Tamoxifen for Breast Cancer Treatment, Prevention

Tamoxifen is a medication that blocks the female hormone estrogen in the body. For more than 30 years, doctors have prescribed it to fight breast cancer tumors that depend on estrogen to grow.

How does tamoxifen work?

Scientists aren’t sure exactly how the drug works. But they do know that some breast cancer cells are sensitive to estrogen. They need it to grow and spread. In order to fuel that growth, estrogen must attach itself to those breast cancer cells.

Doctors think that tamoxifen stops estrogen from attaching to the cell. No estrogen means no growth for these types of breast cancer cells.

Why do doctors prescribe tamoxifen?

There are different reasons, including:

In higher than normal doses, tamoxifen may even kill some breast cancer cells that don’t depend on estrogen.

How does tamoxifen prevent breast cancer?

Women who have no history of breast cancer can take tamoxifen as a way to protect them from getting the disease. This drug reduces the risk of breast cancer in two ways:

  1. It blocks estrogen from working on cancer cells that are sensitive to the hormone.
  2. It blocks estrogen from acting on cells that are not cancerous.

Tamoxifen also keeps healthy breast cells from growing and multiplying. That way, it also reduces the number of cells that could become cancer.

In a study of more than 13,000 women at high risk for breast cancer, scientists found that women who took tamoxifen for 5 years had fewer breast cancers than women who took a placebo. Newer research suggests that taking the drug for 10 years lowers the risk even more.

Another study by the National Cancer Institute found about a 50% reduction in breast cancer cases among women at high risk who took tamoxifen. The study also found another plus for the medicine -- it reduced the chance of fractures of the hip, wrist, and spine from the bone-thinning condition osteoporosis.


What are the side effects of tamoxifen?

When a woman takes tamoxifen to treat breast cancer, most doctors agree that the benefits of the drug are much greater than the possible risks of taking it. Some experts believe that women who take it to prevent the disease should think carefully about the pros and the cons, like the cost of the drug and its side effects, which can be serious.

Side effects of tamoxifen include:

Milder side effects include symptoms that are similar to menopause, such as:

Tamoxifen may also cause irregular periods and sexual problems.

If you’ve been diagnosed with breast cancer or you’re at high risk for the disease, talk with your doctor to see if tamoxifen is right for you. Together, you can weigh the benefits and risks of this drug to make sure you get the healthiest result.

WebMD Medical Reference Reviewed by Melinda Ratini, DO, MS on June 27, 2017



National Cancer Institute: "Tamoxifen: Questions and Answers."

American Cancer Society: "Medicines to Reduce Cancer Risk."

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