Risks
Medicines used for estrogen blocker therapy carry the risk of side effects. Side effects can affect daily life, sexual health and the ability to continue treatment. But many symptoms can be managed.
Estrogen blocker therapy side effects
Side effects of estrogen blocker therapy depend on the medicine you receive. Here are some examples of common side effects and less common but serious side effects.
Common side effects include:
- Joint or muscle pain.
- Hot flashes and night sweats.
- Vaginal discharge or dryness.
- Fatigue.
- Changes in menstrual periods before menopause.
- Sexual health concerns.
Depending on the medicine, less common side effects that can be serious include:
- Blood clots in veins.
- Cataracts.
- Endometrial cancer or uterine cancer.
- Heart disease or stroke.
- Thinning bones.
Your care team can explain which side effects and risks are associated with the medicine you take.
Managing side effects
Tell your care team if side effects get in the way of daily life or make it difficult to continue treatment. There may be ways to manage side effects so that you are more comfortable.
Do not stop estrogen blocker therapy without first talking with your care team. Your care team can help you manage side effects, switch you to another medicine or adjust your treatment plan. Many people are able to continue treatment with the help of their care team.
What you can expect
There are several approaches to estrogen blocker therapy for breast cancer. The medicines and treatment schedule depend on several factors.
These factors include breast cancer stage, whether you've gone through menopause and any treatments you have already had. Other factors include the risk that breast cancer will come back and certain features of the cancer cells, called tumor biomarkers.
During treatment, your care team monitors you for side effects and talks with you about any new symptoms. Routine imaging tests generally aren't needed unless symptoms suggest that the cancer has come back or spread to other parts of the body.
Medicines that block hormones from attaching to cancer cells
One approach is to stop the hormones from attaching to the receptors inside the cancer cells. When the hormones can't access the cancer cells, the cancer's growth may slow and the cells may die.
Medicines that block estrogen receptors are called selective estrogen receptor modulators (SERMs). SERMs stop estrogen from attaching to breast cancer cells. These medicines include:
- Tamoxifen. Tamoxifen is usually taken daily in pill form. It's often used to lower the risk of cancer recurrence after treatment for early-stage breast cancer. It's typically taken for 5 to 10 years. Tamoxifen also may be used to treat advanced cancer. In women, tamoxifen is an option for those who have been through menopause and those who haven't. In men, tamoxifen is usually the first medicine considered for estrogen blocker therapy.
- Toremifene. Toremifene (Fareston) is taken as a daily pill. It's used to treat breast cancer that has spread to other areas of the body. In women, it's approved for those who have been through menopause.
Other medicines called selective estrogen receptor degraders (SERDs) block estrogen receptors in breast cancer cells and cause the cells to break down those receptors over time. Options include:
- Medicine you receive by injection. Fulvestrant (Faslodex) is given as a shot every month after first getting a dose every two weeks for the first month. It's used to treat breast cancer that has spread to other parts of the body. In women, it's only used in those who have been through menopause.
- Medicine you take in pill form. Elacestrant (Orserdu) and imlunestrant (Inluriyo) are pills that you take by mouth. These medicines work in cancers that have specific changes in the estrogen receptor 1 (ESR1) gene. Testing may be done to look for these changes before your care team would recommend this medicine.
Another type of medicine called vepdegestrant (Veppanu) also is used to treat advanced cancers with changes in the ESR1 gene. The medicine is a pill that you take once a day. It works by targeting estrogen receptors in cancer cells and causing the receptors to break down.
Medicines that stop the body from making estrogen
Aromatase inhibitors are medicines that reduce the amount of estrogen in the body. This medicine deprives breast cancer cells of the hormones they need to grow.
In women, aromatase inhibitors are only used in those who have gone through menopause. They cannot be used unless your body is in natural menopause or in menopause induced by medicines or removal of the ovaries. In men, aromatase inhibitors are typically used with other medicines to better block hormones in the body.
Aromatase inhibitors used to treat breast cancer include:
- Anastrozole (Arimidex).
- Exemestane (Aromasin).
- Letrozole (Femara).
These medicines can reduce the risk of cancer recurrence in those who have been treated for early-stage breast cancer. Aromatase inhibitors also can be used to treat advanced breast cancer. And they may be an option for people who no longer benefit from tamoxifen treatment or whose treatment is completed.
Aromatase inhibitors are pills that you take once a day. All three aromatase inhibitors work the same way and reduce the production of estrogen in the body.
