The Women's Health Resource. On the web since 1997.

Basic Information On Tamoxifen

What is Tamoxifen?

Tamoxifen is a drug taken orally in pill form. For over a quarter of a century, physicians have prescribed tamoxifen to help treat patients with advanced breast cancer. In the 1990s, physicians began using tamoxifen to treat early stage breast cancer after breast surgery (lumpectomy or mastectomy). Tamoxifen has been shown to help prevent the original breast cancer from returning after breast surgery while also hindering the development of new cancers in the opposite breast.

In late 1998, tamoxifen became the first drug to be approved by the U.S. Food and Drug Administration (FDA) to prevent breast cancer after research showed it reduced the chances of developing breast cancer by 50% in women at high risk.

How Does Tamoxifen Work?

To grow and reproduce, breast cancer cells require the female hormone estrogen. Tamoxifen is an "anti-estrogen" and works by competing with estrogen to bind to estrogen receptors in breast cancer cells. Tamoxifen is formally known as a selective estrogen receptor modulator (SERM). By blocking estrogen in the breast, tamoxifen helps slow the growth and reproduction of breast cancer cells.

While tamoxifen fights estrogen in breast cancer cells, it also mimics the positive effects of estrogen in other body systems. Post-menopausal women who take tamoxifen may decrease their risk of heart disease or osteoporosis (a degenerative bone disease) without having to use hormone replacement therapy (HRT).

Who is a Candidate for Tamoxifen Therapy?

A woman's physician will determine whether she is a candidate for treatment with tamoxifen. Patients who have been treated with tamoxifen include:

  • Women with Stage III or Stage IV breast cancer (tamoxifen is usually prescribed in conjunction with chemotherapy or other treatment)
  • Women with early stage breast cancer after breast surgery (lumpectomy or mastectomy)
  • Women who have been identified to be at high risk of developing breast cancer

Tamoxifen has been shown to be more beneficial for women whose breast cancer cells contain estrogen receptors (ER). An "ER-positive cell" (one that has estrogen receptors) may contain hundreds or thousands of estrogen receptors near the cell nucleus where the genetic information is stored. Because tamoxifen works by binding to estrogen receptors in cells thus blocking the body's own supply of estrogen from reaching the receptors, women with ER-positive receptors typically respond better to tamoxifen therapy than women with ER-negative receptors. A tissue sample obtained from breast biopsy may be analyzed to determine whether cancer cells are ER-positive or ER-negative. Note: the absence of estrogen receptors in breast cancer cells does not necessarily mean that tamoxifen will be ineffective. Researchers are still investigating how tamoxifen acts on ER-negative cells.

Several studies have also shown the benefit of tamoxifen in preventing breast cancer is age-related. Women under 40 years of age typically show the least response to tamoxifen, women between the ages of 40 and 50 years of age show some response, and women over 50 typically benefit most from tamoxifen. In fact, early-stage breast cancer patients over 50 years of age who are treated with tamoxifen may be able to significantly reduce (or sometimes completely eliminate) the size of their breast tumors so that surgeons may perform lumpectomy instead of mastectomy. Researchers are not certain why young women do not respond as well to tamoxifen but believe it may be related to the amount of estrogen produced in their body. When a woman reaches menopause (typically around 50 years of age), her body stops producing estrogen.

Typical profiles of women who take tamoxifen to help prevent breast cancer:

Age 35 or older and any of the following combination of factors:

  • One first degree relative (such as mother or sister) with a history of breast cancer, 2 or more benign biopsies, and a history of a breast biopsy showing atypical hyperplasia
  • At least 2 first degree relatives with a history of breast cancer, and a personal history of at least one breast biopsy
  • Lobular carcinoma in situ (LCIS, also called lobular neoplasia), a condition that significantly increases the risk for breast cancer

Age 40 or older and any of the following combination of factors:

  • One first degree relative with a history of breast cancer; 2 or more benign biopsies, age 25 or older at first live birth, and age 11 or younger at menarche (first menstruation)
  • At least 2 first degree relatives with a history of breast cancer, and age 19 or younger at first live birth
  • One first degree relative with a history of breast cancer, and a personal history of breast biopsy showing atypical hyperplasia

Age 45 or older and any of the following combination of factors:

  • At least 2 first degree relatives with a history of breast cancer and age 24 or younger at first live birth; or
  • One first degree relative with a history of breast cancer with a personal history of a benign breast biopsy, age 11 or younger at menarche and age 20 or older at first live birth.

Age 50 older and any of the following combination of factors:

  • At least 2 first degree relative with a history of breast cancer; or
  • History of one breast biopsy showing atypical hyperplasia, and age 30 or older at first live birth and age 11 or younger at menarche
  • History of at least two breast biopsies with a history of atypical hyperplasia, and age 30 or older at first live birth.

Age 55 or older and any of the following combination of factors:

  • One first degree relative with a history of breast cancer with a personal history of benign breast biopsy, and age 11 or younger at menarche; or
  • History of at least 2 breast biopsies with a history of atypical hyperplasia, and age 20 or older at first live birth.

Age 60 or older and:

  • Five-year predicted risk of breast cancer 1.67%, as calculated by the Gail Model (the Gail Model was developed by the National Cancer Institute and researchers from the National Surgical Adjuvant Breast and Bowel Project). Click here for a patient version of the Breast Cancer Risk Assessment Tool.

Source: AstraZeneca, manufacturer of Nolvadex (brand name of tamoxifen)