What is tamoxifen?

Tamoxifen (Nolvadex®) is a medication in pill form that has been used for more than 25 years to treat breast cancer in women and men. Tamoxifen is one of the most common endocrine therapy drugs. It has been shown to decrease the chance of recurrence in some early-stage breast cancers and to prevent the development of cancer in the opposite breast. Tamoxifen can also slow or stop the growth of cancer cells present in the body.

There are an estimated 29 million women at increased risk for breast cancer in this country, and tamoxifen may offer another alternative to watchful waiting or prophylactic (preventative) mastectomy.

How does tamoxifen affect breast cancer?

Tamoxifen is classified as a selective estrogen receptor modulator (SERM) and works as an anti-estrogen: While the hormone estrogen promotes the growth of breast cancer cells, tamoxifen works by blocking estrogen from attaching to estrogen receptors on these cells. By blocking the estrogen receptors, it is believed that the growth of the breast cancer cells will be halted.

Can tamoxifen prevent breast cancer?

In 1998, the National Cancer Institute (NCI) funded a large research study designed to determine whether tamoxifen would reduce the incidence of breast cancer in healthy women known to be at high risk. The National Surgical Adjuvant Breast and Bowel Project (NSABP) conducted the study, known as the Breast Cancer Prevention Trial (BCPT). The results of the trial showed a 50 percent reduction in both invasive and non-invasive breast cancer in the treated women.

Tamoxifen has also been shown to substantially reduce the risk of developing invasive breast cancer in women who have had ductal carcinoma in situ (DCIS). DCIS is a group of cancer cells that involve only the lining of a duct and have not spread to other tissues in the breast.

When is tamoxifen prescribed?

Tamoxifen should be prescribed only for women at high risk for breast cancer and after a complete medical evaluation of a woman's individual risk factors. These risk factors include the woman's age, personal health history and family history of breast cancer.

However, tamoxifen may not be suitable for all women at high risk of developing the disease. Women who should definitely not use tamoxifen include pregnant women, women planning to become pregnant, women with a history of blood clots or stroke or those who are taking anticoagulants. An informed discussion between a woman and her physician is essential in determining the appropriateness of this treatment option.

How long should a woman take tamoxifen?

For reducing the risk of developing breast cancer in high-risk women and in women with DCIS, tamoxifen is generally prescribed for 5 years.

Studies have confirmed that taking adjuvant tamoxifen for 5-10 years reduces the risk of recurrence of the original breast cancer as well as the risk of developing a second primary cancer in the other breast. (Adjuvant treatment is a treatment used to decrease the chances of a recurrence or spread of disease.)

The length of time a woman with metastatic breast cancer can take tamoxifen depends on their response to the treatment as well as other factors.

Does tamoxifen have any other beneficial effects?

Because tamoxifen acts like estrogen in tissue other than breast tissue, women who take tamoxifen may derive some of the benefits of estrogen replacement therapy, such as lowered blood cholesterol. Tamoxifen may also increase bone density, which can slow the development of osteoporosis (bone loss) and reduce the incidence of bone fractures.

What are some side effects I might experience when taking tamoxifen?

For women, the side effects of tamoxifen are similar to some of the symptoms of menopause. Two of the most common side effects are hot flashes and vaginal discharge. The information below does not include all possible side effects, and not all patients who take tamoxifen will have these symptoms. Always contact your doctor if you have questions about your personal situation.

Side effects in women may include:

  • Hot flashes
  • Vaginal discharge
  • Vaginal dryness and/or itching
  • Irritation of the skin around the vagina
  • Irregular menstrual periods
  • Headache
  • Nausea and/or vomiting
  • Skin rash
  • Fatigue
  • Fluid retention and/or weight gain

Side effects in men may include:

  • Hot flashes
  • Headache
  • Nausea and/or vomiting
  • Skin rash
  • Impotence
  • Decrease in sexual interest

Will taking tamoxifen bring on early onset of menopause?

If you are premenopausal and are taking tamoxifen, you may experience menstrual irregularities, vaginal bleeding or cessation of menstrual periods entirely. Depending on your age your periods may resume during or after completion of your tamoxifen course. Please discuss your personal healthcare concerns with your healthcare provider.

What are some precautions and recommendations I should know when taking this medication?

Tamoxifen may affect fertility, so it is important to use some form of birth control while you are taking this medication. However, do not use oral contraceptives (the "pill") since they may change the effects of tamoxifen. Tell your doctor right away if you think you may have become pregnant while taking this medication.

Take anti-nausea medications only as prescribed by your doctor. Ask your doctor which over-the-counter medications you can take.

Are there any other concerns I should be aware of while taking tamoxifen?

According to data from large treatment studies as well as the BCPT, women taking tamoxifen may have a slightly increased risk of developing blood clots in the lungs or large veins. This may be especially true for women undergoing chemotherapy (anti-cancer drugs) while taking tamoxifen. Women in the BCPT also had an increased risk of stroke. Additional risks may include:

Endometrial cancer: Tamoxifen may increase a woman's risk of developing endometrial cancer (cancer of the lining of the uterus). However, this risk is less or the same as the risk of postmenopausal women taking single-agent estrogen replacement therapy.

Cataracts: Taking tamoxifen appears to put some women at increased risk for developing cataracts, a clouding of the lens inside the eye. A few patients have also reported eye problems such as corneal scarring or retinal changes.

When should I call my doctor?

Call your doctor immediately if you have a fever of 100° F or higher. You should also call your doctor if you have any questions or concerns or if you experience any of the following:

  • Inability to eat or to drink fluids for 24 hours
  • Uncontrolled vomiting
  • Blurry vision
  • Chest pain
  • Shortness of breath
  • Calf swelling and/or tenderness
  • Vaginal bleeding
  • Any other unusual symptoms

Is tamoxifen used as part of chemotherapy for breast cancer?

Studies have shown that a combination of tamoxifen and chemotherapy drugs after surgery for breast cancer may increase survival rates for some patients. Generally speaking, if both chemotherapy and tamoxifen is appropriate, a woman will receive chemotherapy and then tamoxifen (these are generally not started at the exact same time).

Are there other hormonal therapy medications that can prevent breast cancer?

Other drugs are also available for use in postmenopausal patients with advanced breast cancer. Anastrozole (Arimidex®) and letrozole (Femara®) are among a different class of drugs than tamoxifen called aromatase inhibitors (AIs). Aromatase is an enzyme that helps the body produce estrogen in the fat tissue and adrenal glands. While tamoxifen blocks estrogen receptors, AIs block the action of aromatase. Another drug, exemestane (Aromasin®), is similar to the AIs (and is often included in discussion of AI’s) but is an aromatase inactivator. Instead of blocking the aromatase, aromatase inactivators actually destroy the enzyme. Both tamoxifen and AIs are only effective for treating cancers that use hormones to help the tumors grow.

Are there other SERMs that can prevent breast cancer?

Studies have shown that raloxifene, a SERM (selective estrogen receptor modulator) marketed as Evista®, can also reduce one's risk for breast cancer. It was approved by the FDA for the following very specific indication:

  • For the reduction in risk of invasive breast cancer in postmenopausal women at high risk for invasive breast cancer.

It is important to understand that Evista did NOT receive approval for treatment of invasive breast cancer, reduction of the risk of recurrent breast cancer, or reduction of the risk of noninvasive breast cancer.

Last reviewed by a Cleveland Clinic medical professional on 03/21/2014.


Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy