Medicines to Reduce Breast Cancer Risk

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Aromatase inhibitors and other compounds for lowering breast cancer risk

Aromatase inhibitors (drugs that lower estrogen levels) may someday prove to be as good as or even better than tamoxifen or raloxifene in reducing breast cancer risk, but they haven’t been well studied for this use. More study is needed to see who would most benefit and how long treatment should be continued.

What are aromatase inhibitors?

Aromatase inhibitors lower estrogen levels by stopping an enzyme in fat tissue (called aromatase) from changing other hormones into estrogen. These drugs don’t stop the ovaries from making estrogen. They only lower estrogen levels in women whose ovaries aren’t making estrogen (such as women who have already gone through menopause). The drugs in this class include:

  • Exemestane (Aromasin®)
  • Letrozole (Femara®)
  • Anastrozole (Arimidex®)

Aromatase inhibitors are pills taken once a day.

Can aromatase inhibitors lower the risk of breast cancer?

Aromatase inhibitors are used mainly to treat hormone receptor-positive breast cancer, but are being studied to see if they can lower breast cancer risk. They are not yet approved for this use in the US. Studies have shown that both exemestane and anastrozole can lower the risk of breast cancer in post-menopausal women who are at higher risk of the disease.

In one study, taking exemestane for 3 years lowered the risk of breast cancer overall (invasive cancer plus ductal carcinoma in situ) by about half (47%).

In another study, taking anastrozole for 5 years lowered the risk of breast cancer overall by about half (47%).

Letrozole is being studied now to see if it can lower breast cancer risk.

What are the risks and side effects of aromatase inhibitors?

The most common side effects of aromatase inhibitors are symptoms of menopause, such as hot flashes, night sweats, and vaginal dryness. These drugs can also cause muscle and joint pain. This side effect can be serious enough to cause some women to stop taking the drugs.

Unlike tamoxifen and raloxifene, aromatase inhibitors tend to speed up bone thinning, which can lead to osteoporosis. It is easy for people with osteoporosis to have broken bones.

Aromatase inhibitors do not seem to increase the risk of serious blood clots or cancer of the uterus. Tamoxifen and raloxifene do.

Other compounds being studied to reduce breast cancer risk

Fenretinide, a drug related to vitamin A, is being studied to see if it can lower breast cancer risk. Other clinical trials are looking at breast cancer reduction as an unintended effect of drugs used for other reasons. (This is how raloxifene, used for osteoporosis, was found to be useful in breast cancer.) Drugs currently being researched include bisphosphonates (drugs for osteoporosis), and statins (such as atorvastatin and lovastatin), which are used to lower cholesterol.

Dietary supplements have also been studied to see if they can reduce breast cancer risk. These have included grapeseed extract, folate, omega-3 fatty acids, and vitamins B6 and B12. Although some of these human studies have been completed, very little has been published in the available medical literature to date.

Supplements being studied now include hydroxytyrosol, a component in olive oil, curcumin, omega-3 fatty acids (coupled with weight loss). These studies aren’t looking directly at the effect of these substances on breast cancer risk, though. They are looking at other factors that may reflect risk.


Last Medical Review: 10/21/2014
Last Revised: 04/14/2016