Atypical hyperplasia, the accumulation of abnormal cells in a breast duct or lobule, is a precursor to breast cancer, and women diagnosed with atypical hyperplasia are at high risk of developing breast cancer, according to the Mayo Clinic. Tamoxifen therapy has been shown to significantly reduce risk of breast cancer among women diagnosed with atypical hyperplasia, according to a 2009 review published in the journal "Current Breast Cancer Reports." However, tamoxifen treatment has several serious side effects.
Endometrial Cancer
Patients who take tamoxifen have an elevated risk for endometrial cancer, which is a cancer that starts from the lining of the uterus. According to a 2009 review published in "Current Breast Cancer Reports," endometrial cancer was increased by approximately 2.5 fold in tamoxifen users compared to non-users.
Thromboembolic events
According to a 2008 review published in "Breast Cancer" journal, thromboembolic events were found twice as often in tamoxifen users compared to non-users, based on data from four clinical trials. Thromboembolic events refer to formation of blood clots. Two serious thromboembolic events are formation of blood clots in the lung (pulmonary embolism) and in the brain (stroke). According to the 2009 review published in "Current Breast Cancer Reports," taking tamoxifen increases the risk of having a stroke and pulmonary embolism by 39 percent and 79 percent, respectively.
Cataracts
Tamoxifen use increases the risk of developing cataracts (clouding of the lens of the eye) in postmenopausal women. Based on data from four clinical trials, tamoxifen increases the risk of having cataract surgery by 59 percent, according to a review published in "Current Breast Cancer Reports."
Other Side Effects
Other side-effects of tamoxifen include vaginal irritation, leg cramps, flu-like symptoms and hot flashes, according to the American Cancer Society.
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