Aromatase inhibitors are medications that help lower estrogen levels. They’re also called hormone therapy or endocrine therapy.

For some people with estrogen receptor-positive (ER+) breast cancer (one of the most common types), aromatase inhibitors are a safe, effective treatment. However, they’re not for everyone with ER-positive breast cancer. Some people may find it difficult to tolerate the possible side effects.

Read on as we take a closer look at aromatase inhibitors and who can use them.

A 2020 article in the Journal of Clinical Oncology shows that 79 to 84 percent of breast cancers test positive for estrogen receptors. That means the cancer cells use estrogen to fuel growth.

One goal of aromatase inhibitor treatment is to lower estrogen levels.

Most estrogen comes from the ovaries, but it’s not the only source. A small amount comes from the adrenal glands and other tissues. An enzyme called aromatase takes other hormones and converts them into estrogen.

That means postmenopausal and nonmenstruating people also have some estrogen.

Aromatase inhibitors bind to aromatase and stop the process of conversion to estrogen. This keeps estrogen levels down and helps prevent cancer cells from getting the estrogen they can use to grow and spread.

Aromatase inhibitor therapy does not affect estrogen production in the ovaries.

A 2020 research article published in JCI Insight suggests that aromatase inhibitors can reduce breast cancer recurrence and help improve your outlook. However, the cancer returned in as many as 30 percent of people observed in the study.

Three aromatase inhibitors are used to treat breast cancer. Two are nonsteroidal medications that temporarily stop the conversion process involving aromatase:

Another is a steroidal medication that permanently stops the conversion process involving aromatase:

All three are taken orally every day.

Aromatase inhibitors are typically used to treat people who have already experienced menopause.

These medications do not affect estrogen production in the ovaries. Since that’s where most estrogen comes from, aromatase inhibitors are not usually recommended for people with fully functioning ovaries.

There are some other uses for this medication, too. For example, a doctor might prescribe this therapy for someone who hasn’t yet undergone menopause and is taking medication to reduce the functions of the ovaries, including estrogen production.

A 2015 study found that combining aromatase inhibitors with ovarian suppression medications was effective in reducing the risk of cancer recurrence in premenopausal women with breast cancer.

If you’re receiving chemotherapy, you might stop having periods. That’s not unusual — but it does not mean your ovaries are not functioning or that you’re menopausal. You may be recommended aromatase inhibitors in this case.

The 2020 American Society of Clinical Oncology Guideline also recommends offering an aromatase inhibitor to people assigned male at birth who:

  • cannot take tamoxifen to treat breast cancer
  • have advanced or metastatic ER-positive, HER2-negative breast cancer

Anastrozole is also approved to treat postmenopausal people in the following cases:

  • as an adjuvant, or additional, treatment for early-stage breast cancer
  • as a first-line treatment for ER-positive or unknown hormone receptor status with locally advanced or metastatic breast cancer
  • as a second-line treatment for advanced breast cancer that has continued to spread even with tamoxifen therapy

Letrozole is approved to treat postmenopausal people as:

  • an additional treatment for early breast cancer beyond primary treatments like surgery
  • an extended treatment of early breast cancer after 5 years of tamoxifen therapy
  • a first-line treatment for advanced breast cancer
  • a second-line treatment if cancer continues to spread after anti-estrogen therapy

Exemestane is used in postmenopausal women as:

  • an additional treatment for early breast cancer after 2 to 3 years of tamoxifen
  • a treatment for advanced breast cancer that has progressed after tamoxifen therapy

Studies have found that exemestane and anastrozole may reduce the risk of breast cancer in women who have already gone through menopause and may be at high risk of breast cancer. However, neither medication has been approved for this use.

A large 2020 study suggests a higher risk of heart failure and death from cardiovascular conditions when taking aromatase inhibitors, compared with the risks of taking tamoxifen.

The researchers recommend weighing the benefits of aromatase inhibitors compared with tamoxifen against these possible risks.

If you already have heart disease, discuss all the potential benefits and risks with a doctor.

Some other possible side effects of aromatase inhibitors can include:

Other side effects may include:

Aromatase inhibitors do not work for ER-negative breast cancers and should not be used if you’re:

  • nursing
  • pregnant or trying to get pregnant
  • currently on hormone replacement therapy (HRT) for menopause

Aromatase inhibitors are often used in combination with other treatments, including:

Some drugs lower estrogen by suppressing ovarian function, including:

  • goserelin (Zoladex)
  • leuprolide (Lupron)

Other hormone therapies that can help block the effects of estrogen are:

  • tamoxifen (Nolvadex)
  • toremifene (Fareston)
  • fulvestrant (Faslodex)

Are there natural aromatase inhibitors?

Some foods and dietary supplements may affect estrogen levels. A 2016 study suggests that eating a lot of meats and sugars may contribute to high estrogen levels in postmenopausal women.

Kailey Proctor, MPH, RDN, CSO, says that grape seed extract can act as an aromatase inhibitor due to its high levels of procyanidin B dimers.

“Procyanidin B dimers are a phytochemical that is found in the skin and seeds of grapes. In mice, procyanidin B dimers have been shown to reduce the growth of breast cancer tumors,” explains Proctor, a board-certified oncology dietitian at Leonard Cancer Institute with Providence Mission Hospital in California.

However, there’s not enough research to conclude that breast cancer patients can replace aromatase inhibitors with grape seed extract or other dietary choices, cautions Proctor.

“We still need more human trials. Mice have a different metabolism than humans. It is also important to remember that correlation doesn’t equal causation,” adds Proctor.

Even natural supplements can interact with medications.

“I always advise patients to talk with their oncologist before starting any new supplements or herbs — especially since grape seed extract can increase the risk of bleeding,” adds Proctor.

Aromatase inhibitors help lower estrogen levels, but they don’t stop the ovaries from making estrogen. That’s why they’re mostly used to treat postmenopausal women with ER-positive breast cancer.

While aromatase inhibitors can be an effective part of overall treatment, they’re not for everyone. If you have ER-positive breast cancer, ask a doctor about the potential benefits and risks of taking these medications.