Tamoxifen is a medication with both anti-estrogen and estrogen-like effects, depending on the particular type of tissue it affects. It is classified as a selective estrogen receptor modulator (SERM), along with the medication Evista (raloxifene).
This article will review the indications for tamoxifen, how it reduces the risk of breast cancer recurrence, what side effects can happen with the medication, and how it compares to aromatase inhibitors.
Indications
There are three primary uses for tamoxifen:
In women and men diagnosed with estrogen receptor-positive early-stage breast cancer after surgery, chemotherapy, and/or radiation to reduce the risk of recurrence In women and men diagnosed with hormone-receptor-positive advanced stage or metastatic breast cancer For primary breast cancer prevention in those at an increased risk of developing the disease (a. k. a. cancer previvors)
Tamoxifen isn’t usually beneficial for those who have estrogen- or progesterone-negative breast cancer. If your cancer is estrogen receptor- as well as HER2-positive, your healthcare provider may recommend tamoxifen (or an aromatase inhibitor), a HER2-targeted medication such as Herceptin, or both, depending on your specific situation.
Because the majority of breast cancers in men are estrogen receptor-positive, hormonal therapy is recommended for most male breast cancers. Tamoxifen is the hormonal therapy of choice in early-stage breast cancer (unless the medication is not tolerated or should not be used for some reason) per 2020 guidelines. This is in contrast to women, who have the option of tamoxifen or an aromatase inhibitor (for postmenopausal women or premenopausal women also treated with ovarian suppression therapy).
Benefits
Tamoxifen was approved in 1998 and has since been found effective in treating breast cancer for millions of people. It is still considered an excellent option if you are premenopausal or are postmenopausal and cannot take an aromatase inhibitor.
When used after primary treatment (e.g., surgery), it may reduce your risk of breast cancer recurrence by half if your tumor was estrogen receptor-positive. The drug continues to have this benefit even after you stop taking it.
It can also reduce your risk of developing another cancer in the same breast or new cancer in your other breast by up to 50%.
In contrast to its anti-estrogen effects on breast tissue, tamoxifen has estrogen-like effects on bone. Therefore tamoxifen (as well as Evista) may help slow or stop bone loss. Tamoxifen may also lower cholesterol levels, specifically LDL cholesterol.
Reduction of Late Recurrence
While many people associate surviving 5 years after breast cancer with a cure, this is far from true. In fact, for people who have estrogen-receptor-positive tumors, the cancer is more likely to recur after 5 years than in the first 5 years.
This is true even with very small, node-negative tumors. Fortunately, while chemotherapy does not appear to reduce the risk of late recurrences, tamoxifen does, and a 2019 study found that tamoxifen reduces the risk of recurrence up to 15 years after the initial diagnosis.
How It Works
Estrogen receptor-positive breast cancer cells are fed by estrogen. Estrogen in the body binds to proteins on the surface of these cells (estrogen receptors) to signal the cell to divide and grow. Tamoxifen binds to this receptor, essentially starving the cancer cells.
Tamoxifen is broken down into a metabolite called endoxifen by the cytochrome P450 enzyme CYP2D6. (There are also other enzymes such as CYP3A4, but CYP2D6 is likely most significant.) Endoxifen is 30 to 100 times more effective in suppressing estrogen-related cell growth than tamoxifen and is the principal compound responsible for its effects. You may hear tamoxifen referred to as a “pro-drug” for this reason.
Anything that reduces the activity of CYP2D6 can result in a reduced amount of the active metabolite. Reduced activity of the enzyme may occur if you are taking other medications or if you have particular genetic variations that make the enzyme less active.
The Impact of Your Genetics
There is a spectrum of activity of the CYP2D6 enzyme, and this results in differences in the metabolism of drugs. A review of studies found that extensive metabolizers appear to have better outcomes than poor metabolizers.
There are commercial genotyping kit tests available for the genotyping of CYP2D6, but this testing is not ordinarily done for women on tamoxifen therapy for estrogen receptor-positive breast cancer. In other words, you probably won’t know if this issue applies to you.
This is an area of controversy as well, and some researchers believe that the presence of another enzyme, CYP3A4*22, might compensate for the reduction of endoxifen concentrations related to low CYP2D6 activity.
