Hi Thanks for choosing HCM for your query. I appreciate your concern towards the safety of Tamoxifen. The use of Tamoxifen in the treatment of breast cancer is associated with weight gain due to retention of salt and water in the body caused by the hormonal action of estrogen. As a matter of fact, Tamoxifen has anti estrogenic effect only on breast tissues which express estrogen receptor overactivity. On the other tissues or organs it behaves like an estrogen hormone. So weight gain is a metabolic effect seen with the use of Tamoxifen in addition to uterine fibroids, thromboembolism, hypertension, etc. After discontinuing Tamoxifen, it may take some time to get rid of its side effects. However, you are advised to visit your doctor if the weight continues to increase even after stopping Tamoxifen. Hope your doubts are clarified. kindly rate the answer if you are convinced.