How Does Tamoxifen Work?

Clinically reviewed by Dr. Megan May

Tamoxifen is a hormone therapy doctors use to treat breast cancer. Specifically, it’s used to treat what’s called hormone receptor-positive (or simply hormone-positive) breast cancer. 

Tamoxifen is very effective in reducing the risk that a patient’s cancer will return or spread. Doctors also prescribe daily oral tamoxifen in some instances to lower a person’s risk of developing breast cancer. 

Preventing Cancer from Accessing Hormones

Hormone-positive breast cancers need hormones to grow and multiply. Tamoxifen works as a selective estrogen receptor modulator (SERM). 

The medication attaches to specific proteins in breast cancer cells called hormone receptors. This prevents the cells from getting the hormones they require.

Approved by the Food and Drug Administration (FDA) in 1977, tamoxifen has been very effective in the millions of patients who have received it. For example, studies suggest that it:

• Lowers the risk of breast cancer in the opposite breast by 50%
• Reduces the risk of an initial breast cancer diagnosis by as much as 40%
• Lowers the chance of cancer recurrence in premenopausal women by 30% to 50% and postmenopausal women by 40% to 50%
• Reduces the risk of a diagnosed cancer becoming invasive by up to 50%

Tamoxifen Side Effects and Risks

Like most medications and treatments, tamoxifen has side effects. It also increases the risk of certain medical problems. The side effects include:

• Fatigue
• Nausea
• Swelling in the legs (fluid retention)
• Changes to or loss of menstrual periods
• Hot flashes, vaginal dryness, vaginal bleeding, night sweats, and other symptoms like those in menopause
• Skin rash
• Vaginal discharge

Tamoxifen may also increase the risk of:

• Cataracts and other eye problems
• Strokes
• Uterine cancer
• Blood clots and deep vein thrombosis (DVT)

In addition, tamoxifen can cause birth defects. People who are pregnant or planning to become pregnant shouldn’t take tamoxifen. Your doctor can advise you on non-hormonal birth control options. You must use contraception while taking tamoxifen and for a few months after stopping the medication.

How Long Can You Take Tamoxifen?

Doctors typically prescribe tamoxifen for at least five years. They may prescribe it for up to 10 years, depending on the characteristics of the cancer being addressed.

Patients take tamoxifen — in pill or liquid form — daily by mouth. Ideally, they take it at the same time each day. 

Other Benefits of Tamoxifen

Taking tamoxifen may lower your risk of osteoporosis (bone loss) after menopause.

Learn About Cancer Care at Baptist Health

Baptist Health provides world-class cancer care. If you’ve been diagnosed with breast cancer or have questions about tamoxifen, we’re here for you! 

The same is true for any type of cancer. Being diagnosed with cancer can be frightening. But meeting with a Baptist Health cancer specialist, receiving a treatment plan, and getting support from a skilled and compassionate care team helps take much of the fear out of the situation for you and your loved ones.