How Does Tamoxifen Work for Breast Cancer?
Key takeaways:
Tamoxifen (Soltamox) is a hormone therapy medication that treats hormone-receptor positive breast cancers. It can also prevent breast cancer in certain high-risk individuals.
Tamoxifen works by blocking estrogen receptors on breast cancer cells. This prevents estrogen from binding to cancer cells and helping them grow. Since it only affects estrogen receptors, tamoxifen isn’t effective for breast cancers that aren’t affected by these receptors.
You’ll typically take tamoxifen for 5 to 10 years to prevent breast cancer from appearing or returning. And in some cases, you may need to take it along with other breast cancer treatments.
Table of contents
A breast cancer diagnosis can be scary, and the treatment options can be overwhelming to process. But having a good understanding of your treatment can help you know what to expect and feel more confident throughout the process.
Tamoxifen (Soltamox) is a medication that’s commonly used for hormone receptor-positive (HR+) breast cancer. Depending on your type of breast cancer, tamoxifen may be used alone or along with other treatments.
But how does tamoxifen work? And why can it only treat certain types of breast cancer? Let’s take a closer look.
How does tamoxifen work to treat breast cancer?
Even though tamoxifen treats breast cancer, it‘s not chemotherapy. Instead, tamoxifen is a type of hormone therapy.
Certain types of breast cancer are sensitive to hormones. They contain receptors (chemical binding sites) for hormones like estrogen or progesterone. This means that when a hormone binds to receptors on this type of breast cancer cell, it helps the cancer grow. Hormone therapy blocks this process, which slows down or stops the cancer from growing.
Tamoxifen is a type of hormone therapy called a selective estrogen receptor modulator (SERM). It binds to estrogen receptors on breast cancer cells. This blocks estrogen from binding to them and helping the cancer cells grow. So tamoxifen can both treat and prevent HR+ breast cancer by blocking cancer from growing in the body. It can also help shrink the cancer to make it easier to remove via surgery.
Good to know: Although tamoxifen blocks estrogen receptors in some cells, it can activate estrogen receptors in other cells. This includes cells in your bones and uterus. Activating estrogen receptors in bones can help prevent osteoporosis in postmenopausal women. But it can also raise the risk of uterine cancer.
Does tamoxifen treat all types of breast cancer?
No. Tamoxifen is only effective for breast cancer that tests positive for hormone receptors.
There are three main receptors that can appear on breast cancer cells:
Estrogen receptors
Progesterone receptors
Hormone epidermal growth factor receptor 2 (HER2) receptors
The type of breast cancer you have depends on what receptors it tests positive for. And some breast cancers may test positive for more than one type of receptor. If breast cancer tests positive for estrogen or progesterone receptors, it’s considered hormone positive. If a breast cancer cell tests negative for all receptors, it’s called triple-negative breast cancer.
Keep in mind that tamoxifen isn’t the only type of hormone therapy for hormone-positive breast cancer. Others include aromatase inhibitors, such as anastrozole (Arimidex), and selective estrogen receptors downregulators (SERDs), such as fulvestrant (Faslodex).
How effective is tamoxifen for breast cancer?
Tamoxifen is effective for treating and preventing hormone receptor-positive breast cancer. Keep in mind that the overall effectiveness of tamoxifen depends on many different factors, including the breast cancer’s stage and grade and what other treatments you’re receiving. Your provider can provide more detailed information about what to expect from your specific treatment.
With 5 years of treatment, tamoxifen lowered the risk of breast cancer returning by 47% in clinical studies. And tamoxifen may reduce the risk of breast cancer by up to 50% in those at high risk who take it as a preventative treatment.
That being said, aromatase inhibitors were shown to be more effective than tamoxifen in studies. So in some cases, your provider may prefer an aromatase inhibitor to tamoxifen. Or you may take both an aromatase inhibitor and tamoxifen, one after the other. Tamoxifen is also often prescribed along with other hormone-positive breast cancer treatments, including chemotherapy, to improve long-term outcomes.
How long does it take tamoxifen to work?
Tamoxifen works over time, so you won’t see results right away. Your healthcare provider may use imaging scans to see how your tumor is responding to treatment.
Most of the time, you’ll take tamoxifen for 5 to 10 years to prevent cancer from growing or coming back. So the main sign it’s working is that you remain cancer free or your cancer hasn’t grown. It’s best to stay on top of your appointments during this time so your provider can monitor your progress. And don’t stop taking tamoxifen unless your provider tells you to.
Is tamoxifen safe to take?
Tamoxifen is known to cause a number of side effects. But serious or dangerous side effects from tamoxifen are rare. Since it blocks estrogen, the most common tamoxifen side effects are similar to those seen during menopause, including:
Hot flashes
Vaginal discharge
Irregular periods
Skin changes
Mood changes
Other side effects can include eye problems, nausea, and swelling in the arms and legs. Talk to your provider if you’re experiencing side effects that are bothersome. They can provide options to help you manage them. Seek emergency care for any side effects that seem severe or life-threatening.
Tamoxifen also has a boxed warning for an increased risk of uterine or endometrial cancers, blood clots, or stroke. A boxed warning is the FDA’s most serious warning. But if you’ve been diagnosed with breast cancer, the benefits of tamoxifen outweigh these risks for most people. Your provider can help decide if tamoxifen is a safe treatment option for you based on your medical history.
But most experts agree that if you don’t have breast cancer, you shouldn’t take tamoxifen to prevent it if you:
Have had or are at high risk for a blood clot
Are pregnant or breastfeeding
Are taking estrogen for contraception or menopause treatment
Are taking an aromatase inhibitor
Are younger than age 35
The risks typically aren’t worth the benefits of tamoxifen to prevent breast cancer in these cases. But this decision will be between you and your provider.
The bottom line
Tamoxifen (Soltamox) treats hormone-receptor positive breast cancer. It’s a hormone therapy medication that blocks estrogen from binding to receptors on breast cancer cells. This helps stop breast cancer from growing. Tamoxifen can also help prevent breast cancer if you’re at high risk for it. But it’s only effective for hormone-receptor positive breast cancer.
You may take tamoxifen on its own or in combination with other breast cancer treatments. You’ll typically take it for 5 to 10 years. Since it blocks estrogen, tamoxifen side effects are similar to those seen during menopause. This includes hot flashes and irregular periods.
Tamoxifen may increase your risk of uterine cancer and blood clots. The benefits outweigh these risks for most people with breast cancer. But tamoxifen may not be the best choice to prevent breast cancer if you’re already at a high risk of these complications. Your healthcare provider can decide if tamoxifen is a safe treatment option for you.
Why trust our experts?
References
American Cancer Society. (2021). Deciding whether to use medicine to reduce breast cancer risk.
American Cancer Society. (2021). Tamoxifen and raloxifene for lowering breast cancer risk.
American Cancer Society. (2022). Treatment of breast cancer stages I-III.
Breastcancer.org. (2023). Breast cancer hormone receptor status.
Farrar, M. C., et al. (2023). Tamoxifen. StatPearls.
Howell, A., et al. (2023). Tamoxifen evolution. British Journal of Cancer.
McKesson Corporation dba SKY Packaging. (2023). Tamoxifen citrate [package insert].
Meiser, B., et al. (2017). Motivators and barriers of tamoxifen use as risk-reducing medication amongst women at increased breast cancer risk: A systematic literature review. Hereditary Cancer in Clinical Practice.
National Cancer Institute. (2022). Hormone therapy for breast cancer.
National Comprehensive Cancer Network. (2023). Invasive breast cancer.
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