The Who, What, Where, When and Sometimes, Why.

Aromatase Inhibitors

Aromatase inhibitors are a hormone therapy (also called endocrine therapy). They are used to treat hormone receptor-positive early, locally advanced and metastatic breast cancers.

Learn about aromatase inhibitors and other hormone therapies for metastatic breast cancer.

How do aromatase inhibitors work?

Hormone receptor-positive breast cancers need estrogen (a female hormone) to grow.

Aromatase inhibitors lower estrogen levels in the body by blocking aromatase, an enzyme that converts other hormones into estrogen. This slows or stops the growth of the tumor by preventing the cancer cells from getting the hormones they need to grow.

Aromatase inhibitors include:

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

To learn about a specific aromatase inhibitor, visit the National Institutes of Health’s Medline Plus website.

Aromatase inhibitors and treatment for early and locally advanced breast cancers

For women with hormone receptor-positive breast cancer, treatment with an aromatase inhibitor (alone or after several years of tamoxifen) lowers the risk of [90]:

  • Breast cancer recurrence (a return of breast cancer)
  • 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 greater benefit compared to tamoxifen alone [90,108-111].

Anastrozole, exemestane and letrozole are equally effective and have similar side effects [90,108-111]. However, you may tolerate one drug better than another.

Let your health care provider know about any side effects you have.

Learn more about the side effects of aromatase inhibitors.

Learn about aromatase inhibitors and treatment for metastatic breast cancer.


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

Who can take aromatase inhibitors?

Aromatase inhibitors are used to treat breast cancer in women. In rare cases, they are used to treat breast cancer in men.

Menopausal status

In general, aromatase inhibitors are only used to treat breast cancer in postmenopausal women.

However, some premenopausal women may take an aromatase inhibitor when combined with ovarian suppression.

Ovarian suppression prevents the ovaries from making estrogen, so a woman becomes postmenopausal. Ovarian suppression is usually done with drug therapy, so menopause may be temporary.

How are aromatase inhibitors taken?

Aromatase inhibitors are pills. You take one pill every day.

Postmenopausal women with hormone receptor-positive breast cancer can:

  • Begin hormone therapy with an aromatase inhibitor 
  • Begin hormone therapy with tamoxifen and then after a few years, switch to an aromatase inhibitor

When an aromatase inhibitor is the only hormone therapy given, it’s taken for 5-10 years.

When an aromatase inhibitor is taken after tamoxifen, the drugs are taken for a combined total of 5-10 years.

Talk with your health care provider about how long you should take an aromatase inhibitor.

Find a list of questions on hormone therapy you may want to ask your health care provider.

Length of treatment with an aromatase inhibitor

Randomized clinical trials have compared outcomes for women who used aromatase inhibitors for 10 years versus 5 years [112-114]. Taking an aromatase inhibitor for 10 years [112-114]:

  • Improves disease-free survival (time to breast cancer recurrence or breast cancer death)
  • Lowers the risk of contralateral breast cancer (cancer in the opposite breast)

However, overall survival is the same whether a woman takes an aromatase inhibitor for 5 years or 10 years [112-114]. For most women, the benefit of the extra 5 years of treatment is small [112].

Women who take an aromatase inhibitor for more than 5 years continue to have side effects while taking the drug, including a higher number of bone fractures and a higher rate of osteoporosis [112,114-117].

Talk with your health care provider about how long you should take an aromatase inhibitor.

Learn about the importance of completing treatment with an aromatase inhibitor.

Click on the topics below to learn more.

Possible side effects of aromatase inhibitors include muscle pain, joint pain and menopausal symptoms (such as hot flashes).

Aromatase inhibitors also cause a loss of bone density. You’ll get a baseline measure of your bone density so changes in your bone density can be monitored.

Before you begin taking an aromatase inhibitor, talk with your health care provider about possible side effects and how to manage them.

Learn more about the side effects of aromatase inhibitors.

Aromatase inhibitors don’t normally work in premenopausal women because their ovaries are still making estrogen.

However, some premenopausal women may take an aromatase inhibitor when combined with ovarian suppression, which shuts down the ovaries.

Learn more about ovarian suppression and aromatase inhibitors.

It’s important to follow the treatment plan (for medications and other therapies) prescribed by your health care provider in terms of:

  • Timing
  • Dose
  • Frequency

Breast cancer treatment is most effective when all parts of the treatment plan are followed as prescribed.

Completing aromatase inhibitor therapy

To get the most benefit out of hormone therapy, you need the full recommended course of treatment. People who complete the full course have better survival than those who don’t [95-97].

Aromatase inhibitors are prescribed for 5-10 years [10]. The length of this treatment along with side effects, such as menopausal symptoms, can make it tough to complete treatment. Talk with your health care team about ways to ease these and other side effects.