A 2019 study published in the Journal of Clinical Oncology may be reassuring to those who wonder if they are poor metabolizers. In this study, the researchers found no association between CYP2D6 genotypes or endoxifen concentrations and the clinical outcomes of people with early-stage breast cancer who were treated with tamoxifen.
Tamoxifen vs. Aromatase Inhibitors
Aromatase inhibitors are also used to prevent breast cancer recurrence. Drugs in this class include:
Aromasin (exemestane) Arimidex (anastrozole) Femara (letrozole)
There are several differences between these drugs and tamoxifen, which dictate who can benefit from them and what risks they carry.
Effectiveness in Premenopausal Women
Since aromatase inhibitors do not address estrogen produced by the ovaries, they aren’t as effective before menopause unless a woman is also receiving ovarian suppression therapy. This, however, is not the case with tamoxifen.
Risk of Recurrence
For women who are postmenopausal or those who are premenopausal and have received ovarian suppression therapy, an aromatase inhibitor may offer greater benefits in reducing the risk of recurrence. This is one reason why oncologists may recommend switching to an aromatase inhibitor when you reach menopause or treating you with ovarian suppression therapy to induce menopause.
Bone Loss
Aromatase inhibitors also cause menopausal symptoms but can accelerate bone loss rather than reduce it like tamoxifen. Bone and joint pain can occur with either class of drugs but are much more common with aromatase inhibitors.
Cost
With respect to cost, tamoxifen therapy is usually much less expensive than any of the aromatase inhibitors.
Side Effects
Many of the common side effects of tamoxifen are essentially the same as those that occur during menopause, when there is a reduced amount of estrogen in the body.
Common side effects of tamoxifen include:
Hot flashesNight sweatsVaginal drynessVaginal dischargeReduced libido
Hot flashes are actually linked with better survival from breast cancer.
Risks
Tamoxifen’s actions on endometrial tissue can raise the risk of uterine cancer. The risk is highest for postmenopausal women, but it is still rare. Premenopausal women treated with tamoxifen have no known increased risk of uterine cancer and require no additional monitoring beyond routine gynecologic care.
Tamoxifen can also increase your risk of developing blood clots either in your legs (deep vein thrombosis) or lungs (pulmonary embolism).
Interactions and Contraindications
As with many medications, there are situations in which tamoxifen should not be used or where caution is needed.
Abnormal vaginal bleedingPain in your pelvisLeg pain and/or swellingChest painShortness of breathWeakness, numbness, or tinglingVision problems
Due to how it is metabolized, tamoxifen may interact with both some common prescription and over-the-counter medications. Talk to your healthcare provider about any other medications you use and make sure your pharmacist is aware as well. In particular, several antidepressants, as well as over-the-counter allergy medications, may reduce the effectiveness of tamoxifen.
Due to a relatively high rate of birth defects, tamoxifen should not be used in pregnancy, and the drug should be stopped at least two months before trying to get pregnant.
How Long You Should Take It
Based on clear evidence from two large randomized phase III clinical studies (ATLAS and aTTom), a 10-year rather than a 5-year adjuvant treatment with tamoxifen is associated with a smaller risk of recurrence and a reduction in breast cancer mortality.
This reduction in breast cancer recurrence must be weighed against potential side effects for each person. For example, if your cancer has a relatively high risk of recurrence (such as if lymph nodes are positive), the benefit of longer treatment may clearly outweigh the risk. In contrast, if your tumor has a very low risk of recurrence, the potential adverse effects of tamoxifen (such as blood clots) may outweigh the potential benefit.
For men with early-stage breast cancer, tamoxifen is recommended for 5 years, with the option of continuing the medication for another 5 years for those at high risk of recurrence.
Summary
Tamoxifen is an important medication that is used to reduce the risk of breast cancer recurrence in women who have estrogen receptor positive breast cancer. Although it does have the possibility of causing side effects such as hot flashes, or more seriously, blood clots, there can be significant benefit to reduce breast cancer risk.
A Word From Verywell
Tamoxifen can be a lifesaver for some women with breast cancer. As with all medications, however, there are potential risks and side effects. Working closely with your healthcare provider while you are on this drug will ensure you catch any problems that may arise.
