Hormone therapy is used to treat cancers that are hormone receptor-positive (meaning hormone receptors are present in the cells). Breast cancers can be estrogen receptor-positive (ER-positive), progesterone receptor-positive (PR-positive), or both.

The female hormone estrogen can fuel cancer growth. Several hormone therapy drugs block the effects of estrogen or lower its levels. Common side effects for these drugs include hot flashes, mild nausea, mood swings, tiredness, and vaginal discharge.

Hormone therapy is also used to reduce the risk of the cancer returning after surgery and to treat advanced-stage cancer.

Hormone therapy drugs used to alter hormone levels are called aromatase inhibitors (AIs). The ovaries produce most of the body’s estrogen before menopause. After menopause, fat tissue produces a small amount of estrogen. Aromatase inhibitors stop fat tissue from making estrogen. These drugs are only useful if the ovaries are no longer making estrogen (i.e., after menopause). They work to lessen the chances of cancer recurring in women who have already gone through menopause. Advanced breast cancers can also be treated using aromatase inhibitors.

Possible side effects associated with AIs include muscle or joint pain and bone thinning. Sometimes switching from one AI to another can relieve side effects. Women who are unable to tolerate the side effects of AIs are often switched to one of the estrogen-blocking drugs.

Luteinizing hormone-releasing hormone (LHRH) analogs are used to shut down the ovaries to lower estrogen levels in premenopausal women. LHRH analogs are mostly used to aid other hormone therapy drugs, though they can be used to treat advanced hormone receptor-positive breast cancers on their own. Possible side effects are similar to those of menopause, including hot flashes, mood swings, night sweats, and vaginal dryness.

Surgery to remove the ovaries can be performed in place of using LHRH analogs to lower estrogen levels in women who have not gone through menopause.