Those on tamoxifen also fared worse on information-processing speed than the exemestane users, the researchers found.
No substantial differences were found between any of the three groups in terms of visual memory, working memory, verbal fluency, reaction speed and motor speed, according to the study.
Exactly why the tamoxifen users fared worse is not known, Schilder said. "The mechanisms of action of tamoxifen on the brain are hardly known yet," she said, adding that one possibility is that tamoxifen might counteract possible beneficial action of estrogen on the brain.
Dr. Victor G. Vogel, vice president of research for the American Cancer Society, said the results were not surprising. And though the reason women on tamoxifen had more cognitive decline isn't known, he said a logical though unproven explanation is that "it may just not be good for your brain not to have estrogen."
However, Vogel emphasized that it's important to keep the study results in perspective, noting the life-saving benefits of tamoxifen.
According to the U.S. National Cancer Institute, "the benefits of tamoxifen as a treatment for breast cancer are firmly established and far outweigh the potential risks."
Said Vogel: "I'll take a little memory loss to keep my patients alive."
The American Cancer Society has more on tamoxifen.
SOURCES: Christien Schilder, doctoral student, Netherlands Cancer Institute/Antoni van Leeuwenhoek Hospital, Amsterdam, the Nethe
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