A new prospective study has reported that taking tamoxifen for the recommended five years instead of stopping after two years reduces the risk of recurrence during the first 15 years after starting treatment. It also lowers the risk of cardiovascular disease and death from heart disease, particularly among women age 50 to 59 years. Tamoxifen is an anti-estrogen typically prescribed to treat hormone receptor positive breast cancer.

The study was designed to compare the long-term outcomes of taking tamoxifen for five years compared to two years among postmenopausal women with early stage breast cancer. The study included 3,449 women with operable breast cancer enrolled at ages 50 to 81 years in the Cancer Research UK “Over 50s” trial. Between 1987 and 1997, recurrence-free participants, all of whom had been taking 20 mg of tamoxifen for two years, were randomly assigned to either stop or continue tamoxifen for another three years. The study results are reported as of a median follow-up period of 10 years. Data on breast cancer recurrences, new tumors, mortality, and cardiovascular events (such as heart attack) were collected as of April 2010.

A total of 1,103 recurrences, 755 breast cancer-specific deaths, 621 cardiovascular events, and 236 deaths as a result of heart disease were recorded. After 15 years of treatment, 5.8 fewer women experienced recurrence for every 100 women who took tamoxifen for five years compared to those who took tamoxifen for only two years. The risk of contralateral breast cancer (cancer in the opposite or untreated breast) was also 30% lower among women who took tamoxifen for five years. Restricting the analysis to women aged 50 to 59 years, a 35% reduction in cardiovascular events and a 59% reduction in death as a result of a heart disease was found in the women who took tamoxifen for five years - this effect was much smaller and not statistically significant in older women. In a separate interview, lead author Allan Hackshaw said, “Our study provides conclusive evidence that taking tamoxifen for five years offers women the best chance of surviving breast cancer. Women diagnosed with early stage breast cancer who are prescribed tamoxifen are recommended to take the drug for five years, but we know that many stop after two or three. Worryingly, our results suggest that by doing this, they could increase their risk of cancer coming back.” He also commented that this is the first major study "to look at the long-term benefits of five years of tamoxifen and show it has the extra benefit of substantially reducing a woman’s risk of developing heart disease. This effect was greatest among women in their 50s, perhaps because the way plaques tend to build up in the arteries with age may be easier for tamoxifen to reverse in younger women.”

Please see our article on what to eat during tamoxifen treatment for more information on how to optimize treatment with tamoxifen and reduce side effects.