Aromatase inhibitors are another type of hormone therapy that can be used to treat hormone receptor-positive breast cancers. They lower estrogen levels in the body by blocking an enzyme called aromatase that helps produce estrogen. This slows or stops the growth of the tumor by preventing the cancer cells from getting the hormones they need to grow.

In general, aromatase inhibitors are only used to treat breast cancer in postmenopausal women. However, some premenopausal women may take them along with treatments that suppress ovarian function.

Aromatase inhibitors include anastrozole, commonly known as Arimidex; letrozole, or Femara and exemestane, also known as Aromasin. Anastrozole, exemestane and letrozole are equally effective and have similar side effects. But you may tolerate one drug better than another. These drugs are all pills.

For hormone receptor-positive early breast cancer, treatment with an aromatase inhibitor, used alone or after several years of tamoxifen, lowers the risk of breast cancer recurrence, reduces the chance of breast cancer in the opposite breast and improves survival.

Among postmenopausal women with hormone receptor-positive breast cancer, aromatase inhibitors, either alone or after tamoxifen, offer the same or slightly greater benefit compared to tamoxifen alone.

Studies have found that taking an aromatase inhibitor for 10 years improves disease-free survival and lowers the risk of cancer in the opposite breast when compared to taking one for 5 years.

However, overall survival is the same whether a woman takes an aromatase inhibitor for 5 years or 10 years. This means the treatment lowers the risk of dying from breast cancer or another cause by the same amount.

For most women, there’s minimal benefit with 5 extra years of treatment. Those who take an aromatase inhibitor for more than 5 years will continue to have side effects from the drug, including a higher number of bone fractures and a higher rate of osteoporosis.

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

Common side effects of aromatase inhibitors include hot flashes, night sweats, joint and muscle pain, vaginal dryness or itching, loss of sex drive and loss of bone mineral density, which may lead to osteoporosis or bone fractures. Less common or rare side effects include carpal tunnel syndrome, hair thinning, heart problems, increased blood pressure and cholesterol, mood swings and depression.

Once you begin an aromatase inhibitor and throughout the time you’re taking it, talk to your health care team about side effects and how to manage them because it’s really important to complete the full course of treatment as prescribed to get the most benefit.

For more information, visit the Aromatase Inhibitors page within the Treatment section of komen.org.