omen with a high risk of breast cancer are declining to take tamoxifen because they are afraid of side effects, even though the drug can protect them from the cancer, researchers reported on Tuesday.

Tamoxifen can reduce by nearly 50 percent the risk of breast cancer in women considered highly vulnerable. But the drug, which somewhat mimics the hormone estrogen, can raise the risk of endometrial cancer, as well as blood clots and hot flashes.

“While the availability of tamoxifen is a significant advance in breast cancer prevention, it also presents a complicated decision for women at high risk for the disease,” said Sharon Bober, a psychiatrist at Harvard Medical School, the Beth Israel Deaconess Medical Center and Dana-Farber Cancer Institute.

Bober and colleagues studied 129 women eligible to take tamoxifen to prevent breast cancer because they had an estimated risk of 1.7 percent or more over five years.

Writing in the Journal of Clinical Oncology, the researchers found that after two months, 56 percent of the women opted to take either tamoxifen or to enroll in a trial that included tamoxifen or a similar drug. Forty four percent declined treatment or were undecided.

After four months, 35 percent had declined treatment, and 14 percent remained undecided.

Women with a history of breast lumps were more likely to take the drug, while those with a family history of the disease that put them at high risk were more likely to opt out. Those who refused treatment were more likely to be worried about side effects and to doubt that tamoxifen would help them.

“Our study underscores the need to address psychological factors that may influence decision-making, in order to help women feel confident and satisfied with their treatment choice,” Bober said.

Tamoxifen is available generically and also under the brand name Nolvadex, made by AstraZeneca Plc. A new class of breast cancer drugs called aromatase inhibitors may also reduce the risk of breast cancer, with different side effects, but doctors have not finished the studies that would show this.