Tamoxifen tablets are often used as an adjuvant treatment for breast cancer after surgery, ovarian cancer, and endometrial cancer, and their main effect is to competitively antagonize estradiol. Tamoxifen tablets can cause some adverse effects in women, the most common being gastrointestinal reactions such as nausea and dry heaving. Tamoxifen tablets can also cause water and sodium retention in the body, leading to weight gain. So taking tamoxifen tablets may cause women to gain weight, but it is simply weight gain, not increased fatty tissue deposits.