Tamoxifen is a selective estrogen receptor modulator (SERM) that is often used as a treatment for breast cancer in women who already have the disease. It blocks the effects of estrogen — a reproductive hormone that influences the growth and development of many breast tumors. In the uterus, tamoxifen acts like estrogen and encourages growth of the lining of the uterus.

Can tamoxifen reduce breast cancer risk?

Tamoxifen is approved for reducing the risk of breast cancer in women known to have an increased risk. Other compounds, including dietary supplements, are also being studied to find out if they might help reduce risk. However, to date, none have been shown to be helpful.

The effect of tamoxifen on breast cancer risk has varied in different studies. It has been shown to lower the risk of breast cancer recurrence (cancer that comes back after treatment) and lengthen the lives of women with early-stage breast cancer. Tamoxifen has been shown to lower risk by 13 to 48 percent. It lowers the risk of both invasive breast cancer and ductal carcinoma in situ.

(Read also: Are blood pressure medications linked to breast cancer?)

What are tamoxifen’s side effects?

You may experience some side effects while taking tamoxifen, which could include hot flashes and vaginal discharge, and a higher risk of developing uterine cancer, blood clots and stroke. Other side effects are night sweats and vaginal dryness. In rare cases, taking tamoxifen can cause blood clots, endometrial cancer or uterine cancer, cataracts and stroke.

The risk of uterine cancer in premenopausal women taking tamoxifen is very low, compared with those who have undergone menopause. The benefits of tamoxifen outweigh the risks in premenopausal women who have an increased risk of breast cancer due to a strong family history of the disease or a personal history of precancerous breast changes.

Who can use tamoxifen?

Tamoxifen is mostly used in women aged 35 and older who have a high risk of invasive breast cancer, whether or not they have gone through menopause.

In some cases, you might not be able to take this medication, so always talk to your doctor first. You may be prescribed this medication if you have a strong family history of breast cancer or you're at high risk of developing breast cancer. For instance, if you've had a breast biopsy that found a precancerous condition such as lobular carcinoma in situ (LCIS), atypical ductal hyperplasia or atypical lobular hyperplasia; your Gail model risk score is greater than 1.66 percent (the Gail model is a tool doctors use to predict the future risk of developing breast cancer, based on factors such as age, reproductive history and family history); and you don't have a history of blood clots.

In women who have undergone menopause, the benefits of tamoxifen may outweigh the risks in women who have an increased risk of breast cancer and have also had surgery to remove the uterus (hysterectomy).

Talk to your doctor if you have a history of blood clots, stroke, or have been immobilized (unable to move around) for a long time, if you are pregnant or breast-feeding, or may become pregnant. You may not be able to use this medication.

Tamoxifen is 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 10 years after a patient stops taking tamoxifen. (kes)