Aromatase Inhibitors for Lowering Breast Cancer Risk

Aromatase inhibitors (AIs) may someday prove to be as good as or even better than tamoxifen or raloxifene in reducing breast cancer risk, but they haven’t been studied as much for this use. More research is needed to see how effective they are, who would most benefit from them, and how long treatment should be continued.

What are aromatase inhibitors?

Aromatase inhibitors lower estrogen levels by stopping an enzyme in fat tissue (called aromatase) from changing other hormones into estrogen. 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 include:

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

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 large 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 medical organizations include them as options along with tamoxifen and raloxifene to reduce breast cancer risk in women who are past menopause. 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 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.

Cuzick J, Sestak I, Forbes JF, et al. Anastrozole for prevention of breast cancer in high-risk postmenopausal women (IBIS-II): An international, double-blind, randomised placebo-controlled trial. Lancet. 2014;383:1041-1048.

Goss PE, Ingle JN, Alés-Martínez JE, et al. Exemestane for breast-cancer prevention in postmenopausal women. N Engl J Med. 2011;364(25):2381−2391.

National Comprehensive Cancer Network (NCCN). Practice Guidelines in Oncology: Breast Cancer Risk Reduction. V.1.2017. Accessed at on August 9, 2017.

Visvanathan K, Hurley P, Bantug E, et al. Use of pharmacologic interventions for breast cancer risk reduction: American Society of Clinical Oncology clinical practice guideline. J Clin Oncol. 2013;31:2942-2962.


Last Medical Review: September 6, 2017 Last Revised: September 6, 2017

American Cancer Society medical information is copyrighted material. For reprint requests, please see our Content Usage Policy.