If you have trouble remembering to take your medicine, a pillbox or setting an alarm on your watch or phone (you may be able to download an app) may help.

However, you don’t need to worry if you miss a day or two.

Learn more about the importance of following your breast cancer treatment plan.

Learn about ways to manage hot flashes and other menopausal symptoms.

Findings from randomized controlled trials have shown the aromatase inhibitors exemestane and anastrozole may lower the risk of developing breast cancer in postmenopausal women at high risk [118-119].

The American Society for Clinical Oncology (ASCO), the National Comprehensive Cancer Network (NCCN) and the U.S. Preventive Services Task Force list exemestane and anastrozole as risk-lowering drug options for postmenopausal women at high risk of breast cancer.

However, aromatase inhibitors are not FDA-approved for use as risk-lowering drugs.

Learn more about risk-lowering drugs for women at high risk of breast cancer.

Treatment guidelines

Although the exact treatment for breast cancer varies from person to person, evidence-based guidelines help make sure high-quality care is given. These guidelines are based on the latest research and agreement among experts.

The National Comprehensive Cancer Network (NCCN) and the American Society of Clinical Oncology (ASCO) are respected organizations that regularly review and update their guidelines.

In addition, the National Cancer Institute (NCI) has treatment overviews.

Talk with your health care team about which treatment guidelines they follow.

After you get a recommended treatment plan from your health care team, study your treatment options. Together with your health care team, make thoughtful, informed decisions that are best for you. Each treatment has risks and benefits to consider along with your own values and lifestyle.

Prescription drug assistance

Hormone therapy drug costs can quickly become a financial burden for you and your family.

Medicare and many insurance companies offer prescription drug plans. One may already be included in your policy, or you may be able to buy an extra plan for prescriptions.

Aromatase inhibitors are pills, so they’re covered under your health insurance plan’s prescription drug benefit rather than the plan’s medical benefit. This means there are usually out-of-pocket costs, which can add up over time.

You may qualify for programs that help with drug costs or offer low-cost or free prescriptions.

Aromatase inhibitors are all available in a generic form. Generic drugs cost less than name brand drugs but are just as effective.

Many cancer centers have financial counselors who can discuss insurance and cost coverage with you.

Learn more about insurance plans and prescription drug assistance programs.  

Learn about other financial assistance programs.

Komen Financial Assistance Program

Susan G. Komen® created the Komen Financial Assistance Program to help those struggling with the costs of breast cancer treatment by providing financial assistance to eligible individuals.

To learn more about this program and other helpful resources, call the Komen Breast Care Helpline at 1-877 GO KOMEN (1-877-465-6636) or email

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Susan G. Komen®‘s position on fairness in oral cancer drug coverage

Insurance coverage of oral cancer drugs

Cancer medications given through an IV into a vein or by an injection (under the skin or into a muscle) are usually covered under a health insurance plan’s medical benefit. However, oral cancer drugs (cancer medications that are pills) are usually covered under a health insurance plan’s prescription drug benefit.

As a result, people often find themselves facing high out-of-pocket costs when filling prescriptions for oral cancer drugs. Sometimes these costs can be thousands of dollars a month.

The impact of high cost-sharing

High prescription drug costs and the resulting out-of-pocket burden on patients are a barrier to care. They can prevent people from getting the medications prescribed by their health care providers.

No one should be forced to get less appropriate treatment because an insurer gives more coverage for IV and injectable drugs than for pills.

Efforts to increase fairness in drug coverage

Komen supports state and federal efforts to require insurers to provide the same or better coverage for oral cancer drugs as they do for IV and injectable cancer drugs. This would help make sure patients have access to affordable, appropriate treatment.

Become a Komen Advocacy Insider

Sign up to be a Komen Advocacy Insider and get informed when action is needed on drug coverage issues at the state or national level.

Susan G. Komen® Support Resources

  • Do you need help? We’re here for you. The Komen Patient Care Center is your trusted, go-to source for timely, accurate breast health and breast cancer information, services and resources. Our navigators offer free, personalized support to patients, caregivers and family members, including education, emotional support, financial assistance, help accessing care and more. Get connected to a Komen navigator by contacting the Breast Care Helpline at 1-877-465-6636 or email to get started. All calls are answered Monday through Thursday, 9 a.m to 7 p.m. ET and Friday, 9 a.m. to 6 p.m. ET. Se habla español.
  • The Komen Breast Cancer and Komen Metastatic (Stage IV) Breast Cancer Facebook groups are places where those with breast cancer and their family and friends can talk with others for friendship and support.
  • Our fact sheets, booklets and other education materials offer additional information.

Updated 04/08/24