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Aromatase inhibitors

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The aromatase inhibitors, or AIs, are a class of medicines that reduce the risk of breast cancer returning in postmenopausal women with hormone receptor-positive, early-stage breast cancer. They are also a standard treatment for metastatic disease.

There are three FDA approved aromatase inhibitors for all stages of breast cancer:

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How aromatase inhibitors work

After menopause, your body stops making estradiol, the main estrogen produced by the ovaries. But your body still makes estrone, an estrogen created by converting a male sex hormone found in the adrenal glands, fat cells, and muscle. The enzyme aromatase converts that hormone into estrogen. Fat cells and other body parts also do so as well.

Aromatase inhibitors interfere with the enzyme aromatase to decrease the female hormones in your body. It then can reduce the risk of breast cancer returning, or control the growth of metastatic disease.

Anastrozole, exemestane, and letrozole work similarly to one another. Talk with your doctors about why they recommend one AI over another.

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Who gets aromatase inhibitors

All three aromatase inhibitors are given to postmenopausal women with hormone receptor-positive breast cancer. Any of the AIs may be the first or primary hormonal therapy your doctor prescribes if you have early-stage disease because aromatase inhibitors are the standard hormonal therapy for postmenopausal women.

If you have metastatic breast cancer, you may receive any of the three aromatase inhibitors as a first treatment after your stage IV diagnosis. Your doctor may also recommend:

  • anastrozole if the cancer grows despite treatment with tamoxifen
  • exemestane if the cancer grows despite treatment with tamoxifen or another aromatase inhibitor
  • letrozole if the cancer grows despite treatment with other hormonal therapies
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How aromatase inhibitors are given

The AIs are all given as daily pills. In early-stage disease they are usually given after surgery, radiation or chemotherapy. They are FDA approved to be given for 5 years after surgery, or for 5 years after up to 5 years of tamoxifen for a total of 10 years of hormonal therapy.

In 2014, findings from a clinical trial suggested premenopausal women who use LHRH agonists to temporarily stop the function of their ovaries may be able to safely take exemestane. Exemestane is not yet FDA approved for this purpose but your doctor may discuss this option with you if you have not yet gone through menopause.

In metastatic breast cancer, any of the AIs may be the first treatment given if you are postmenopausal and have hormone-positive disease. They may be paired with other treatments or given alone. You may take one medicine for as long as it works at controlling the cancer. Your doctor may then recommend you try a different medicine.

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Side effects & things to remember


With hormonal therapies, we don’t know why some women experience difficult side effects when others, taking the same medicine, have mild or no problems. Always talk with your providers about what you’re feeling.

If side effects from a certain aromatase inhibitor become too difficult or interfere with your ability to function, your doctor might suggest another AI for you. Even though the AIs work in similar ways, a different aromatase inhibitor may not cause the same side effects for you. It’s likely you can switch to another medicine and see if that helps. Consider a second opinion if your doctor won’t discuss your concerns with you. Never stop treatment on your own without speaking with your providers first.

Possible side effects of the AIs are:

  • stiffness, and joint and bone pain
  • bone thinning and fractures
  • high blood pressure
  • higher cholesterol
  • fatigue and insomnia
  • nausea and vomiting
  • hot flashes
  • weakness
  • headache
  • vaginal dryness
  • loss of appetite
  • weight gain
  • mood changes
  • constipation or diarrhea
  • dry skin
  • hair thinning


You can also go to our section on side effects for more information.

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Doctors should inform us about expected side effects and dedicate time to discuss mitigation strategies. With a clearer understanding of what to expect, more women might complete their prescribed course of AIs, because we know they have powerful anti-cancer effects. Don't hesitate to ask your doctor for guidance.

Diane Heditsian, patient advocate, Managing treatment side effects of anti-estrogen therapies

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Reviewed and updated: August 31, 2015

Reviewed by: Jennifer Winn, MD, MS

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