MANILA, July 15 — For women across the Philippines and the wider region facing hormone receptor-positive breast cancer, a medication called tamoxifen has long been a cornerstone of treatment. But understanding exactly how this drug works — and the risks it carries — is crucial for families and communities navigating a breast cancer diagnosis. Tamoxifen is a selective estrogen receptor modulator, or SERM.
Its primary action is to bind to estrogen receptors on breast cancer cells. By doing so, it blocks estrogen from stimulating tumor growth.
In breast tissue, the drug acts as an estrogen antagonist, effectively shutting down the hormone’s ability to fuel the cancer. But the story does not end there. In other parts of the body, tamoxifen behaves differently.
In the uterus, for example, the drug can act as an estrogen agonist. This means it promotes cell growth in the endometrial lining.
It is this estrogen-like effect on the uterus that increases the risk of endometrial cancer and uterine sarcoma — a serious consideration for women on long-term therapy. The drug is typically taken orally as a 20 mg tablet daily for five to ten years. It is used for both premenopausal and postmenopausal women with hormone receptor-positive breast cancer.
That means many women of childbearing age, as well as those past menopause, rely on this medication as part of their treatment plan.
How the body processes tamoxifen
The body does not simply absorb tamoxifen and put it to work. The drug is metabolized in the liver by the cytochrome P450 enzyme system, specifically an enzyme called CYP2D6. This process converts tamoxifen into active metabolites, the most important of which is endoxifen.
Genetic variations in CYP2D6 can affect how well tamoxifen works. Some women carry gene variants that make them poor metabolizers, meaning their bodies produce less of the active endoxifen.
This can reduce the drug’s efficacy and also influence the side effects a patient experiences.
What this means for patients
For a woman starting tamoxifen, the message is clear: this is a powerful drug with a well-understood mechanism, but it is not without trade-offs. The same drug that blocks estrogen in the breast can stimulate growth in the uterus. That dual action is why doctors monitor patients closely, especially for any signs of uterine changes.
The five-to-ten-year treatment course also means a long-term commitment. For families across the region, supporting a loved one through years of daily medication — and managing potential side effects — is part of the broader journey of cancer care. Consult your doctor for medical advice.
