Breast cancer chemoprevention: Drugs that reduce risk

Preventive medications can reduce breast cancer risk. Learn about how these drugs work, who should consider taking them, plus their side effects and health risks.

By Mayo Clinic Staff

If you're at high risk of breast cancer, you may be able to improve your odds of staying cancer-free by taking certain medicines — an approach known as chemoprevention or preventive therapy.

Medications for breast cancer chemoprevention are the subject of much ongoing research.

Here's a look at what's known about each of these medications, including how they may work to prevent breast cancer and the possible side effects and health risks.


How it works

Tamoxifen blocks the effects of estrogen — a reproductive hormone that influences the growth and development of many breast cancers.

Tamoxifen belongs to a class of drugs known as selective estrogen receptor modulators (SERMs). The drug reduces the effects of estrogen in most areas of the body, including the breast. In the uterus, however, tamoxifen acts like an estrogen and encourages the growth of the lining of the uterus.

Tamoxifen is usually prescribed as a pill taken once a day by mouth. For breast cancer risk reduction, tamoxifen is typically taken for a total of five years. The risk reduction benefit continues for five additional years after you stop taking tamoxifen. In total, you could receive up to 10 years of benefit.

Who it's for

Tamoxifen is used to reduce the risk of invasive breast cancer if you're at high risk and you're 35 and older, whether or not you've gone through menopause.

You and your health care provider might consider whether chemoprevention with tamoxifen is right for you if:

  • Your Gail model risk score is greater than 1.7%. The Gail model is a tool that health care providers use to predict future risk of developing breast cancer. The risk score is based on factors such as age, reproductive history and family history.
  • You're at high risk of developing breast cancer. For instance, you've had a breast biopsy that found a precancerous condition such as lobular carcinoma in situ, atypical ductal hyperplasia or atypical lobular hyperplasia.
  • You have a strong family history of breast cancer.
  • You don't have a history of blood clots.

Common side effects

Common side effects of tamoxifen include:

  • Hot flashes
  • Night sweats
  • Vaginal discharge
  • Vaginal dryness


Rarely, taking tamoxifen may cause:

  • Blood clots
  • Endometrial cancer or uterine cancer
  • Cataracts
  • Stroke

If you haven't undergone menopause, the risk of uterine cancer when taking tamoxifen is very low compared with the risk for those who have already undergone menopause. In this situation, the benefits of tamoxifen may outweigh the risks if you have an increased risk of breast cancer due to a strong family history or a personal history of precancerous breast changes.

If you've undergone menopause, the benefits of tamoxifen may outweigh the risks if you have an increased risk of breast cancer and have also had surgery to remove your uterus (hysterectomy).


How it works

Raloxifene (Evista) is another drug in the class known as SERMs. It's also prescribed in pill form, to be taken by mouth once a day for five years.

Like tamoxifen, raloxifene works by blocking estrogen's effects in the breast and other tissues. Unlike tamoxifen, raloxifene doesn't exert estrogen-like effects on the uterus.

Who it's for

Raloxifene is used to reduce the risk of invasive breast cancer if you're at high risk and you've undergone menopause (postmenopausal). You're considered at high risk if you score greater than 1.7% on the Gail model.

Raloxifene is also used for prevention and treatment of the bone-thinning disease osteoporosis in those who've undergone menopause.

Common side effects

Common side effects of raloxifene include:

  • Hot flashes
  • Vaginal dryness or irritation
  • Joint and muscle pain
  • Weight gain


Health risks associated with raloxifene are similar to those associated with tamoxifen.

Both drugs carry an increased risk of blood clots, though the risk may be lower with raloxifene. Raloxifene may be associated with fewer instances of endometrial and uterine cancers than is tamoxifen.

Raloxifene may also be linked to fewer strokes than tamoxifen in people at average risk of heart disease. But if you have heart disease or you have multiple risk factors for heart disease, raloxifene may increase your risk of strokes.

Although tamoxifen may be slightly better than raloxifene at reducing the risk of breast cancer, the risk of blood clots and uterine cancer are lower with raloxifene. For this reason, raloxifene may be a preferred option if you've undergone menopause and haven't had a hysterectomy or have osteoporosis.

Aromatase inhibitors

Aromatase inhibitors are commonly used to treat breast cancer that's hormone receptor positive. These drugs are also an option for breast cancer chemoprevention.

How they work

Aromatase inhibitors are a class of medicines that reduce the amount of estrogen in your body, depriving breast cancer cells of the fuel they need to grow and thrive.

Three aromatase inhibitors are currently approved in the United States for breast cancer treatment: anastrozole (Arimidex), exemestane (Aromasin) and letrozole (Femara).

These medications are used to treat breast cancer that's estrogen- or progesterone-responsive in those who've undergone menopause.

Who they're for

Aromatase inhibitors have been studied and shown to be effective to treat breast cancer and to prevent breast cancer recurrence after menopause. Aromatase inhibitors are not intended for preventing breast cancer recurrence if you still have menstrual cycles.

Aromatase inhibitors, specifically exemestane and anastrozole, have also been studied to see if they may reduce the risk of breast cancer in people at high risk, such as those with a family history of breast cancer or a history of precancerous breast lesions. Studies have shown promise in reducing breast cancer risk in these individuals.

Based on these results, you and your health care provider may choose to use aromatase inhibitors to reduce the risk of breast cancer, though these drugs aren't approved by the U.S. Food and Drug Administration for this use.

Additional studies are underway to determine whether aromatase inhibitors may reduce the risk of breast cancer in those with genetic mutations that increase the risk of breast cancer.

Common side effects

Common side effects of aromatase inhibitors include:

  • Hot flashes
  • Vaginal dryness
  • Joint and muscle pain
  • Headache
  • Fatigue


Aromatase inhibitors increase the risk of osteoporosis.

Aromatase inhibitors aren't associated with an increased risk of blood clots or uterine cancer, as tamoxifen and raloxifene are. Because aromatase inhibitors are a newer class of medications, more research needs to be done about long-term health risks, such as heart disease and broken bones.

As more results from research studies become available, health care providers will have a better idea of the long-term health implications for these drugs and their effectiveness in breast cancer chemoprevention.

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Oct. 19, 2021 See more In-depth

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