Tamoxifen, the generic name of Nolvadex, is the oldest and most-prescribed SERM. Tamoxifen is approved by the U.S. Food and Drug Administration (FDA) to treat:
- women and men diagnosed with hormone-receptor-positive, early-stage breast cancer after surgery (or possibly chemotherapy and radiation) to reduce the risk of the cancer coming back (recurring)
- women and men diagnosed with advanced-stage or metastatic hormone-receptor-positive disease
Tamoxifen also is used to:
- reduce breast cancer risk in women who haven't been diagnosed but are at higher-than-average risk for disease
Tamoxifen won't work on hormone-receptor-negative breast cancer.
Tamoxifen is a pill taken once a day. Most doctors recommend taking tamoxifen at the same time each day. Tamoxifen is also available as a liquid under the brand name Soltamox.
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 post-menopausal women, tamoxifen is the first choice for pre-menopausal women and is still a good choice for post-menopausal women who can't take an aromatase inhibitor.
- reduce the risk of breast cancer coming back by 40% to 50% in post-menopausal women and by 30% to 50% in pre-menopausal 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 post-menopausal women
- lower breast cancer risk in women who have a higher-than-average risk of disease but have not been diagnosed
Tamoxifen offers other health benefits that aren't related to treating cancer. Because it's a SERM, it selectively either blocks or activates estrogen's action on specific cells. While tamoxifen blocks estrogen's action on breast cells, it activates estrogen's action in bone and liver cells. So tamoxifen can:
- help stop bone loss after menopause
- lower cholesterol levels
Some people may not get the full benefit of tamoxifen
The body uses an enzyme called CYP2D6 to convert tamoxifen into its active form. Two things can interfere with the body’s ability to make this happen: a flaw in the CYP2D6 enzyme and certain medications that block the effectiveness of this enzyme.
- Abnormal CYP2D6 enzyme: About 10% of people have a CYP2D6 enzyme that doesn’t function as well as it should. Having an abnormal CYP2D6 enzyme might keep a person from getting the full benefit of tamoxifen. Ask your doctor about being tested for this enzyme abnormality if you are seriously considering taking tamoxifen.
Medications that can interfere with CYP2D6: There are medications that can block the activity of CYP2D6 to varying degrees (usually stated as “strong” or “moderate” inhibitors of CYP2D6). These medications include some of the antidepressants known as serotonin-specific reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs). There are also other types of commonly prescribed medications, such as Cardioquin (chemical name: quinidine), Benadryl (chemical name: diphenhydramine), and Tagamet (chemical name: cimetidine), that can block CYP2D6. Blocking the activity of CYP2D6 can interfere with the activation of tamoxifen — reducing its effectiveness as an anti-cancer treatment. Most doctors recommend that you avoid taking strong and moderate inhibitors of CYP2D6 while you’re on tamoxifen.
If you have already finished tamoxifen and you were taking other medications at the same time, make an appointment to talk with your doctor about whether any of your other medications may have interfered with CYP2D6 and the potential benefit you received from tamoxifen. Your doctor may recommend no additional therapy or extended hormonal therapy (with tamoxifen or an aromatase inhibitor) depending on your risk of recurrence, your overall medical condition, and your preferences.
If you were taking tamoxifen because you are at high risk but have never been diagnosed, and you were also taking a CYP2D6 inhibitor, your doctor may now recommend additional anti-estrogen therapy with tamoxifen or Evista (chemical name: raloxifene), depending on your menopausal status. Talk to your doctor about what’s best for your situation.
If you had progression of breast cancer while on both tamoxifen and a strong or moderate CYP2D6 inhibitor, you can’t assume that tamoxifen failed to work. Rather, it’s possible that tamoxifen never had a fair chance at getting the cancer under control because its action was blocked by the other medicine. Moving forward, tamoxifen, without a CYP2D6 inhibitor, may still provide significant benefit.
For those taking raloxifene (brand name: Evista)
The CYP2D6 enzyme is not needed to activate raloxifene (brand name: Evista, a sister medicine to tamoxifen, used to reduce the risk of developing hormone-receptor-positive breast cancer and treat osteoporosis in postmenopausal women).
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