Aromatase Inhibitors
An aromatase inhibitor is a type of hormonal therapy medicine that may be recommended for you if you’ve been diagnosed with hormone receptor-positive breast cancer.
There are three aromatase inhibitors doctors use to treat breast cancer:
Arimidex (chemical name: anastrozole)
Aromasin (chemical name: exemestane)
Femara (chemical name: letrozole)
How do aromatase inhibitors work?
Aromatase inhibitors work by blocking the enzyme aromatase, which turns the hormone androgen into small amounts of estrogen in the body. Hormone receptor-positive breast cancer relies on estrogen to grow. By lowering estrogen in the body, breast cancer cells can’t multiply.
Aromatase inhibitors don’t work on hormone receptor-negative breast cancer because these cells don’t rely on estrogen to grow.
Who gets aromatase inhibitors?
Before menopause, the ovaries make most of the estrogen in the body. Aromatase inhibitors can’t stop the ovaries from making estrogen. As a result, doctors prescribe these medicines mainly to people who’ve had their ovaries removed or those who are post-menopausal.
If you haven’t gone through menopause, it’s still possible that your doctor would recommend an aromatase inhibitor. Research suggests that an aromatase inhibitor can be combined with medicine that stops the ovaries from working (ovarian suppression medicine). This approach is sometimes recommended for people at high risk of recurrence who are pre-menopausal.
How long do you take an aromatase inhibitor?
All the aromatase inhibitors are pills taken once a day. How long you take an aromatase inhibitor depends on the stage of the cancer and the medicine recommended for you.
If you’ve been diagnosed with early-stage, hormone receptor-positive breast cancer, you usually take an aromatase inhibitor for at least five years after surgery to reduce the risk of recurrence. In some cases, another five years of the aromatase inhibitor may be recommended, for a total of 10 years of hormonal therapy.
If you’ve already taken tamoxifen for two to three years because you were pre-menopausal when first diagnosed, your doctor may recommend that you take an aromatase inhibitor for another two or three years — for a total of five years of hormonal therapy. Your doctor may then recommend that you continue the aromatase inhibitor for another five years after, for a total of 10 years of hormonal therapy.
If you’ve been diagnosed with advanced-stage, hormone receptor-positive breast cancer, your doctor will likely recommend that you take any of the aromatase inhibitors for as long as you’re getting benefits from the medicine.
Side effects of aromatase inhibitors
Like all hormonal therapy medicines, aromatase inhibitors can cause side effects. Bone and joint pain and stiffness are the most common.
If side effects from taking an aromatase inhibitor are bothering you, tell your doctor right away. You may be able to take a different hormonal therapy medicine. If you’re considering stopping hormonal therapy early or skipping doses, talk to your doctor before you do anything. Explain your concerns and issues and ask about possible solutions.
Listen to the episode of The Breastcancer.org Podcast featuring Ashish Khanna, MD, explaining how and why aromatase inhibitors cause side effects and how those side effects can be eased.
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Which aromatase inhibitor is best?
The short answer is: the one that works best for you.
Few studies have compared all three aromatase inhibitors head-to-head. The most recent was a 2025 study that compared recurrence rates and overall survival among more than 148,000 post-menopausal French women taking either Arimidex, Aromasin, or Femara for early-stage hormone receptor-positive breast cancer. After about five years of follow-up, recurrence rates for women taking Arimidex and Femara were slightly lower (about 2%) than they were for women taking Aromasin. Similarly, overall survival rates for women taking Arimidex and Femara were slightly higher (about 2%) than they were for women taking Aromasin.
The three aromatase inhibitors can cause similar side effects, but these side effects are different for each person. One person may have very mild side effects from Arimidex, while for another person, the bone and joint pain may be intolerable. This is why it’s so important to talk to your doctor as soon as you start having any side effects. In many cases, you can switch to a different aromatase inhibitor that causes you fewer or milder side effects.
You can talk with others taking an aromatase inhibitor or other hormonal therapy medicine in the Breastcancer.org Community.
