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Aromatase Inhibitors



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Aromatase inhibitors and breast cancer treatment  

Aromatase inhibitors lower estrogen levels in the body by blocking aromatase, an enzyme that converts other hormones into estrogen.  

Like tamoxifen, aromatase inhibitors are used to treat metastatic and early breast cancers that are hormone-receptor positive. Aromatase inhibitors include:

  • Anastrozole (Arimidex)
  • Exemestane (Aromasin)
  • Letrozole (Femara)

Unlike tamoxifen, aromatase inhibitors are only used to treat postmenopausal women.  

Postmenopausal women with hormone receptor-positive breast cancer either begin hormone therapy with an aromatase inhibitor or begin with tamoxifen and after a few years, switch to an aromatase inhibitor.  

Aromatase inhibitors come in a pill form and are taken every day. When aromatase inhibitors are the only hormone therapy given, they are taken for five years. When women switch to an aromatase inhibitor after taking tamoxifen for a few years, they take these drugs for a combined total of at least five years. The number of years that aromatase inhibitors should be taken to get the most treatment benefit is still under study.

Benefits of aromatase inhibitors

Treatment with an aromatase inhibitor (alone or after tamoxifen) lowers the risk of [61]:

  • Breast cancer recurrence 
  • Breast cancer in the opposite breast
  • Death from breast cancer

Among postmenopausal women with hormone receptor-positive breast cancer, aromatase inhibitors (alone or after tamoxifen) offer the same or slightly better treatment benefit compared to treatment with tamoxifen alone [61,69-72]. The aromatase inhibitors anastrozole, exemestane and letrozole all have a similar treatment benefit [61,69-72]. They also have similar side effects. However, you may tolerate one drug better than another.


For a summary of research studies on aromatase inhibitors and early breast cancer, visit the Breast Cancer Research section.


Side effects of aromatase inhibitors  

The side effects of aromatase inhibitors differ from those of tamoxifen. Learn more about the side effects of aromatase inhibitors.

Issues still under study  

At present, we still do not know how long women should take aromatase inhibitors. There are no data on the benefits or safety of taking aromatase inhibitors for longer than five years.  

Another question is whether aromatase inhibitors benefit women who were premenopausal when diagnosed with breast cancer, but became menopausal due to treatment. New and ongoing studies should help answer such questions. Because aromatase inhibitors only work in postmenopausal women, tamoxifen should be used if there is any chance you are still premenopausal.  

Aromatase inhibitors are being studied in the chemoprevention (risk-lowering drugs) setting as well. Findings from a recent randomized controlled trial showed the aromatase inhibitor exemestane lowered the risk of developing breast cancer in postmenopausal women at high risk [73]. Exemestane does not yet have FDA-approval for use in the chemoprevention setting. Learn more about chemoprevention for breast cancer.  

To learn more about aromatase inhibitors, visit the National Institutes of Health's Medline Plus website.

Importance of following your breast cancer treatment plan

The importance of adherence (compliance)

Breast cancer treatment is most effective when all parts of the treatment plan are followed. Adherence (also called compliance) is how closely people follow the treatment plan (for medications and other therapies) prescribed by their health care providers in terms of:

  • Timing
  • Dose
  • Frequency

Completing hormone therapy with aromatase inhibitors 

Hormone therapy used to treat breast cancer is typically prescribed for at least five years. The length of treatment coupled with side effects can make adherence to hormone therapy difficult. Although the menopausal symptoms related to hormone therapy can be hard to deal with, there are treatments that may ease these side effects. If you have side effects, talk to your health care provider about ways to treat them.  

To get the most benefit out of hormone therapy, you need to take the full course of treatment. Women who complete the full course have higher rates of survival [63].  

If you have trouble remembering to take your hormone therapy, a daily pillbox or setting an alarm on your watch or phone may help [3]. However, you do not need to panic if you miss a day or two.   

Learn more about adherence.  


For more information on aromatase inhibitors, visit the National Comprehensive Cancer Network (NCCN) or the American Society for Clinical Oncology (ASCO).

Updated 03/28/14

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