Hormone therapy for breast cancer is a treatment for breast cancers that are sensitive to hormones. The most common forms of hormone therapy for breast cancer work by blocking hormones from attaching to receptors on cancer cells or by decreasing the body's production of hormones.

Hormone therapy is only used for breast cancers that are found to have receptors for the naturally occurring hormones estrogen or progesterone.

Hormone therapy for breast cancer is often used after surgery to reduce the risk that the cancer will return. Hormone therapy for breast cancer may also be used to shrink a tumor before surgery, making it more likely the cancer will be removed completely.

If your cancer has spread to other parts of your body, hormone therapy for breast cancer may help control it.

Hormone therapy for breast cancer is only used to treat cancers that are hormone sensitive (hormone receptor positive breast cancers).

Doctors refer to these cancers as estrogen receptor positive (ER positive) or progesterone receptor positive (PR positive). This means that these breast cancers are fueled by the natural hormones estrogen or progesterone.

A doctor who specializes in analyzing blood and body tissue (pathologist) determines if your cancer is ER positive or PR positive by analyzing a sample of your cancer cells to see if they have receptors for estrogen or progesterone.

Hormone therapy for breast cancer can help to:

• Prevent cancer from coming back
• Decrease the risk of cancer developing in other breast tissue
• Slow or stop the growth of cancer that has spread
• Reduce the size of a tumor prior to surgery

Side effects of hormone therapy for breast cancer include:

• Hot flashes
• Vaginal discharge
• Vaginal dryness or irritation
• Fatigue
• Nausea
• Joint and muscle pain
• Impotence in men with breast cancer

Less common, more-serious side effects of hormone therapy may include:

• Blood clots in veins
• Endometrial cancer or uterine cancer
• Cataracts
• Stroke
• Osteoporosis
• Heart disease

There are several approaches to hormone therapy.

Medications that block hormones from attaching to cancer cells

One approach to hormone therapy is to stop the hormones from attaching to the receptors on the cancer cells. When the hormones can't access the cancer cells, the tumor growth may slow and the cells may die.

Breast cancer medications that have this action include:

• Tamoxifen. Tamoxifen is usually taken daily in pill form. It's often used to reduce the risk of cancer recurrence in women who have been treated for early-stage breast cancer. In this situation, it's typically taken for five to 10 years.

  Tamoxifen may also be used to treat advanced cancer. Tamoxifen is appropriate for both premenopausal women and postmenopausal women.

• Toremifene (Fareston). Toremifene is taken as a daily pill. It's used to treat breast cancer that has spread to other areas of the body. Toremifene is approved for use in postmenopausal women.
• Fulvestrant (Faslodex). Fulvestrant is administered as a shot every month after first getting a dose every two weeks for the first month. It's used in postmenopausal women to treat advanced breast cancer.

Medications that stop the body from making estrogen after menopause

Aromatase inhibitors are a class of medicines that reduce the amount of estrogen in your body, depriving breast cancer cells of the hormones they need to grow.

Aromatase inhibitors are only used in women who have undergone menopause. They cannot be used unless your body is in natural menopause or in menopause induced by medications or removal of the ovaries.

Aromatase inhibitors used to treat breast cancer include:

• Anastrozole (Arimidex). Anastrozole is used to reduce the risk of cancer recurrence in women who have been treated for early-stage breast cancer. It can also be used to treat advanced breast cancer.
• Exemestane (Aromasin). Exemestane is used to reduce the risk of cancer recurrence in women who have been treated for early-stage breast cancer. It's sometimes used after taking tamoxifen for two or three years. It can also be used to treat advanced breast cancer in women for whom tamoxifen is no longer working.
• Letrozole (Femara). Letrozole is used to reduce the risk of cancer recurrence in women who have been treated for early-stage breast cancer. It can be used alone or given after completing tamoxifen treatment. Letrozole is also used to treat advanced breast cancer.

Aromatase inhibitors are given as pills you take once a day. All three aromatase inhibitors work the same and reduce the production of estrogen in your body.

How long you continue aromatase inhibitors depends on your specific situation. Current research suggests that the standard approach would be to take these medications for up to 10 years, but every person is different and you and your doctor should carefully assess how long you should take them.

Treatments to stop ovarian function in premenopausal women

Women who haven't undergone menopause — either naturally or as a result of cancer treatment — may opt to undergo treatment to stop their ovaries from producing hormones.

Options may include:

• Surgery to remove the ovaries (oophorectomy)
• Radiation therapy aimed at the ovaries
• Medications, such as goserelin (Zoladex)

Treatments to stop ovarian function may allow premenopausal women to take medications only available to postmenopausal women.

Combining targeted therapy with hormone therapies

Hormone therapy for cancer that spreads to other parts of the body (metastatic breast cancer) sometimes combines hormone therapies with targeted therapy. Targeted therapy drugs attack specific weaknesses in cancer cells. The combination can make hormone therapy more effective.

Medications used in this way include:

• Abemaciclib (Verzenio)
• Palbociclib (Ibrance)
• Ribociclib (Kisqali)
• Everolimus (Afinitor)

You'll meet with your cancer doctor (oncologist) regularly for follow-up visits while you're taking hormone therapy for breast cancer. Your oncologist will ask about any side effects you're experiencing. Many side effects can be controlled.

Hormone therapy following surgery, radiation or chemotherapy has been shown to reduce the risk of breast cancer recurrence in people with early-stage hormone-sensitive breast cancers. It can also effectively reduce the risk of metastatic breast cancer growth and progression in people with hormone-sensitive tumors.

Depending on your circumstances, you may undergo tests to monitor your medical situation and watch for cancer recurrence or progression while you're taking hormone therapy. Results of these tests can give your doctor an idea of how you're responding to hormone therapy, and your therapy may be adjusted accordingly.