Aromatase Inhibitors for Lowering Breast Cancer Risk

For women with a higher than average risk of breast cancer who are considering taking medicine to lower their risk, drugs called aromatase inhibitors (AIs) may be an option instead of tamoxifen or raloxifene.

What are aromatase inhibitors?

Aromatase inhibitors lower estrogen levels by stopping an enzyme in fat tissue (called aromatase) from changing other hormones into estrogen. (Estrogen can fuel the growth of breast cancer cells.)

These drugs don’t stop the ovaries from making estrogen. They only lower estrogen levels in women whose ovaries aren’t making estrogen (such as women who have already gone through menopause). Because of this, they are used mainly in women who are past menopause.

The drugs in this class that have been shown in studies to lower breast cancer risk include:

  • Anastrozole (Arimidex)
  • Exemestane (Aromasin)

AIs are pills taken once a day.

Can aromatase inhibitors lower the risk of breast cancer?

AIs are used mainly to treat women with hormone receptor-positive breast cancer. But some studies of anastrozole and exemestane have also found that they can lower breast cancer risk in postmenopausal women who are at increased risk.

These drugs are not yet approved in the US to lower breast cancer risk. However, some expert groups include them as options (along with tamoxifen and raloxifene) to reduce breast cancer risk in postmenopausal women who are at increased risk. For example, they might be a reasonable option for women who have an increased risk of blood clots and therefore should not take tamoxifen or raloxifene.

When used to lower the risk of breast cancer, these drugs are typically taken daily for 5 years.

What are the risks and side effects of aromatase inhibitors?

The most common side effects of AIs are symptoms of menopause, such as hot flashes, night sweats, and vaginal dryness.

These drugs can also cause muscle and joint pain. This side effect can be serious enough to cause some women to stop taking the drugs.

Unlike tamoxifen and raloxifene, AIs tend to speed up bone thinning, which can lead to osteoporosis. People with osteoporosis are more likely to have broken bones.

AIs may raise cholesterol. Women with pre-existing coronary heart disease who take an AI may be at risk of having a heart problem.

The American Cancer Society medical and editorial content team

Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

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Last Medical Review: September 10, 2019 Last Revised: October 10, 2019

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