Tamoxifen

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

Risks

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).