Hormone Therapy for Breast Cancer
Many breast cancers are hormone-receptor positive, meaning estrogen or progesterone helps the cancer grow. Hormone therapy, also called endocrine therapy, blocks or reduces this hormonal stimulation.
For appropriate patients, hormone therapy can substantially reduce the risk of recurrence and death from breast cancer.
The main treatments are tamoxifen and aromatase inhibitors, including anastrozole, letrozole, and exemestane. Treatment commonly lasts five years, although some women benefit from longer treatment.
The important question is not simply whether hormone therapy works, but how much benefit it is likely to provide for an individual woman compared with its side effects.
Tamoxifen or an Aromatase Inhibitor?
Tamoxifen blocks estrogen from attaching to receptors in breast cells. It can be used before or after menopause. Common side effects include hot flashes; less commonly, it increases the risk of blood clots and cancer of the uterine lining.
Aromatase inhibitors (AIs) reduce estrogen production after menopause. They are often preferred for postmenopausal women because, on average, they reduce recurrence somewhat more than tamoxifen.
AIs can cause joint and muscle pain and loss of bone density, increasing the risk of osteoporosis and fractures.
Premenopausal women generally cannot use an AI alone because their ovaries continue producing estrogen. For women at higher risk, an AI may be combined with ovarian suppression.
The choice depends on menopausal status, recurrence risk, bone health, blood-clot risk, side effects, and personal preferences.
How Much Does Hormone Therapy Help?
The benefit varies considerably from one woman to another.
It is useful to distinguish relative risk reduction from absolute benefit. Cutting a 20% recurrence risk to 10% is a 10-percentage-point benefit. Cutting a 4% risk to 2% is the same 50% relative reduction, but only a 2-percentage-point benefit.
A woman's likely benefit depends on tumor size and grade, lymph-node involvement, age, genomic test results, and other characteristics.
A particularly useful question is:
“What is my estimated risk of recurrence with hormone therapy and without it?”
Five Years—or Longer?
For many women, five years of hormone therapy provides substantial protection. But hormone-receptor-positive breast cancers can sometimes recur many years later.
In the ATLAS trial, continuing tamoxifen for 10 rather than five years further reduced recurrence and breast-cancer mortality.
In the MA.17R trial, extending aromatase-inhibitor treatment improved disease-free survival and reduced new cancers in the opposite breast but did not improve overall survival.
Extended treatment also means longer exposure to side effects. Women with positive lymph nodes or other higher-risk features may be more likely to benefit than women with low-risk cancers.
Longer treatment is therefore not automatically better.
At five years, a useful question is:
“If I stop now, what is my risk of recurrence, and how much would continuing treatment reduce it?”
Managing Side Effects
Side effects sometimes make hormone therapy difficult to continue.
Aromatase inhibitors can cause joint and muscle pain and bone loss. Bone-density monitoring, weight-bearing and resistance exercise, adequate calcium and vitamin D, and sometimes bone-protecting medications may be appropriate.
Tamoxifen commonly causes hot flashes and other menopausal symptoms. Less commonly, it increases the risk of blood clots and endometrial cancer.
Both treatments can affect sexual health and quality of life.
Side effects should be discussed rather than simply endured. Switching drugs or treating particular symptoms can sometimes make therapy much more tolerable.
What If You Want to Stop?
Stopping hormone therapy early can reduce its protection against recurrence, but the effect is not the same for every woman.
Before stopping, possible alternatives include switching to another aromatase inhibitor, changing to tamoxifen, treating specific side effects, or reconsidering how long therapy is needed.
The decision involves balancing reducing recurrence risk with maintaining quality of life.
A useful question is:
“If I stop now, how much does my absolute risk of recurrence increase?”
Questions to Ask Your Doctor
What is my risk of recurrence with and without hormone therapy?
Why do you recommend this particular drug for me?
What side effects should I expect?
Should I have a bone-density test?
How long should I take hormone therapy?
How much additional benefit would I receive beyond five years?
If side effects become difficult, could I switch drugs?
If I stop early, how much would my recurrence risk increase?
Learn More
Overview of tamoxifen, aromatase inhibitors, ovarian suppression, benefits, and common side effects.
A patient-friendly explanation of who receives hormone therapy, the main drugs, treatment duration, and side effects.
A major randomized trial showing that continuing tamoxifen to 10 years further reduced breast-cancer recurrence and mortality, while increasing some risks.
A randomized trial in which extended letrozole 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 receiving extended letrozole or placebo.