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What Does Tamoxifen Do

what does tamoxifen do


Tamoxifen (Nolvadex) has been used for over 40 years to treat hormone-receptor positive early, locally advanced and metastatic breast cancers.

Learn about tamoxifen and other hormone therapies for metastatic breast cancer.

How does tamoxifen work?

Hormone receptor-positive breast cancers need estrogen and/or progesterone (female hormones produced in the body) to grow.

Tamoxifen attaches to the hormone receptor in the cancer cell, blocking estrogen from attaching to the receptor.

This slows or stops the growth of the tumor by preventing the cancer cells from getting the hormones they need to grow.

Why do doctors prescribe tamoxifen?

  • There are different reasons, including:
  • To treat breast cancer after surgery or radiation
  • To treat cancer in one breast and reduce the risk of the disease in the other breast
  • To prevent invasive breast cancer in women at high risk
  • To treat the earliest stage of breast cancer, called ductal carcinoma in situ (DCIS), after surgery and radiation
  • To prompt ovulation in women with fertility problems
  • To treat other cancers such as ovarian cancer, uterine cancer, melanoma, and brain tumors
  • In higher than normal doses, tamoxifen may even kill some breast cancer cells that don’t depend on estrogen.

Benefits of tamoxifen

Since its approval in 1998, tamoxifen has been used to treat millions of women and men diagnosed with hormone-receptor-positive breast cancer. While an aromatase inhibitor is the first hormonal therapy medicine choice for postmenopausal women, tamoxifen is the first choice for premenopausal women and is still a good choice for postmenopausal women who can't take an aromatase inhibitor.

Tamoxifen can:

  • reduce the risk of breast cancer coming back by 40% to 50% in postmenopausal women and by 30% to 50% in premenopausal women
  • reduce the risk of a new cancer developing in the other breast by about 50%
  • shrink large, hormone-receptor-positive breast cancers before surgery
  • slow or stop the growth or advanced (metastatic) hormone-receptor-positive breast cancer in both pre- and postmenopausal women
  • lower breast cancer risk in women who have a higher-than-average risk of disease but have not been diagnosed.

Medicines to avoid while taking tamoxifen

In the list below, the medications under the headings “Strong Inhibitors” and “Moderate Inhibitors” can inhibit CYP2D6 and interfere with the effectiveness of tamoxifen. The medications under the heading “Not Inhibitors” do not block the CYP2D6 enzyme and will not interfere with tamoxifen treatment.

This list is incomplete and subject to change over time. Use it as a starting place and ask your doctor if any medications you are taking or that are recommended to you are compatible with tamoxifen.

Side effects of tamoxifen

Tamoxifen's selective estrogen activation effects can cause some serious side effects, including blood clots, stroke, and endometrial cancer. If you and your doctor are considering tamoxifen as part of your treatment plan, tell your doctor if you smoke or have a history of blood clots or heart attack. If you're taking tamoxifen, call your doctor immediately if you have any of these symptoms:

  • abnormal vaginal bleeding or discharge
  • pain or pressure in the pelvis
  • leg swelling or tenderness
  • chest pain
  • shortness of breath
  • weakness, tingling, or numbness in your face, arm, or leg
  • difficulty speaking or understanding
  • vision problems
  • dizziness
  • sudden severe headache