Study: Ten Years of Tamoxifen Better than Five

The anti-estrogen drug tamoxifen is widely used to treat certain types of breast cancer. Taking the drug for 5 years—the usual treatment for pre-menopausal women and some post-menopausal women—cuts the chances of the cancer coming back by about half and helps women live longer. Now a new study shows that taking the drug for 10 years even further reduces the likelihood of recurrence and death from breast cancer. The study was published online December 5, 2012 in the Lancet and simultaneously presented at the San Antonio Breast Cancer Symposium.

An international group of researchers studied 6,846 women with estrogen receptor (ER)-positive breast cancer who took part in a large, long-term trial. All the women had been taking tamoxifen daily for 5 years. As part of the study, half the women were assigned to a control group that stopped taking the drug, while the other half continued taking tamoxifen for an additional 5 years.

Estrogen is a hormone that fuels breast cancer in some patients—those whose breast cancer cells contain hormone receptors. Tamoxifen works by interfering with the activity of estrogen. It can shrink tumors, reduce the likelihood of recurrence, and help patients live longer. Tamoxifen is generally safe, but it can rarely cause some serious side effects including increased risk of endometrial cancer and stroke.

The researchers found that taking tamoxifen for 10 years produced more reductions in breast cancer recurrence and death than taking tamoxifen for 5 years. In the 10 years after the study started, the risk of recurrence was 21.4% in the continuation group compared with 25.1% in the control group. Death from breast cancer was 12.2% in the continuation group compared with 15% in the control group.

The most significant side effect was an increase in risk of endometrial cancer, but its impact on survival was relatively small. The increased risk of death from endometrial cancer was 0.4% in the continuation group compared to 0.2% in the control group. There was no evidence in the study of an increased risk of stroke.

Researcher Christina Davies, MD, said “Five years of adjuvant tamoxifen is already an excellent treatment that substantially reduces the 15-year risk for recurrence and death from estrogen receptor (ER)-positive breast cancer, but [the trial] now shows that 10 years of tamoxifen is even more effective.”

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Long-term effects of continuing adjuvant tamoxifen to 10 years versus stopping at 5 years after diagnosis of oestrogen receptor-positive breast cancer: ATLAS, a randomised trial. Published online December 5, 2012 in the Lancet. First author: Christina Davies, MBChB, University of Oxford, United Kingdom.

American Cancer Society news stories are copyrighted material and are not intended to be used as press releases. For reprint requests, please see our Content Usage Policy.