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.
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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:
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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:
Aromatase inhibitors can cause severe joint pain. If you have this side effect, ask your healthcare provider for help. They may recommend:
Call your healthcare provider if:
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%.
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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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Last reviewed on 05/07/2025.
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