Nolvadex-D (tamoxifen citrate) 20 mg is a selective estrogen receptor modulator (SERM) commonly used in the treatment and prevention of breast cancer. Tamoxifen works by blocking estrogen receptors on breast cancer cells, helping slow or stop the growth of cancerous cells that depend on estrogen. It is often prescribed as part of hormone therapy for patients with estrogen receptor-positive (ER+) breast cancer and may also be used to reduce the risk of breast cancer recurrence or as a preventive measure in high-risk individuals.