"We have clearly demonstrated that tamoxifen can prevent breast cancers, and that is a big step forward," lead researcher Jack Cuzick, PhD, tells WebMD. "But we still have more work to do to learn how to do that safely with a minimum number of side effects."
In Cuzick's study, some 7,000 women without breast cancer but at high risk for the disease took either tamoxifen or a placebo for five years. The frequency of breast cancer fell by one third among women taking tamoxifen, compared with those taking a placebo. But there was also an increase in potentially dangerous blood clots in the tamoxifen group.
Tamoxifen is routinely used to help prevent breast cancer from coming back. But Cuzick says there are too many unanswered questions about tamoxifen's safety to recommend it for routine use in women who do not already have breast cancer.
Cuzick's preliminary study, reported in the Sept. 14 issue of The Lancet, offers a conflicting profile of tamoxifen's safety.
In the past, tamoxifen has been linked to an increase in uterine cancer. But this was not the case in the present study. The numbers of women who developed uterine cancer in both groups were not significantly different. And when uterine cancer did strike, it was always in the early stages and cured by hysterectomy.
But more women on tamoxifen suffered blood clots-- 43 women vs. 17 women taking a placebo. The blood clots tended to occur following surgery, leading Cuzick to recommend that women on tamoxifen be taken off the drug and put on blood-thinning agents prior to major surgery.
The increase in blood clots may account for the fact that women taking tamoxifen had a higher risk of dying from any cause -- not just from breast cancer -- than women taking placebo.
Cuzick says it may be several years before the long-term risks and benefits of tamoxifen are clarified for women taking it to prevent first-time breast cancers. That is because these women are only now beginning to complete the recommended five-year treatment protocol.
"The best possible scenario is that there would be a continued reduction of cancers with a cessation of side effects," he says. "But we just don't know. And there are hints that tamoxifen may increase the long-term risk of [hard to treat] tumors. We need to look at that more carefully."
Oncologist Herman Kattlove, MD, tells WebMD that though tamoxifen does appear to prevent breast cancers in women without breast cancer, it is not yet clear whether its use in high-risk women will save lives.
Most women don't die of breast cancer, and the cancers that tamoxifen prevents are very receptive to treatment, Kattlove tells WebMD.
Kattlove, who now serves as a medical editor for the American Cancer Society, has used tamoxifen to treat women with breast cancer. He says most of his patients found the drug's more common side effects, such as hot flashes and vaginal dryness, difficult to tolerate.
"A lot of my patients were unhappy on this drug," he says. "This is not some benign therapy. Most women experience side effects while taking it, and even if those side effects aren't life threatening, they are troubling."
Several large trials are now under way assessing tamoxifen's usefulness in preventing breast cancers in women who have not had the disease but are at high risk for it.