Aromatase inhibitors for breast cancer

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Aromatase inhibitors for breast cancer


Generic NameBrand Name

Aromatase inhibitors are available as tablets. Follow your doctor's orders or the directions on the label.

How It Works

Aromatase inhibitors interfere with how much estrogen the body's tissues can make. This limits the amount of estrogen available in the body.

An aromatase inhibitor cannot lower estrogen levels made by the ovaries. That is why an aromatase inhibitor only works after menopause, when a woman's ovaries have stopped making estrogen and other hormones.1

Why It Is Used

Aromatase inhibitors are used to treat early estrogen receptor-positive (ER+) breast cancer. They are also used to treat metastatic or recurrent ER+ breast cancer. An aromatase inhibitor can be used alone or after tamoxifen treatment.

Some doctors may use aromatase inhibitors "off-label" to treat infertility and endometriosis. This means that Health Canada has not approved this use.

How Well It Works

Several studies have shown that the use of aromatase inhibitors after menopause make ER+ breast cancer less likely to come back. Studies suggest that this is true for:

  • First-time localized breast cancer.2
  • New breast cancer that develops while tamoxifen is being taken.3
  • Metastatic breast cancer.

Aromatase inhibitors may be given to post-menopausal women who have breast cancer, either at the beginning of treatment or after they are given tamoxifen. Studies have shown that for post-menopausal women with ER+ breast cancer, aromatase inhibitors work slightly better than tamoxifen alone to keep the cancer from coming back.4

Endometriosis. In small studies, aromatase inhibitors have been shown to reduce both pain and the chance of endometriosis growths coming back. Aromatase inhibitors may help women who have endometriosis but who have not had relief with hormonal treatments. Aromatase inhibitors are used with a hormonal treatment (such as birth control hormones or progestin). Long-term use of aromatase inhibitors may cause bone loss. More research needs to be done before it is known how well this treatment works and what the side effects are.5

Side Effects

Possible side effects of an aromatase inhibitor include:

  • Hot flashes.
  • Muscle or body aches.
  • Mild nausea.
  • Diarrhea or constipation.
  • A general feeling of illness.
  • Weakness and fatigue.
  • Bone thinning. Aromatase inhibitors increase the rate of bone thinning that occurs normally in post-menopausal women.

See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

For breast cancer treatment, aromatase inhibitors should be given only under the supervision of a medical oncologist.

Further study is needed to determine how long to continue treatment with aromatase inhibitors. The follow-up time of recent studies is limited, and the benefits of aromatase inhibitors must be weighed against the potential for side effects.6

Complete the new medication information form (PDF) (What is a PDF document?) to help you understand this medication.



  1. Smith IE, Chua S (2006). ABC of breast diseases. Medical treatment of early breast cancer. I: Adjuvant treatment. BMJ, 332(7532): 34–37.
  2. ATAC Trialists' Group (2005). Results of the ATAC (Arimidex, Tamoxifen, Alone or in Combination) trial after completion of 5 years' adjuvant treatment for breast cancer. Lancet, 365(9453): 60–62.
  3. Abramowicz M (2005). Drugs for breast cancer. Treatment Guidelines From the Medical Letter, 3(29): 1–6.
  4. National Cancer Institute (2008). Breast Cancer PDQ: Treatment—Health Professional Version. Available online:
  5. Attar E, Bulun S (2006). Aromatase inhibitors: The next generation of therapeutics for endometriosis? Fertility and Sterility, 85(5): 1307–1318.
  6. Goss PE, et al. (2003). A randomized trial of letrozole in postmenopausal women after five years of tamoxifen therapy for early-stage breast cancer. New England Journal of Medicine, 349(19): 1793–1802.


By Healthwise Staff
Primary Medical Reviewer Joy Melnikow, MD, MPH - Family Medicine
Specialist Medical Reviewer Douglas A. Stewart, MD - Medical Oncology
Specialist Medical Reviewer Brian D. O'Brien, MD - Internal Medicine
Last Revised October 20, 2009

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