Background

Hormonal therapy, also called endocrine therapy, for hormone receptor-positive breast cancer is an adjuvant therapy, which is treatment given after surgery, chemotherapy, and/or radiation therapy. The goal of adjuvant therapy is to lower the risk of recurrence (cancer that comes back after treatment). Hormone receptor-positive breast cancer means that a patient's breast cancer depends on hormones called estrogen and/or progesterone to grow. About 75% to 80% of breast cancers are hormone receptor-positive. Common hormonal therapy for hormone receptor-positive breast cancer includes:

• Aromatase inhibitors (AIs), which reduce the amount of estrogen in a woman's body by stopping tissues and organs other than the ovaries from producing estrogen. In women who have gone through menopause, AIs can further reduce estrogen levels by more than 90%. AIs are not used for premenopausal women (those who have not gone fully into menopause). These drugs are known as anastrozole (Arimidex), exemestane (Aromasin), and letrozole (Femara). All work the same way even though they are three different brands, and research suggests that they are equally effective and have similar side effects.
• Tamoxifen, which blocks the effects of estrogen on tumor growth and has been proven to lower the risk of breast cancer recurrence and improve survival for women with early-stage breast cancer. Tamoxifen has been studied for over 30 years, and the benefits and side effects are well known. It is effective for women who have gone through menopause and those who have not.

Both AIs and tamoxifen have specific side effects, but neither appears to have fewer or more side effects. The side effects of AIs may include increased cholesterol, heart disease, weakening bones and bone breaks, and bone and joint pain or stiffness. The side effects of tamoxifen include blood clots, increased risk of uterine cancer, and hot flashes.