ASCO Updates Guideline on Length of Tamoxifen Treatment

Breast Cancer News
September 23, 2014
Robin Warshaw, Contributing Writer
Reviewed By: 
Robert A. Somer, MD

After evaluating long-term results from recent studies, the American Society of Clinical Oncology, ASCO, issued a guideline update for tamoxifen therapy for hormone receptor-positive breast cancer.

The update advises doctors to offer up to 10 years of tamoxifen treatment, instead of the previous standard 5-year duration.


Tamoxifen, a hormonal therapy, is used to lower the risk of hormone receptor-positive breast cancer recurrence, or cancer returning. It is usually taken after surgery and chemotherapy (if needed), a use called adjuvant treatment.

Women who are premenopausal, or still have their monthly period, and perimenopausal, or have their period only sometimes, are often given tamoxifen. If a woman starts tamoxifen but then permanently stops having her period and becomes postmenopausal, she may switch to an aromatase inhibitor, another form of anti-estrogen therapy.

For decades, tamoxifen was given for 5 years after finishing other treatment. That length of time was supported by many studies and earlier ASCO guidelines that recommended tamoxifen for pre-and perimenopausal women, and AIs for postmenopausal women. More recently, postmenopausal women have been able to take an aromatase inhibitor instead of tamoxifen, but they can take either medicine. AIs do not work in premenopausal women.

Though early studies found no benefit to taking tamoxifen for more than the standard 5 years, recent studies found that taking tamoxifen for 10 years lowered recurrence rates, reduced breast cancer risk in the opposite breast, and improved survival.

Why and How the Guideline was Updated

ASCO guidelines give doctors the most up-to-date information and treatment recommendations. To update guidelines, a committee of experts reviews recent research and determines whether changes are needed.

The studies reviewed for this guideline showed no new serious side effects when women took tamoxifen for more than 5 years. Some, such as cancer in the uterus, were more common with extended therapy, but deaths remained rare. Quality-of-life side effects, such as menopausal symptoms, were not measured.

This update focuses only on tamoxifen treatment for longer than 5 years. It changes some of the advice in ASCO’s 2010 guideline on hormonal therapy.


For women with stage I to III, hormone receptor-positive breast cancer who are pre- or perimenopausal when they start hormonal therapy, the new guideline recommends tamoxifen for the first 5 years, then either tamoxifen or an AI for up to a total of 10 years, depending on menopausal status. It recommends

  • women pre- or perimenopausal after 5 years should be offered continued tamoxifen for up to 10 years; women postmenopausal after 5 years should be offered continued tamoxifen for up to 10 years, or switch to up to 5 years of an AI
  • if menopausal status can’t be determined, women should be given up to 10 years of tamoxifen

If postmenopausal at start of hormonal therapy, women should be offered

  • tamoxifen for 10 years, or
  • AI for 5 years, or
  • tamoxifen for 5 years, then switch to AI for up to 5 years, or
  • tamoxifen for 2 to 3 years, then switch to AI for up to 5 years

The guideline does not recommend that AIs be taken for more than 5 years, but future research will examine longer AI duration.

What This Means for You

Your treatment plan may include tamoxifen after your primary therapy is completed. If you are already on tamoxifen, talk with your oncologist about whether this update might affect how long you stay on the medicine. The benefit of extended therapy may depend on your individual diagnosis and treatments you have had. You may also want to consider if you want to become pregnant in the future. Pregnancy is not advised while on tamoxifen because it can damage the fetus.

If you have already completed five years of tamoxifen, ask your doctor if you should consider resuming it now.

Be sure to let your healthcare providers know about any side effects you may experience from tamoxifen or AIs, especially those that significantly disrupt your quality of life. Your doctor or nurse should be able to help with those side effects so you can continue to take hormonal therapy to reduce your risk of recurrence.

Read this Insight article to learn more about tamoxifen and how women reacted to a large 2012 study showing benefit in extending therapy for up to 10 years.

Burstein, HJ, Temin, S, Anderson, H et al. Adjuvant Endocrine Therapy for Women With Hormone Receptor-Positive Breast Cancer: American Society of Clinical Oncology Clinical Practice Guideline Focused UpdateJournal of Clinical Oncology, 2014; DOI: 10.1200/JCO.2013.54.2258

This article was supported by Cooperative Agreement Number DP11-1111 from The Centers for Disease Control and Prevention. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the Centers for Disease Control and Prevention

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Hormone Receptor-Positive