Aromatase inhibitors can lower bone density and increase the risk of fractures. Some studies suggest aromatase inhibitors also may increase the risk of some heart and blood vessel conditions.
Your care team may recommend a bone density test and ways to help protect your bones. These steps may include regular physical activity, calcium, vitamin D or medicines, such as bisphosphonates or denosumab, to help protect bone health.
Treatments to stop ovarian function in premenopausal women
Women who haven't gone through menopause, either naturally or as a result of cancer treatment, can have treatment to stop their ovaries from producing hormones.
Options to stop ovarian function include:
- Medicines such as goserelin or leuprolide. Their effects generally are temporary.
- Surgery to remove the ovaries, called oophorectomy.
- Radiation therapy aimed at the ovaries.
Treatments to stop ovarian function may allow those who haven't been through menopause to take medicines only available to those who have been through menopause.
Surgery and radiation therapy stop ovarian function permanently. Your care team can help you decide whether one of these treatments is right for you. The decision depends on your age, treatment goals, the risk that the cancer may return and your plans for future fertility.
Combining targeted therapy with estrogen blocker therapy
Estrogen blocker therapy sometimes is combined with targeted therapy. Targeted medicines act on specific features that help cancer cells grow. Adding targeted therapy can make estrogen blocker therapy more effective for some people.
Targeted medicines used with estrogen blocker therapy include:
- CDK4/6 inhibitors. CDK4/6 inhibitors are medicines that help slow or stop breast cancer cells from growing and dividing. They may be used for some early-stage breast cancers or metastatic breast cancers. Examples include abemaciclib (Verzenio), palbociclib (Ibrance) and ribociclib (Kisqali).
- Medicines that target a cell signaling pathway. The PIK3CA/AKT/mTOR pathway is a chain of signals inside cancer cells. Medicines that block parts of that signal chain can help slow or stop cancer cells from growing. These medicines may be options for people with metastatic breast cancer. Biomarker testing may help determine whether certain medicines, such as alpelisib (Piqray) or capivasertib (Truqap), are treatment options. Other medicines that target this pathway include everolimus (Afinitor) and gedatolisib (Revtorpyk).
Biomarker testing can help your care team decide which targeted medicines are most likely to work. Biomarkers may include changes in cancer-related genes or proteins, such as ESR1 or PIK3CA.
How long treatment lasts
Estrogen blocker therapy given after surgery, called adjuvant therapy, usually continues for at least five years. Some people may benefit from treatment lasting 7 to 10 years.
Longer treatment may lower the risk of breast cancer coming back or developing a new cancer in the other breast. But longer treatment also means living with possible side effects for a longer time. The added benefit may be small for people with a lower risk that breast cancer will come back.
When deciding how long treatment should be, your care team considers these factors:
- Whether cancer is in any lymph nodes.
- Features of your breast cancer.
- Whether you've gone through menopause.
- Treatments you've already had.
- How well you tolerate the medicine.
Some treatment plans include switching from one estrogen blocker medicine to another. For example, you may take tamoxifen for part of your treatment and later switch to an aromatase inhibitor. The order and timing depend on your menopausal status, risk that the cancer might return, side effects and how well you tolerate treatment.
For some early-stage breast cancers, a genomic test called a recurrence-risk assay may be done. This test looks at the activity of certain genes in the cancer cells. The results can help your care team estimate how likely it is that cancer will come back. Test results also can help guide treatment decisions.
Results
You'll meet with your cancer doctor, called an oncologist, regularly for follow-up visits while you're taking estrogen blocker therapy for breast cancer. Your oncologist will ask about any side effects you're experiencing. Many side effects can be controlled.
Estrogen blocker therapy can help lower the risk that hormone receptor positive breast cancer will come back after surgery. It also may lower the risk of a new breast cancer in the other breast. For advanced or metastatic breast cancer, it can help slow or stop cancer growth.
To get the most benefit, take the medicine for as long as your care team recommends. Stopping early or missing doses may increase the risk that early-stage breast cancer will come back and may affect survival.
Depending on your situation, your care team may recommend exams or tests during follow-up visits. If you've been treated for early-stage breast cancer, follow-up focuses on checking for signs or symptoms that the cancer may have returned. If you have advanced breast cancer, tests may be used to see how your cancer is changing during treatment. Your care team may use this information to decide whether your treatment plan needs to change.
How much the treatment helps varies from person to person. Your care team can explain what benefit you may expect from estrogen blocker therapy based on your cancer and overall health.
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