Aromatase inhibitors (AIs) are drugs used in hormonal therapy to prevent aromatase — an enzyme in fat tissue — from transforming hormones into estrogen.

This medication does not prevent the ovaries from creating estrogen. Instead, AIs help to slow down the process and lower estrogen levels in ovaries that are no longer producing the hormone. This is most often the case in postmenopausal women because the ovaries don’t produce estrogen and progesterone after menopause.

Aromatase inhibitors can be used in hormonal therapy to treat hormone receptor-positive breast cancers in women. This is because many breast cancers require estrogen to grow. Aromatase inhibitors lower estrogen levels and can therefore slow down tumor growth and treat breast cancer.

Many scientists believe AIs are more effective than tamoxifen, an antiestrogen drug also used to treat breast cancer. Tamoxifen has many side effects that AIs don’t, such as increasing your risk for developing uterine cancer.

Scientists are also working to determine if AIs can be used to reduce the risk of breast cancer. Using AIs preventatively is not yet approved in the United States.

When you’re diagnosed with breast cancer, your doctor will test your cancer cells to see if they’ve attached to estrogen or progesterone proteins. Breast cells and some cancer cells depend on these receptors or proteins to grow.

If they’ve attached to one or both of these proteins, your cancer will be referred to as:

  • ER-positive (ER+). This indicates that your breast cancer hormone status is positive for estrogen receptors.
  • PR-positive (PR+). This indicates that your hormone status is positive for progesterone receptors.

If your breast cancer cells test negative for either hormone, hormone therapy is unlikely to work. In this case, doctors recommend pursuing more traditional cancer treatment options.

Aromatase inhibitors are only recommended for postmenopausal women. Tamoxifen is recommended for men as well as pre- and postmenopausal women.

The FDA has approved three forms of prescription aromatase inhibitors. The drugs are available as oral tablets:

  • anastrozole (Arimidex)
  • exemestane (Aromasin)
  • letrozole (Femara)

Treatment typically begins after the initial cancer treatment, such as surgery, chemotherapy, or radiation.

Aromatase inhibitors can be taken alongside tamoxifen, a similar drug that blocks estrogen from attaching to receptors. Hormone therapy typically lasts 5–10 years. Depending on your individual case, your doctor may recommend using AIs with tamoxifen.

Most scientific research notes that the main advantages of AIs are that they don’t involve many of the negative side effects of tamoxifen. These include:

  • longer times to disease progression (42 weeks on letrozole compared with 23 weeks on tamoxifen)
  • better tolerability
  • positive effects during therapy that lasts longer than five years
  • no increased risk of thromboembolism or endometrial cancer

If you decide to pursue aromatase hormone therapy, be aware of potential side effects:

AI therapy is also shown to increase your risk for developing the following more serious conditions:

Bone thinning and fracture are the most common of these risks. Your risk is heightened while taking the medication, but it decreases after you stop taking it. One study evaluated bone loss, comparing people taking AIs with a control group:

  • 8.7 percent in the AI group experienced bone loss, compared with 7.1 percent in the control group
  • 13.5 percent in the AI group experienced fractures, compared with 10.3 percent in the control group

This treatment is not recommended for women in perimenopause because aromatase inhibitor drugs do not stop ovaries from producing estrogen hormones that contribute to cancer growth.

Breast cancer is treatable if caught early and treated properly. If you have hormone receptor-positive breast cancer and you’ve already gone through menopause, aromatase inhibitor therapy may be a successful treatment option for you. Talk with your doctor about how you can use traditional cancer treatment methods and AI hormone therapy to treat your breast cancer.