Letrozole is an oral non-steroidal aromatase inhibitor primarily used to treat hormone receptor-positive breast cancer in postmenopausal women. It works by blocking the enzyme aromatase, which converts androgens into estrogens, thereby significantly reducing estrogen production in the body. This mechanism helps suppress the growth of estrogen-dependent breast tumors, making it effective for early-stage breast cancer after tamoxifen therapy, as well as advanced or metastatic cases. Letrozole is a third-generation inhibitor that minimally impacts cortisol, aldosterone, or thyroxine levels. It is also studied for other estrogen-sensitive cancers and sometimes used off-label in fertility treatments to induce ovulation.