced tamoxifen activation --> failure of tamoxifen

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