Medicines to Reduce Breast Cancer Risk

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What are the risks in taking these drugs?


Tamoxifen is a complex drug. It acts like an anti-estrogen in some tissues, but acts like estrogen in others. Its anti-estrogen effects cause the most common side effects, such as hot flashes and night sweats. But because it also acts like an estrogen in some tissues, tamoxifen may increase a woman’s chance of some rare but serious health problems.

Cancer of the uterus

Estrogens and agents that act like estrogens are known to raise the risk of cancer of the uterus when they are taken by women after menopause. In the Breast Cancer Prevention Trial (BCPT), the women taking tamoxifen had a higher risk of uterine cancer. Most of these were endometrial cancer (cancer of the lining of the uterus), but a few were uterine sarcoma (cancer of the muscle layer of the uterus).

Overall, the risk of uterine cancer was low. Over 7 years, the risk was higher than that seen in the placebo group, but still less than 2%. This came out to about 16 cases of uterine cancer out of 1,000 women in the tamoxifen group, compared with around 5 out of 1,000 in the placebo group. Most of these cancers were found at a very early stage.

The higher risk seemed to affect the women over 50 and not the younger women. For some women over 50, the increased risk of endometrial cancer may offset the reduction in invasive breast cancer. The higher risk of endometrial cancer seemed to drop after the women finished taking tamoxifen.

It is especially important for women who have taken or are taking tamoxifen to talk about the risk of uterine cancer with their doctors. These women should tell their doctors about any vaginal bleeding or spotting after menopause. Bleeding, spotting, or discharge could be symptoms of these cancers. Women should also talk to their doctors about the possible benefits, risks, and limitations of testing for early endometrial cancer.

Endometrial cancer usually can be found at an early stage, when it hasn’t spread and treatment works best. Most endometrial cancers are found when women with symptoms (such as spotting or bleeding) are checked by their doctors. Further testing, with endometrial biopsy and transvaginal ultrasound, is done to see if cancer is present. For more information on this, see our document called Endometrial (Uterine) Cancer.

The American Cancer Society recommends that women taking tamoxifen learn about their testing options for endometrial cancer so that they can make informed decisions. But at this time we do not recommend routine testing for these women. This is because studies have not shown that routine testing helps find endometrial cancer at a more curable stage. Also, many studies have found that routine testing for endometrial cancer can lead to unnecessary surgery to check out false-positive test results.

Women who have had a hysterectomy (surgery to remove the uterus) are not at risk for endometrial cancer or uterine sarcoma and do not have to worry about these cancers.

Major blood clots

Tamoxifen is known to increase the risk of major blood clots. In the Breast Cancer Prevention Trial, women taking tamoxifen had a higher risk of a blood clot such as a pulmonary embolism (a blood clot that travels to the lung) or a deep vein thrombosis (a blood clot in a large vein in the leg) than the women on the placebo. These clots can sometimes cause serious problems, and even death. Still, the risk was less than 1%..

Women in the tamoxifen group may have also have been more likely to have a stroke than those the placebo group. But the differences were so small that they may have been due to chance rather than the tamoxifen.

Other possible side effects

Women who take tamoxifen may have a slightly higher risk for cataracts (a clouding of the lens of the eye). This was seen in the BCPT study, but not in the IBIS-I. And as women get older, they are more likely to develop cataracts whether or not they take tamoxifen.

The most common side effects of tamoxifen are the symptoms of menopause. These include hot flashes, night sweats, and vaginal dryness. Some women also report problems with a vaginal discharge.

Tamoxifen use may also cause menopause to start earlier than it may have otherwise. This is more likely in women who were close to menopause when they started taking the drug. In most women who take tamoxifen before menopause, the ovaries work normally and make female hormones (estrogens) in the same or slightly increased amounts. But in some women, menstrual periods will stop.


Raloxifene has many of the same side effects as tamoxifen, but the risks are not so high.

Problems with menopausal symptoms, such as hot flashes, night sweats, and vaginal dryness occur at about the same rate. But more serious side effects occur less often.

In the Study of Tamoxifen And Raloxifene trial, the risk of uterine cancer was almost half as much in the raloxifene group as in the tamoxifen group. The risk of serious blood clots was also 25% lower than in women taking tamoxifen. Raloxifene also does not seem to be linked to the development of cataracts.

Last Medical Review: 06/04/2013
Last Revised: 07/17/2013