Aromatase Inhibitors for Breast Cancer

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An aromatase inhibitor (AI) is a type of hormone therapy for cancer. Healthcare providers use aromatase inhibitors to treat ER-positive breast cancer. This therapy reduces the risk that breast cancer will come back after surgery. If you’re at an increased risk of this form of breast cancer, taking an aromatase inhibitor may reduce that risk.

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An aromatase inhibitor (AI) is a common type of hormone therapy for cancer. Healthcare providers use aromatase inhibitors to treat estrogen receptor-positive (ER+) breast cancer. ER-positive breast cancer often affects females ages 50 and older who’ve gone through menopause.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Aromatase inhibitors work by reducing the amount of estrogen in your body. They block the enzyme aromatase, which turns other hormones into estrogen. Estrogen is an essential hormone that supports your sexual and reproductive health. It also regulates other important processes that affect your body. But high estrogen levels may increase your risk of developing ER-positive breast cancer.

Providers use aromatase inhibitor therapy as an initial treatment for ER-positive breast cancer. They may prescribe aromatase inhibitor therapy if:

You’ve had surgery for ER-positive breast cancer (to reduce the risk that the cancer will come back)
You have ER-positive breast cancer that’s spread to nearby lymph nodes
You have metastatic ER-positive breast cancer that’s spread to your organs (breast cancer may spread to your lungs, bones, brain or liver)
You develop ER-positive breast cancer before you’ve gone through menopause (in that case, your ovaries still make estrogen, but you may have additional treatment to block these estrogens)
You’re at increased risk of developing ER-positive breast cancer — studies show certain aromatase inhibitors reduce breast cancer risk (the U.S. Food and Drug Administration hasn’t approved this use)
You’re a male with ER-positive breast cancer

While estrogen is considered a female hormone, males have estrogen, just at lower levels than in females. In this case, your provider may combine aromatase inhibitor therapy with estrogen-blocking therapy.

The three most common drugs are:

Aromatase inhibitors are pills you take every day. You may start treatment with an aromatase inhibitor. In some cases, you might take tamoxifen for a few years and then start aromatase inhibitor therapy.

After treatment, you may be considered in remission if five years have passed, you don’t have cancer symptoms and tests don’t find signs of cancer.

Most people take aromatase inhibitors for five years, stopping treatment if they don’t have signs of recurring or new breast cancer.

That said, studies show breast cancer can come back as long as 20 years after treatment. This is late recurrence breast cancer. If you’re receiving treatment for ER-positive breast cancer, ask your healthcare provider to explain your risk.

The most significant benefit is being free of ER-positive breast cancer. Studies show 95% of people who receive aromatase inhibitor therapy after breast cancer surgery don’t have breast cancer signs five years after completing treatment.

But estrogen helps keep your bones and heart healthy. If you take aromatase inhibitors, you may have the following side effects:

Increased risk of osteoporosis and bone fractures
Muscle pain (myalgia)
Joint pain
Hot flashes
Vaginal dryness
Cardiovascular disease

Care at Cleveland Clinic

Aromatase inhibitors can cause severe joint pain. If you have this side effect, ask your healthcare provider for help. They may recommend:

Anti-inflammatory medications, including nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen
Switching your medication to another aromatase inhibitor
Substituting tamoxifen

Call your healthcare provider if:

Your side effects become more severe than you expected
Your side effects don’t improve with the medication your provider recommended
You notice changes in your body that may be symptoms of recurrent breast cancer (ask your provider about watching for specific changes)

Aromatase inhibitors help lower the chance of hormone-positive breast cancer coming back. Studies show that people who take an aromatase inhibitor after surgery have a much lower risk of dying from breast cancer — about 40% lower after 10 years — compared to those who don’t take hormone-blocking medicine at all.

Aromatase inhibitor therapy significantly reduces the risk that ER-positive breast cancer will come back. One early study compared outcomes of people who took the aromatase inhibitor letrozole after tamoxifen to those of people who didn’t. The study showed that taking letrozole reduced the risk of dying from breast cancer by 24% to 35%.

Aromatase inhibitor therapy is one of the most effective ways to treat ER-positive breast cancer in women who’ve gone through menopause. Studies show that most women who take aromatase inhibitors after breast cancer surgery remain free of breast cancer for five or more years after treatment.

Still, aromatase inhibitor therapy has side effects that may affect your quality of life. If you’re taking an aromatase inhibitor, ask your healthcare provider about ways to reduce side effects so you can continue treatment that helps you live free of breast cancer.

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Cleveland Clinic’s health articles are based on evidence-backed information and review by medical professionals to ensure accuracy, reliability and up-to-date clinical standards.

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Peptide Therapy Guide Editorial Team