The risk of breast cancer death five to 14 years after the diagnosis was slightly over 12 percent among those who stayed on the drug versus 15 percent among those who stopped.
The follow-up is continuing, Peto said. "We are going to follow for another five years, to see what happens," he said. "I think there will probably be future gain."
Will tamoxifen end up being a lifetime drug, once it is prescribed? "No, because tamoxifen has side effects," Peto explained.
The side effects are well documented -- an increased risk of cancer of the uterus and blood clots in the lungs are major ones. In the study, those who continued on the drug had higher risks of both conditions. However, the risks were outweighed by the reduction in death risk, the researchers added.
The research will be practice-changing, said Dr. Joann Mortimer, director of the Women's Cancers Program at City of Hope Comprehensive Cancer Center, in Duarte, Calif.
"Whether women will allow it to change practice remains to be seen," she said, referring to the reluctance of women to take anti-cancer drugs like tamoxifen. Compliance is ''atrocious," she said.
Women stop the drug for many reasons, she said, including side effects.
Even so, she said, the new research is reason enough for women who may benefit from tamoxifen to talk to their doctor about it, Mortimer said.
The study was funded by AstraZeneca, which makes tamoxifen, as well as Cancer Research U.K., Medical Research Council, the U.S. Army and the European Union.
To learn more about tamoxifen, visit the U.S. National Library of Medicine.
SOURCES: Joanne Mortimer, M.D., director, women's cancers program, City of Ho
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