N-Desmethyltamoxifen is primarily used in the field of medical research and treatment, particularly in the study and management of breast cancer. As an active metabolite of tamoxifen, it plays a crucial role in hormone therapy by acting as an estrogen receptor modulator. It helps in blocking the effects of estrogen in breast tissue, which is essential in preventing the growth of estrogen receptor-positive breast cancer cells. Additionally, it is often monitored in clinical settings to assess patient adherence to tamoxifen therapy and to evaluate the effectiveness of the treatment. Its presence and concentration in the body can provide valuable insights into the pharmacokinetics and therapeutic outcomes of tamoxifen-based treatments.