Some cancers grow in response to natural hormones: most breast cancers are estrogen- and/or progesterone-receptor positive, and prostate cancer is driven by androgens (like testosterone). Hormone therapy either lowers the amount of the hormone in the body or blocks its effect on the cancer. In breast cancer this includes tamoxifen (blocks estrogen receptors) and aromatase inhibitors (cut estrogen production). In prostate cancer, androgen-deprivation therapy (ADT) and androgen-receptor blockers reduce or block testosterone. It is used to shrink tumors, prevent recurrence after surgery, or control advanced disease, often for years.
As daily pills (e.g., tamoxifen, aromatase inhibitors, androgen-receptor blockers) and/or injections/implants that suppress hormone production (e.g., GnRH agonists for prostate or premenopausal breast cancer). Often continued for years. In some cases surgery to remove hormone-producing organs (ovaries or testicles) achieves the same effect.
Because it is usually taken long-term, tolerability matters. Menopausal-type symptoms (breast) or reduced-testosterone effects (prostate) are common. Bone health is monitored, since several of these drugs can weaken bone.