Oncologist-approved cancer information from the American Society of Clinical Oncology
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Hormonal Therapy for Hormone Receptor-Positive Breast Cancer

To help doctors give their patients the best possible care, the American Society of Clinical Oncology (ASCO) developed evidence-based recommendations for treatment with hormonal therapy for early-stage breast cancer. In 2002, ASCO developed a clinical practice guideline about the use of aromatase inhibitors for early-stage breast cancer. Since then, this guideline was updated several times based on clinical trials that included aromatase inhibitors for early-stage breast cancer. In 2010, this guideline was further updated to provide guidance on the use of tamoxifen (Nolvadex) and aromatase inhibitors. This guide for patients is based on the most recent ASCO recommendations.

Key Messages:

  • Adjuvant hormonal therapy is offered to patients with hormone receptor-positive breast cancer after surgery and/or radiation therapy or chemotherapy to lower the risk of the cancer returning.
  • Hormonal therapy options for postmenopausal women (those who have gone through menopause) with hormone receptor-positive breast cancer include aromatase inhibitors and tamoxifen.
  • These drugs have different side effects. Women should speak with their doctors about hormonal therapy options, including how long therapy will last, the risks and benefits of the available drugs, and the cost of hormonal therapy.


Hormonal therapy, also called endocrine therapy, for hormone receptor-positive breast cancer is an adjuvant therapy, which is treatment given after surgery, chemotherapy, and/or radiation therapy. The goal of adjuvant therapy is to lower the risk of recurrence (cancer that comes back after treatment). Hormone receptor-positive breast cancer means that a patient's breast cancer depends on hormones called estrogen and/or progesterone to grow. About 75% to 80% of breast cancers are hormone receptor-positive. Common hormonal therapy for hormone receptor-positive breast cancer includes:

  • Aromatase inhibitors (AIs), which reduce the amount of estrogen in a woman's body by stopping tissues and organs other than the ovaries from producing estrogen. In women who have gone through menopause, AIs can further reduce estrogen levels by more than 90%. AIs are not used for premenopausal women (those who have not gone fully into menopause). These drugs are known as anastrozole (Arimidex), exemestane (Aromasin), and letrozole (Femara). All work the same way even though they are three different brands, and research suggests that they are equally effective and have similar side effects.
  • Tamoxifen, which blocks the effects of estrogen on tumor growth and has been proven to lower the risk of breast cancer recurrence and improve survival for women with early-stage breast cancer. Tamoxifen has been studied for over 30 years, and the benefits and side effects are well known. It is effective for women who have gone through menopause and those who have not.

Both AIs and tamoxifen have specific side effects, but neither appears to have fewer or more side effects. The side effects of AIs may include increased cholesterol, heart disease, weakening bones and bone breaks, and bone and joint pain or stiffness. The side effects of tamoxifen include blood clots, increased risk of uterine cancer, and hot flashes.


Women with hormone receptor-positive breast cancer who have gone through menopause at the time of diagnosis have three options for adjuvant hormonal therapy:

  • They can begin treatment with either an AI or tamoxifen for five years.
  • They can begin treatment with tamoxifen for two to three years and then switch to an AI for up to five years.
  • Some women may switch to an AI after five years of treatment with tamoxifen; this is called extended therapy.

Because AIs only work in postmenopausal women, it is important that a woman is actually postmenopausal before considering an AI. At this time, it is not known which treatment option is better. Women who cannot tolerate one of these options should talk with their doctors about switching treatments.

Women who might be advised to consider using an AI only:

  • Women who cannot take tamoxifen because of a high risk of side effects
  • Women who tried tamoxifen and stopped therapy because of side effects
  • Any woman who has gone through menopause and has hormone receptor-positive breast cancer who would prefer to take an AI instead of tamoxifen as the initial treatment

Women who might be advised NOT to consider using an AI:

  • Women who experience intolerable side effects while taking an AI, or who are concerned about the side effects of AI therapy.
  • Women who have not gone through menopause.
What This Means for Patients
Women with breast cancer should discuss options for hormonal treatment with their doctor. This conversation should cover the risks and benefits of different treatment options, including the likely side effects, how long treatment will last, and the costs of treatment. Each woman's specific medical circumstances should be carefully considered when discussing adjuvant treatment options. For some women, the risks and side effects of an AI may outweigh the potential benefits.
Questions to Ask the Doctor

For women who are considering adjuvant hormonal therapy:
  • Is my breast cancer hormone receptor-positive or hormone receptor-negative? What does this mean?
  • How many years of adjuvant hormonal therapy do I need?
  • Are there any reasons to believe I might not benefit from adjuvant hormonal therapy?
  • What are the benefits and risks of AIs?
  • What are the benefits and risks of tamoxifen?
  • How do the side effects of AIs compare with tamoxifen?

For women who were premenopausal when diagnosed

  • What is my menopausal status now, and how does this affect my options for adjuvant treatment?

For women who have completed two to three years of either tamoxifen or AI therapy:

  • Is there a reason I should switch to a different hormonal therapy?
  • How many more years of hormonal therapy do I need if I continue to take my current hormonal therapy? If I switch to a different hormonal therapy?
  • How will the side effects change if I switch to a different hormonal therapy?

For women who have completed five years of either tamoxifen or AI therapy:

  • Do you recommend further hormonal therapy? If so, with what drug and how long would I continue treatment?
  • If I start a different drug, what are the possible side effects?
Decision Aid
ASCO has created a Decision Aid to help women talk with their doctors about the benefits and risks of adjuvant hormonal therapy.
Helpful Links
Read the entire clinical practice guideline at www.asco.org/guidelines/endocrinebreast

Cancer.Net Guide to Breast Cancer

ASCO Expert Corner: Hormone Therapy Options for Early-Stage Breast Cancer

Menopause and Cancer Risk and Treatment

What to Know: The ASCO and CAP Guideline on Estrogen and Progesterone Receptor Testing for Breast Cancer

What to Know: ASCO's Guideline on Follow-Up Care for Breast Cancer

About ASCO's Guidelines

To help doctors give their patients the best possible care, ASCO asks its medical experts to develop evidence-based recommendations for specific areas of cancer care, called clinical practice guidelines. Due to the rapid flow of scientific information in oncology, new evidence may have emerged since the time a guideline or assessment was submitted for publication. As a result, guidelines and guideline summaries, like this one, may not reflect the most recent evidence. Because the treatment options for every patient are different, guidelines are voluntary and are not meant to replace your physician's independent judgment. The decisions you and your doctor make will be based on your individual circumstances. These recommendations may not apply in the context of clinical trials.

The information in this guide is not intended as medical or legal advice, or as a substitute for consultation with a physician or other licensed health care provider. Patients with health care-related questions should call or see their physician or other health care provider promptly and should not disregard professional medical advice, or delay seeking it, because of information encountered in this guide. The mention of any product, service, or treatment in this guide should not be construed as an ASCO endorsement. ASCO is not responsible for any injury or damage to persons or property arising out of or related to any use of this patient guide, or to any errors or omissions.


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