By Mayo Clinic Staff

If you are at high risk of breast cancer, certain medicines may help lower your chances of getting the disease. This approach is called risk-reducing therapy or preventive therapy.

Here's an overview of these medicines, including how they may help prevent breast cancer and the possible side effects and health risks.

Tamoxifen blocks the effects of estrogen. Estrogen is a hormone that plays a role in the growth and development of most breast cancers.

Tamoxifen is a type of medicine called a selective estrogen receptor modulator (SERM). It lessens the effects of estrogen in most areas of the body, including the breast tissue. In the uterus, however, tamoxifen acts like estrogen and can promote the growth of the lining of the uterus.

Tamoxifen is typically taken once a day, but that can vary based on your health and risk factors. There are two common dosage options: 20 milligrams (mg) daily for five years or 10 mg every other day for three years. Currently, the usual advice is to start with 20 mg daily. If that dose causes issues, it can be lowered. Your healthcare team will help find the best approach for you.

Even after you stop taking tamoxifen, it may continue to offer protection for up to eight years.

Tamoxifen helps lower the risk of breast cancer if you're at high risk and you're 35 or older. This applies whether or not you've gone through menopause.

You and your healthcare team might consider whether preventive therapy with tamoxifen is right for you if:

• You're at high risk of developing breast cancer. For example, you've had a breast biopsy that showed a condition that raises the risk of breast cancer. This might include 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.
• Your risk of developing breast cancer is considered high according to risk models.

Two common risk models are used to estimate the risk of breast cancer. The models consider risk factors such as family history, breast biopsy history and reproductive history.

The Gail model also is known as the Breast Cancer Risk Assessment Tool (BCRAT). It helps healthcare professionals predict your risk of developing breast cancer in the next five years and over your lifetime. If your five-year risk of getting breast cancer is greater than 1.7%, your healthcare professional may discuss preventive medicines with you.

The International Breast Intervention Study model also is known as the Tyrer-Cuzick model. It helps healthcare professionals predict your risk of developing breast cancer in the next 10 years. If your 10-year risk of getting breast cancer is greater than or equal to 5%, your healthcare professional may discuss preventive medicines with you.

Common side effects of taking 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 yet gone through menopause, tamoxifen is less likely to cause uterine cancer or endometrial cancer. If you have a higher risk of breast cancer because of your family history or a history of breast changes that increase the risk of cancer, the benefits of tamoxifen may be greater than the risks.

If you've gone through menopause and had your uterus removed, tamoxifen may still be a good option if you're at higher risk of breast cancer.

Like tamoxifen, raloxifene (Evista) is a kind of medicine known as a selective estrogen receptor modulator (SERM). It is typically taken by mouth once a day for five years.

Raloxifene works by blocking estrogen's effects in the breast and other tissues. However, unlike tamoxifen, raloxifene doesn't have estrogen-like effects on the uterus and does not increase the risk of uterine cancer.

Raloxifene helps lower the risk of breast cancer if you are at high risk and have gone through menopause.

Raloxifene also is used to prevent and treat osteoporosis, a disease that weakens bones, in those who've gone through menopause.

Common side effects of raloxifene include:

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

The health risks of raloxifene are similar to those of tamoxifen. Both medicines raise the risk of blood clots, but raloxifene is considered lower risk than tamoxifen. Unlike tamoxifen, raloxifene does not increase the risk of endometrial and uterine cancers.

Both raloxifene and tamoxifen can raise the risk of strokes. However, raloxifene is less likely to cause strokes than tamoxifen.

Tamoxifen may be slightly better than raloxifene at reducing breast cancer risk. But raloxifene has lower risks of blood clots and uterine cancer. So raloxifene may be a better choice if you've gone through menopause and haven't had a hysterectomy or if you have osteoporosis.

Both raloxifene and tamoxifen are approved by the U.S. Food and Drug Administration (FDA) to reduce the risk of breast cancer. However, raloxifene is recommended only for people who have gone through menopause.

Aromatase inhibitors are medicines often used to treat breast cancer that's hormone receptor positive. They also can be used to help lower risk in people at increased risk of breast cancer.

Aromatase inhibitors reduce the amount of estrogen in the body. This helps stop breast cancer cells from growing. Aromatase inhibitors are usually taken by mouth once daily for five years.

In the United States, three aromatase inhibitors are approved to treat breast cancer: anastrozole (Arimidex), exemestane (Aromasin) and letrozole (Femara).

These medicines are used to treat breast cancers that respond to estrogen or progesterone in people who have gone through menopause.

Aromatase inhibitors are effective in treating breast cancer and preventing its return after menopause. Aromatase inhibitors are not used to prevent breast cancer from returning if you still have menstrual cycles.

Studies have looked into whether certain aromatase inhibitors, such as exemestane and anastrozole, can reduce the risk of breast cancer in people at high risk. This includes people with a family history of breast cancer or those who have had changes in the breast, called lesions, that could raise the risk of cancer. Research suggests that in these groups, certain aromatase inhibitors may help reduce breast cancer risk.

You and your healthcare team may decide to use aromatase inhibitors to help lower your risk of breast cancer. These medicines have not been approved by the U.S. Food and Drug Administration for this use, however.

More studies are being done to see if aromatase inhibitors can lower the risk of breast cancer in those with DNA changes that increase their breast cancer risk.

Common side effects of aromatase inhibitors include:

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

Aromatase inhibitors increase the risk of osteoporosis and may affect the heart.

Unlike tamoxifen and raloxifene, aromatase inhibitors do not raise the risk of blood clots or uterine cancer. Because aromatase inhibitors are a newer type of medicine, more research is needed to understand long-term health risks, such as heart conditions and bone loss.

As more is learned through research studies, healthcare professionals will better understand the long-term health effects of these medicines and how well they work to prevent breast cancer.