The synthetic hormone tamoxifen is used in breast cancer therapies to prevent recurrence of disease or to prevent onset in women at high risk for this cancer, but it may increase the risk of endometrial cancer, strokes, and blood clots.

DES (diethylstilbestrol) is another synthetic hormone that was prescribed to pregnant women in the 1940s, 1950s, and 1960s. DES use was discontinued after scientists discovered that women taking it had an increased risk of breast cancer, and that girls born to women taking DES had an increased risk of rare types of vaginal and cervical cancer. Most physical or structural differences associated with exposure to DES are found in the reproductive tract, including a "hood" or collar on the cervix and a T-shaped uterus.

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