Older women taking the breast cancer drug tamoxifen may have an increased risk of developing diabetes, a new study suggests.
The findings, reported in the journal Cancer, do not prove that tamoxifen directly leads to diabetes in some women.
But researchers say it is plausible that in women with known risk factors for diabetes—like obesity or family history of the disease—tamoxifen furthers the risk somewhat.
The study, of more than 14,000 breast cancer survivors age 65 and up, found that 10 percent were diagnosed with diabetes over five years.
Those odds were one quarter higher among women who were currently on tamoxifen, versus those who were not. (Women prescribed tamoxifen for breast cancer typically take it for five years.)
That increase is small and the findings should not "alarm" women taking tamoxifen, said study leader Lorraine L. Lipscombe, of Women's College Hospital and the University of Toronto in Canada.
"Tamoxifen is a very important drug," Lipscombe said in an interview. "I don't want women to think they should stop taking it."
And if tamoxifen does affect the odds of developing diabetes, it may only do so in certain women, according to Lipscombe.
She and her colleagues speculate that tamoxifen may boost diabetes risk in women already predisposed to the disease.
Tamoxifen, sold in the U.S. as Nolvadex and Soltamox, works by inhibiting the hormone estrogen. And animal research suggests that estrogen plays a role in blood sugar control. Adult-onset, or type 2, diabetes arises when the body can no longer properly use the hormone insulin, which regulates blood sugar.
So in theory, tamoxifen's effects on estrogen may add to any problems in blood sugar regulation.
Still, this is the first study to show a link between tamoxifen and diabetes. "So we definitely recommend that more studies be done," Lipscombe said.
And that, she noted, should include studies of younger women to see if the same association exists for them.
For now, she said, women on tamoxifen may want to pay particular attention to controlling any of the established diabetes risk factors they may have. That includes maintaining a healthy weight through a balanced diet and exercise — "just like any other woman should," Lipscombe noted.
The researchers did find that women who had used tamoxifen in the past, but not currently, were at no increased risk of diabetes.
"We did not see the risk persist after women stopped taking the drug," Lipscombe said.
Nor did the researchers see a connection between diabetes and another class of breast cancer drugs called aromatase inhibitors—which also inhibit estrogen, though by a different mechanism than tamoxifen.
Lipscombe said it's possible that the lack of a link is due to the fact that few women in the study were using an aromatase inhibitor. More studies are needed to answer that question too, she said.
If tamoxifen does increase the risk of diabetes, it would not be the only side effect of the drug. It is known to carry small risks of blood clots, stroke, uterine cancer and cataracts, according to the U.S. National Cancer Institute.
However, the agency says, the benefits of tamoxifen in treating breast cancer are "firmly established and far outweigh the potential risks."
Tamoxifen is the drug of choice for women with early-stage breast tumors that are estrogen-receptor-positive—meaning the hormone fuels their growth. Those tumors account for about 70 percent of breast cancer cases.
Along with treating cancer, tamoxifen is sometimes prescribed to lower breast cancer risk in women at higher-than-average risk of the disease.