Here is some moderately good news for women with breast cancer: taking Tamoxifen for 10 years rather than the prescribed five reduces the risk of cancer recurrence and, ultimately, death from the disease.

Tamoxifen is normally prescribed to pre-menopausal women, in particular, with estrogen-receptor positive breast cancer, the most common kind, to minimize the risk of recurrence. It is taken every day for five years. The drug has already been shown to work more effectively over five years than over two.

In a study published in The Lancet, researchers tracked 6,846 women with estrogen-receptor positive breast cancer who had been taking Tamoxifen for the past five years. About half of the group was told to continue with the drug for another five years; the other half was told to stop taking the drug.

They found that women who took the drug had modestly better health.

Eight years after the 10-year group had stopped taking the Tamoxifen, 617 women had seen recurrences in cancer, while 711 women who had stopped taking the drug had recurrences as well.

In the group that stopped, 397 died; in the drug group, 331 died.

Researchers found that, five to 14 years after the women had stopped taking Tamoxifen, the risk of death declined from 15 percent in the five-year group to 12 percent in the 10-year group.

The benefits were most seen 10 to 15 years after the cessation of treatment, since Tamoxifen lowers the risk of recurrence for up to 15 years after treatment.

While researchers are encouraged that the benefits of Tamoxifen keep increasing the longer it is taken, Tamoxifen will never become a lifetime drug. The cancer drug comes with a lot of side effects and can simply not be tolerated by some women. Taking Tamoxifen carries a heightened risk of endometrial cancer and blood clots in the lungs.

Indeed, 3 percent of the women who took Tamoxifen for 10 years developed endometrial cancer.

However, researchers say that endometrial cancer is rather easy to treat, and that the risks outweigh the benefits.

It is also more common in women over the age of 60.

For pre-menopausal women whose cancer has not spread to their lymph nodes, it may be better to just take Tamoxifen for five years. These women have a low risk of recurrence.

The picture becomes much less clear for post-menopausal women with breast cancer. They are normally prescribed aromatase inhibiters in addition to Tamoxifen. In general, these women are prescribed two years of Tamoxifen, followed by five years of aromatase inhibiters.

Doctors wonder how the new research should affect their prescription of these medications. Should women receive 10 years of both? Five years of inhibiters followed by five years of Tamoxifen? And is an even longer time better than 10 years?

Like much new research, these findings prompt just as many as new questions as they answer.