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Tamoxifen is available in 10 mg and 20 mg tablets. When used for early-stage breast cancer, tamoxifen is usually prescribed at 20 mg a day for 5 years. Tablets may be taken once or twice a day.

How It Works

Tamoxifen is the most commonly used hormone therapy for the treatment of breast 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 of developing the 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.

In rare cases, tamoxifen is used to treat breast pain (mastalgia), because it reduces estrogen levels that cause breast swelling.5 This is an unlabelled use.

How Well It Works

When used by itself, tamoxifen is most effective when taken for 5 years.2 Even after you stop taking tamoxifen, it still helps protect against breast cancer for several more years.3

Tamoxifen is highly effective in lowering the risk of breast cancer recurrence. In women who have already had breast cancer, tamoxifen lowers the risk of breast cancer in the opposite breast (contralateral) by 40% to 50%.2

Newer research shows that for post-menopausal women, a two-stage treatment is more effective than tamoxifen only. After 2 to 3 years of tamoxifen, you take an aromatase inhibitor, such as anastrozole (Arimidex), exemestane (Aromasin), or letrozole (Femara).4

When used to prevent breast cancer among high-risk women, tamoxifen lowers the risk of breast cancer about as much as raloxifene does.1

Tamoxifen is sometimes prescribed for women with breast pain (mastalgia). This is an off-label use. In one study, tamoxifen relieved pain in more than two-thirds of women who had a history of severe breast pain. Experts disagree about the use of tamoxifen for breast pain, because it has important side effects and risks.5

Side Effects

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

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

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 exams 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. Discuss birth control with your doctor if you are taking tamoxifen and could become pregnant.

Complete the new medication information form (PDF) (What is a PDF document?) to help you understand this medication.



  1. Vogel VG, et al. (2006). Effects of tamoxifen vs raloxifene on the risk of developing invasive breast cancer and other disease outcomes: The NASBP study of tamoxifen and raloxifene (STAR) P-2 trial. JAMA, 295(23): 2727–2741.
  2. Smith IE, Chua S (2006). ABC of breast diseases. Medical treatment of early breast cancer. I: Adjuvant treatment. BMJ, 332(7532): 34–37.
  3. Cuzick J, et al. (2007). Long-term results of tamoxifen prophylaxis for breast cancer—96-month follow-up of the randomized IBIS-I trial. Journal of the National Cancer Institute, 99(4): 272–282.
  4. Jonat W, et al. (2006). Effectiveness of switching from adjuvant tamoxifen to anastrozole in postmenopausal women with hormone-sensitive early-stage breast cancer: A meta-analysis. Lancet Oncology, 7(12): 991–996.
  5. Fentiman IS (2004). Management of breast pain. In JR Harris et al., eds., Diseases of the Breast, 3rd ed., pp. 57–62. Philadelphia: Lippincott Williams and Wilkins.


By Healthwise Staff
Primary Medical Reviewer Joy Melnikow, MD, MPH - Family Medicine
Specialist Medical Reviewer Douglas A. Stewart, MD - Medical Oncology
Last Revised October 26, 2009

This information does not replace the advice of a doctor. Healthwise, Incorporated disclaims any warranty or liability for your use of this information.