Women with an increased risk of developing breast cancer [1] may consider taking tamoxifen to reduce their chance of developing this disease. However, long-term use of tamoxifen to prevent breast cancer [1] may raise the odds of developing uterine cancers.

Tamoxifen is often called an “anti-estrogen.” However, while tamoxifen acts against the effects of estrogen in breast tissue, it acts like estrogen in other tissue. According to the National Cancer Institute, tamoxifen increases the risk of two types of cancer [1] that can develop in the uterus: endometrial cancer [1], which arises in the lining of the uterus, and uterine sarcoma.

And, according to a study from the Netherlands Cancer Institute in Amsterdam, the odds of endometrial cancer [1] rose with longer use of tamoxifen. Women who took the drug for two to five years had double the risk of developing endometrial cancer [1], while those taking it for five years or longer had seven times the normal risk. More troublesome findings included the fact that more well-established tumors grew in women who had used the drug for longer periods.

Women who have had a hysterectomy and are taking tamoxifen are not at increased risk for these cancers. The risk of endometrial cancer [1] in women taking tamoxifen was the same or less than the risk in postmenopausal women taking single-agent estrogen replacement therapy (HRT). This risk is about 2 cases of endometrial cancer [1] per 1000 women taking tamoxifen each year. For the rarer uterine sarcoma, there are about 2 cases per 10,000 women taking tamoxifen each year.

The decision to take tamoxifen is an individual one: a woman and her doctor must carefully consider whether the potential benefits outweigh the potential risks of therapy.