Drug Information




Generic Name

How It Works

Tamoxifen is the most commonly used hormone therapy  for the treatment ofbreast cancer .

Many women have breast cancer that tests positive for estrogen receptors (ER+) . This means that estrogen  promotes the growth of the breast cancer cells. Tamoxifen blocks the effects of estrogen on these cells. It is often called an “anti-estrogen.”

Tamoxifen slows or stops the growth of cancer cells that are already present in the body. It helps keep the original breast cancer from coming back and helps prevent new cancer in the opposite breast. It also reduces the risk of breast cancer in women who have a high risk for this disease.

Why It Is Used

Tamoxifen is used to prevent and treat breast cancers that test positive for estrogen receptors (ER+). It blocks the effects that the hormone estrogen has on cancer cells and lowers the chance that breast cancer will grow. Tamoxifen:

  • Has been used for decades to treat patients with advanced breast cancer.
  • Is used after surgery or radiation therapy  for early-stage breast cancer.
  • Is used to prevent breast cancer in women who are high-risk for ER+ breast cancer.
  • Can be used by women of all ages, both before and after menopause .
  • Is being studied for the treatment of other types of cancer.
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In rare cases, tamoxifen is used to treat breast pain (mastalgia), because it reduces estrogen levels that cause breast swelling. This is an unlabelled use .

How Well It Works

Tamoxifen is highly effective in lowering the risk of breast cancer recurrence. In women who have already had breast cancer, tamoxifen also lowers the risk of breast cancer in the opposite breast (contralateral).

For post-menopausal  women, a two-stage treatment using tamoxifen and then an aromatase inhibitor, such as anastrozole (Arimidex), exemestane (Aromasin), or letrozole (Femara) may work better than only taking tamoxifen.

Side Effects

The most common side effects caused by tamoxifen are hot flashes; vaginal dryness, discharge, or irritation; and reduced interest in sex. These side effects are not usually serious, but they can be bothersome.

Other side effects are rare but are more dangerous. These include:

  • Overgrowth of the lining of the uterus ( endometrial hyperplasia ) and cancer of the lining of the uterus ( endometrial cancer ).
  • An increased risk of blood clots in the legs ( deep vein thrombosis ) and the lungs ( pulmonary embolism ). Changes in the blood’s ability to clot have been reported in patients receiving tamoxifen.
  • A small increased chance of stroke .
  • Ovarian cysts.
  • An increased risk of cataract  formation and the need for surgery for cataracts.
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See Drug Reference for a full list of side effects. (Drug Reference is not available in all systems.)

What To Think About

Because tamoxifen can cause changes in the lining of the uterus, women who use it should have yearly pelvic examinations and should be evaluated further if they experience any abnormal uterine bleeding.

Tamoxifen may not work as well if a woman is also taking some types of medicine to treat hot flashes or depression. If you take tamoxifen, talk with your doctor about any other medicines you are taking.

Tamoxifen should not be used if you are pregnant or breast-feeding. Talk with your doctor about birth control if you are taking tamoxifen and could become pregnant.

Last Revised: June 7, 2013

Author: Healthwise Staff

Medical Review: Sarah Marshall, MD – Family Medicine & Anne C. Poinier, MD – Internal Medicine & Kirtly Jones, MD – Obstetrics and Gynecology