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Aromatase Inhibitors to Lower Breast Cancer Risk
Aromatase inhibitors are medicines used to lower breast cancer risk after menopause. For people who have gone through menopause with a higher-than-average risk of breast cancer, these medicines may be an option instead of tamoxifen or raloxifene.
What are aromatase inhibitors?
Aromatase inhibitors lower estrogen levels. They do this by stopping an enzyme in fat tissue from changing other hormones into estrogen. The enzyme is called aromatase. This lowers the risk of breast cancer by reducing estrogen, which 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. Because of this, they are used in people who have gone through menopause already.
The medications that have been shown to lower breast cancer risk include:
- Anastrozole (Arimidex)
- Exemestane (Aromasin)
Like tamoxifen, these medicines are more often used to treat certain breast cancers, such as hormone receptor-positive breast cancer and advanced or recurrent endometrial and ovarian cancers.
When used to lower breast cancer risk, these drugs are typically taken for 5 years. They are pills taken once a day.
Can aromatase inhibitors lower the risk of breast cancer?
In large studies, both anastrozole and exemestane have been shown to lower breast cancer risk in postmenopausal women who are at increased risk.
These drugs are not FDA approved 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 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.
What are the risks and side effects of aromatase inhibitors?
Aromatase inhibitors can cause several common side effects.
- Menopausal symptoms: These include hot flashes, night sweats, vaginal dryness, hair loss, and changes in sexual desire and function.
- Muscle and joint pain: This side effect can be serious enough to cause some people to stop taking the medicine.
- Lower bone density: Unlike tamoxifen and raloxifene, AIs tend to speed up bone thinning. This can lead to osteoporosis. People with osteoporosis are more likely to have broken bones. Because of this, doctors often recommend checking bone density before starting one of these drugs.
- High cholesterol: People with heart disease who take an AI may be at higher risk of having a heart problem.
- High blood pressure: High blood pressure is more common in people taking these medications.
More resources
Learn more about using medicines to lower breast cancer risk.
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- References
Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).
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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 Clinical Practice Guidelines in Oncology: Breast Cancer Risk Reduction Version 1.2026 – August 29, 2025. Accessed at https://www.nccn.org/professionals/physician_gls/pdf/breast_risk.pdf on March 18, 2026.
Nelson HD, Fu R, Zakher B, Pappas M, McDonagh M. Medication use for the risk reduction of primary breast cancer in women: Updated evidence report and systematic review for the US Preventive Services Task Force. JAMA. 2019;322(9):868-886.
Sharma P. Selective estrogen receptor modulators and aromatase inhibitors for breast cancer prevention. UpToDate. 2026. Accessed at https://www.uptodate.com/contents/selective-estrogen-receptor-modulators-and-aromatase-inhibitors-for-breast-cancer-prevention on March 30, 2026.
US Preventive Services Task Force, Owens DK, Davidson KW, Krist AH, et al. Medication use to reduce risk of breast cancer: US Preventive Services Task Force recommendation statement. JAMA. 2019;322(9):857-867.
Visvanathan K, Fabian CJ, Bantug E, et al. Use of endocrine therapy for breast cancer risk reduction: ASCO clinical practice guideline update. J Clin Oncol. 2019;37(33):3152-3165.
Last Revised: March 31, 2026
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