Taking tamoxifen may increase your risk of uterine cancer, stroke, or a blood clot in the lung, which can be fatal. Talk with your doctor about your specific risks in taking tamoxifen. FDA pregnancy category D. This medication can cause harm to an unborn baby. Do not use tamoxifen without your doctor?s consent if you are pregnant. Tell your doctor if you become pregnant during treatment. Use a non-hormonal of birth control (such as a condom or a diaphragm and spermicide) while you are using tamoxifen and for 2 months after your treatment ends. It is not known whether tamoxifen passes into breast milk or if it could harm a nursing baby. This medication may also slow breast milk production. Do not use tamoxifen without telling your doctor if you are breast-feeding a baby.