Many breast cancers are hormone-receptor positive, meaning that estrogen or progesterone helps the cancer grow. Hormone therapy, also called endocrine therapy, reduces or blocks this hormonal stimulation.
For appropriate patients, hormone therapy can substantially reduce the risk that breast cancer will return and can reduce the risk of dying from breast cancer.
The most commonly used treatments are tamoxifen and drugs called aromatase inhibitors, such as anastrozole, letrozole, and exemestane.
Treatment commonly lasts five years, but some women benefit from longer treatment. The best choice depends on the cancer's risk of recurrence, menopausal status, age, overall health, and the treatment's side effects.
The important question is not simply whether hormone therapy works, but how much benefit it is likely to provide for an individual woman and how that benefit compares with its side effects.
The two main types of hormone therapy for early breast cancer are tamoxifen and aromatase inhibitors (AIs). The choice depends partly on whether a woman has gone through menopause.
Tamoxifen blocks estrogen from attaching to receptors in breast cells. It can be used in both premenopausal and postmenopausal women. Important potential side effects include hot flashes and, less commonly, blood clots and cancer of the uterine lining.
Aromatase inhibitors—including anastrozole, letrozole, and exemestane—reduce the amount of estrogen the body produces after menopause. They are often preferred for postmenopausal women because, on average, they reduce breast-cancer recurrence somewhat more than tamoxifen.
Aromatase inhibitors have different drawbacks. They can cause joint and muscle pain and loss of bone density, increasing the risk of osteoporosis and fractures.
Premenopausal women cannot generally use an aromatase inhibitor alone because their ovaries continue to produce estrogen. For women at higher risk, an aromatase inhibitor may sometimes be combined with ovarian suppression.
There is no universally best choice. The decision should consider the expected reduction in recurrence, menopausal status, bone health, risk of blood clots, side effects, and personal preferences.
Hormone therapy can substantially reduce the risk of recurrence for women with hormone-receptor-positive breast cancer. But the size of the benefit varies considerably from one woman to another.
It is important to distinguish between relative risk reduction and absolute benefit. For example, cutting a 20% risk of recurrence to 10% is a large absolute benefit—10 percentage points. Cutting a 4% risk to 2% is the same 50% relative reduction, but the absolute benefit is only 2 percentage points.
A woman's likely benefit depends on factors such as tumor size and grade, lymph-node involvement, age, genomic test results, and other characteristics of the cancer.
This is especially important when weighing treatment against side effects. Rather than asking only “Does hormone therapy reduce my risk?”, a useful question is:
“What is my estimated risk of recurrence with hormone therapy and without it?”
Having both numbers can make the potential benefit much easier to understand.
For many women with hormone-receptor-positive early breast cancer, five years of hormone therapy provides substantial protection against recurrence. But these cancers can sometimes recur many years after the original diagnosis.
Major randomized trials have examined longer treatment. In the ATLAS trial, continuing tamoxifen for 10 years rather than stopping at five further reduced breast-cancer recurrence and mortality in women with estrogen-receptor-positive breast cancer. In the MA.17R trial, extending treatment with the aromatase inhibitor letrozole improved disease-free survival and reduced new cancers in the opposite breast, but did not improve overall survival.
The additional benefit is usually smaller than the benefit from the first five years, and longer treatment also means longer exposure to side effects.
Women with positive lymph nodes or other higher-risk features are more likely to benefit from extended treatment. Women with low-risk, node-negative cancers may receive little additional benefit and may reasonably stop after five years.
Depending on the drug and individual risk, total treatment may last 7 to 10 years. Longer treatment is not automatically better.
A useful question at the five-year point is: “What is my estimated risk of recurrence if I stop now, and how much would continuing treatment reduce that risk?”
Hormone therapy can be very effective, but side effects sometimes make it difficult to continue treatment.
Aromatase inhibitors can cause joint and muscle pain and can accelerate bone loss, increasing the risk of osteoporosis and fractures. Monitoring bone density, resistance and weight-bearing exercise, adequate calcium and vitamin D, and sometimes medications to protect bone may be appropriate.
Tamoxifen commonly causes hot flashes and other menopausal symptoms. Less commonly, it increases the risk of blood clots and, particularly in postmenopausal women, endometrial cancer.
Both types of treatment can affect sexual health and quality of life, including vaginal dryness and discomfort.
Side effects should not simply be endured without discussion. Sometimes switching to another aromatase inhibitor, changing from an aromatase inhibitor to tamoxifen, or addressing individual symptoms can make treatment much more tolerable.
If side effects are significant, a useful discussion is: “How much benefit am I getting from this treatment, and what alternatives would preserve most of that benefit with fewer side effects?”
Some women consider stopping hormone therapy because of joint pain, hot flashes, sexual side effects, fatigue, bone loss, or other quality-of-life concerns.
Stopping treatment early can reduce its protection against breast-cancer recurrence, but that does not mean every woman faces the same risk. The consequences depend on factors including the original risk of recurrence, lymph-node involvement, how long treatment has already been taken, and the particular drug being used.
Before simply stopping, it is worth discussing alternatives. These may include switching to another aromatase inhibitor, changing from an aromatase inhibitor to tamoxifen, treating specific side effects, or reconsidering the planned duration of therapy.
The decision ultimately involves balancing two things that matter: reducing the risk of breast cancer returning and maintaining quality of life.
A particularly useful question is: “If I stop now, how much does my absolute risk of recurrence increase?”
How much does hormone therapy reduce my absolute risk of recurrence?
What is my estimated risk of recurrence with and without hormone therapy?
Why do you recommend tamoxifen or an aromatase inhibitor for me?
What side effects should I expect, and which ones should I report?
Should I have a bone-density test before or during treatment?
How long should I take hormone therapy—five years, seven years, or longer?
How much additional benefit would I receive by continuing beyond five years?
If side effects become difficult, could I switch to another drug?
If I stop treatment early, how much would my risk of recurrence increase?
Are there treatment alternatives that would reduce side effects while preserving most of the benefit?
Overview of tamoxifen, aromatase inhibitors, ovarian suppression, benefits, and common side effects.
A patient-friendly explanation of who receives hormone therapy, the main drugs used, treatment duration, and side effects.
A major randomized trial showing that continuing tamoxifen to 10 years further reduced breast-cancer recurrence and mortality in women with estrogen-receptor-positive disease, while also increasing some risks such as endometrial cancer and pulmonary embolism.
A randomized trial of extended letrozole after about five years of prior aromatase-inhibitor therapy. Extended treatment improved disease-free survival and reduced new cancers in the opposite breast, but did not improve overall survival and increased bone-related problems.
A companion analysis examining quality of life in women randomized to extended letrozole or placebo.