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Common Breast Cancer Medication Is Increasingly Effective Over Time

Tamoxifen has been found to further reduce breast cancer recurrence and mortality over the course of 10 years.

--by Michael Harkin The molecular structure of tamoxifen

The Gist

Tamoxifen is one of the most widely used treatments for ER-positive breast cancer. It is typically prescribed for five years after surgical removal of the cancer, and is effective at reducing the mortality rate during treatment and for 15 years after diagnosis. However, it was previously unknown whether continuing treatment beyond the five-year mark would help further reduce breast cancer’s recurrence rate during that 15-year period.

New findings published today in The Lancet by the researchers behind the ATLAS (Adjuvant Tamoxifen Longer Against Shorter) trial show that 10 years of tamoxifen treatment reduces the recurrence rate and death rate of breast cancer even further than five years of treatment. However, side effects of the medication were found to increase slightly over this time. That said, the researchers consider these side effects to be relatively negligible in light of the increased benefits of longer-term treatment.

The Expert Take

In a telephone interview with Healthline, Sir Richard Peto of the University of Oxford, one of the primary investigators of the ATLAS study, discussed these new findings and their implications.

Tamoxifen, explains Peto, “has side effects, but it prevents about 30 times as many deaths as it’s causing.” The most common side effects, as explained by, include mood swings, headaches, thinning hair, nausea, and loss of libido. In the absence of these side effects, or if the side effects do not prove uncomfortable, women often choose to stay on tamoxifen treatment for 10 years, as opposed to five. This study’s findings support the choice to pursue longer treatment if the patient and her doctor feel comfortable doing so. 

One of the most troublesome aspects of breast cancer is its tendency to recur, even after treatment. “These diseases keep coming back over years and years,” says Peto. Additionally, breast cancer can strike young, affecting women at a much younger age than other forms of cancer. This makes understanding the long-term effectiveness of treatments all the more important. As Peto explains, “thinking about [breast cancer] on a longer time scale is crucial.”

Source and Method

The ATLAS trial has been examining whether long-term treatment with tamoxifen is more effective than treatment of a shorter duration. For this study, 12,984 women who had taken tamoxifen for five years were studied for evidence of side effects. Tamoxifen targets ER-positive (estrogen receptor-positive) breast cancer in particular, so out of this larger group, the 6,847 women with ER-positive cancer were divided up randomly into two groups, one of which stopped tamoxifen treatment after five years, and the other of which continued treatment to ten years.

Continuing to the 10-year mark was found to reduce rates of breast cancer recurrence and mortality. However, it took some extra years for the benefits to emerge. The risk of recurrence after 15 years was 21.4 percent in the group that continued to take tamoxifen, while it was 25.1 percent in the group that stopped after five years. Additionally, mortality was reduced to 12.2 percent in those who continued tamoxifen treatment, versus 15 percent in the group who stopped after five years.

As for the side effects, they had only a small effect on survival. Researchers found an increased risk among postmenopausal women of developing endometrial cancer, which is cancer of the lining of the womb. This form of cancer is usually curable, however, and only posed a 0.2 percent mortality risk by year 15 after diagnosis among those who participated in the study.

The Takeaway

The results of this study represent more of a detail than an actual breakthrough, but it is an encouraging detail nonetheless.

In a statement accompanying the study, research group leader Dr. Christina Davies said, “Our results, taken together with results from previous trials of five years of tamoxifen versus none, suggest that 10 years of tamoxifen treatment can approximately halve breast cancer mortality during the second decade after diagnosis.” Conclusive evidence regarding the effects of 10 years of treatment on 20-year recurrence and mortality will have to wait until current studies yield those results.

This new information regarding the long-term effectiveness of tamoxifen may have implications for the future standard of care for treating breast cancer, a life-threatening and frequently recurring condition. Further findings from the ATLAS study are to be finalized and published next year, and will likely add credence to the findings now emerging.

Other Research

The ATLAS trial previously concluded that five-year treatment with tamoxifen reduced the recurrence rate of breast cancer more than did two-year treatment. 

In addition to ATLAS, there are currently other parallel trials underway, says Peto, that will likely corroborate his findings. Follow-ups with current ATLAS trial participants will continue, and the results of these follow-ups will help provide a longer-range look at the recurrence of breast cancer in women receiving tamoxifen treatment.

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Tags: Awareness , Drugs , Treatments , Latest Studies & Research

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The Healthline Editorial team writes about the latest health news, policy, and research.