Aromatase inhibitors are hormone therapy drugs and can be used to treat hormone receptor-positive breast cancers. Aromatase inhibitors lower estrogen levels in the body by blocking aromatase, an enzyme that converts other hormones into estrogen. This slows or stops the growth of the tumor by preventing the cancer cells from getting the hormones they need to grow.

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. However, you may tolerate one drug better than another. These drugs are all pills.

For women with hormone receptor-positive early breast cancer, treatment with an aromatase inhibitor (alone or after several years of tamoxifen) lowers the risk of breast cancer recurrence and death from breast cancer.

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

In general, aromatase inhibitors are only used to treat breast cancer in postmenopausal women. However, some premenopausal women may take an aromatase inhibitor when combined with ovarian suppression.

Randomized clinical trials have compared outcomes for women who used aromatase inhibitors for 10 years versus 5 years. Taking an aromatase inhibitor for 10 years improves disease-free survival and lowers the risk of cancer in the opposite breast.

However, overall survival is the same whether a woman takes an aromatase inhibitor for 5 years or 10 years. For most women, the benefit of the extra 5 years of treatment is small. Women who take an aromatase inhibitor for more than 5 years continue to have side effects from the drug, including a higher number of bone fractures and a higher rate of osteoporosis. Talk with your health care provider about how long you should take an aromatase inhibitor.

Common side effects of aromatase inhibitors include hot flashes, vaginal dryness, joint and muscle pain, loss of bone mineral density, fatigue and sleeping problems.