The hormones estrogen and progesterone make some breast cancers grow. They are called hormone-sensitive breast cancers. Most breast cancers are sensitive to hormones.

Estrogen and progesterone are produced in the ovaries and other tissues such as fat and skin. After menopause, the ovaries stop producing these hormones. But the body continues to make a small amount.

Hormone therapy only works on hormone-sensitive cancers. To see if hormone therapy may work, doctors test a sample of the tumor that has been removed during surgery to see if the cancer might be sensitive to hormones.

Hormone therapy can work in two ways:

• By blocking the estrogen from acting on cancer cells
• By lowering estrogen levels in a woman's body

Some drugs work by blocking estrogen from causing cancer cells to grow.

Tamoxifen (Nolvadex) is a drug that prevents estrogen from telling cancer cells to grow. It has a number of benefits:

• Taking Tamoxifen for 5 years after breast cancer surgery cuts the chance of cancer coming back by half. Some studies show that taking it for 10 years may work even better.
• It reduces the risk that cancer will grow in the other breast.
• It slows the growth and shrinks cancer that has spread.
• It reduces the risk of getting cancer in women who are at high risk.

Other drugs that work in a similar way are used to treat advanced cancer that has spread:

• Toremifene (Fareston)
• Fulvestrant (Faslodex)
• Elacestrant (Ordserdu)

Some drugs, called aromatase inhibitors (AIs), stop the body from making estrogen in tissues such as fat and skin. But, these drugs do not work to make the ovaries stop making estrogen. For this reason, they are used mainly to lower estrogen levels in women who have been through menopause (postmenopausal). Their ovaries no longer make estrogen.

Premenopausal women can take AIs if they are also taking drugs that stop their ovaries from making estrogen.

Aromatase inhibitors include:

• Anastrozole (Arimidex)
• Letrozole (Femara)
• Exemestane (Aromasin)

This type of treatment only works in premenopausal women who have functioning ovaries. It can help some types of hormone therapy work better. It is also used to treat cancer that has spread.

There are three ways to lower estrogen levels from the ovaries:

• Surgery to remove the ovaries
• Radiation to damage the ovaries so they no longer function, which is permanent
• Drugs such as goserelin (Zoladex) and leuprolide (Lupron) that temporarily stop the ovaries from making estrogen

Any of these methods will put a woman into menopause. This causes symptoms of menopause:

• Hot flashes
• Night sweats
• Vaginal dryness
• Mood swings
• Depression
• Loss of interest in sex

The side effects of hormone therapy depend on the drug. Common side effects include hot flashes, night sweats, and vaginal dryness.

Some drugs can cause less common but more serious side effects, such as:

• Tamoxifen. Blood clots, stroke, cataracts, endometrial and uterine cancers, mood swings, depression, and loss of interest in sex.
• Aromatase inhibitors. High cholesterol, heart attack, bone loss, joint pain, mood swings, and depression.
• Fulvestrant. Loss of appetite, nausea, vomiting, constipation, diarrhea, stomach pain, weakness, and pain.

Deciding on hormonal therapy for breast cancer can be a complex and even difficult decision. The type of therapy you receive may depend on whether you have gone through menopause before treatment for breast cancer. It also may depend on whether you want to become pregnant. Talking with your health care provider about your options and the benefits and risks for each treatment can help you make the best decision for you.

Hormonal therapy - breast cancer; Hormone treatment - breast cancer; Endocrine therapy; Hormone-sensitive cancers - therapy; ER positive - therapy; Aromatase inhibitors - breast cancer

American Cancer Society website. Hormone therapy for breast cancer. www.cancer.org/cancer/breast-cancer/treatment/hormone-therapy-for-breast-cancer.html. Updated January 31, 2023. Accessed April 2, 2024.

Burstein HJ, Somerfield MR, Barton DL, et al. Endocrine Treatment and Targeted Therapy for Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Metastatic Breast Cancer: ASCO Guideline Update. J Clin Oncol. 2021;39(35):3959-3977. PMID: 34324367 pubmed.ncbi.nlm.nih.gov/34324367/.

Henry NL, Shah PD, Haider I, Freer PE, Jagsi R, Sabel MS. Cancer of the breast. In: Niederhuber JE, Armitage JO, Kastan MB, Doroshow JH, Tepper JE, eds. Abeloff's Clinical Oncology. 6th ed. Philadelphia, PA: Elsevier; 2020:chap 88.

National Cancer Institute website. Breast cancer treatment (PDQ®) - health professional version. www.cancer.gov/types/breast/hp/breast-treatment-pdq. Updated January 19, 2024. Accessed July 3, 2024.

Updated by: Todd Gersten, MD, Hematology/Oncology, Florida Cancer Specialists & Research Institute, Wellington, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.