The effect on heart disease among women 50 to 59 years old was even stronger -- a 35 percent reduction in cardiovascular events and a 59 percent reduction in deaths from cardiovascular problems.
However, among older women the heart effect was much smaller and not statistically significant.
Tamoxifen, used for 30 years to treat breast cancer, inhibits the ability of estrogen-receptor positive cancers (the majority of breast cancers) to grow by disrupting estrogen activity.
It's not clear how the drug protects against heart disease, Hackshaw said. "But there is evidence that tamoxifen reduces lipid levels [for example, cholesterol], which we know in turn reduces cardiovascular risk," he explained.
It's possible that the protective effect declined in older women because damage to the arteries had already occurred, he speculated.
The new research is a timely reminder about the benefits of tamoxifen, said Dr. Joanne Mortimer, vice chair of medical oncology at the City of Hope Comprehensive Cancer Center, Duarte, Calif., and director of its women's cancers program.
Although many doctors prescribe medications known as aromatase inhibitors for breast cancer instead of tamoxifen, Mortimer said tamoxifen is still widely prescribed.
"Maybe for those who have problems with an aromatase inhibitor, they would be comforted by the fact that tamoxifen is an alternative and has a favorable effect on normal tissues, like bone and heart muscles," Mortimer said.
While not discounting the effectiveness of aromatase inhibitors, Hackshaw said tamoxifen is much less expensive.
A month's supply of 20-milligram tablets, the dose used in the Hackshaw study, is about $100. Brand-name versions of aromatase inhibitors can cost more than $500 for 30 pills, although cheaper generic versions are also available.
In an editorial accompanying the study, Dr. Kathleen Pritchard, of Sunnybrook Od
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