Taking the cancer drug tamoxifen for five years drives down the incidence of breast cancer in women at high risk for the disease by close to 30 percent, researchers have found. And the medication's protective effects against breast cancer appear to last, unabated, for as long as 16 years after a woman stops taking it, a new study says.

The long-awaited IBIS-I trial (short for International Breast Cancer Intervention Study-I) found that tamoxifen was even more effective in preventing breast cancer in women who did not take hormone replacement medications. Women who took tamoxifen and did not take concurrent replacement hormones had a 38 percent decline in breast cancer diagnoses of any genetic variety over roughly 16 years of follow-up. And new diagnoses of estrogen-sensitive breast cancers - the most common kind - dropped by 45 percent among these women.

That means that for every 22 women who took tamoxifen for five years, there would be one fewer diagnosis of estrogen-sensitive breast cancer over a 20-year period. In an editorial published alongside the IBIS study, Rowan T. Chlebowski, a leading breast cancer researcher at Los Angeles BioMedical Research Institute, called that a "very favorable number."

This may be good news for the estimated 15 percent of women between the ages of 35 and 75 who have a roughly 20 percent lifetime risk of developing breast cancer (double the average woman's risk). While public attention has focused on prophylactic mastectomies, medications such as tamoxifen offer women at elevated breast cancer risk a vastly less drastic way to drive down the odds of developing such malignancies.

For young women with a family history of the disease, or who carry genetic variations such as BRCA-1 and BRCA-2, the findings are especially reassuring, said Jack Cuzick, an epidemiologist at Queens College London who is one the study's principal investigators.

Premenopausal women who take tamoxifen now can be assured that their five-year stint on the drug will confer substantial protection from developing breast cancer well into midlife - and possibly beyond, said Cuzick.

Cuzick cautioned that it would take longer studies still to determine whether breast cancer "chemoprevention" actually drives down breast cancer deaths.

Only 5 percent of the 7,154 IBIS subjects - who were between 35 and 70 at the study's start - have died. The reduction in new cancer diagnoses during the study's roughly 16-year follow-up was "striking" among those who took tamoxifen, said Cuzick.

The IBIS-I study was published Thursday in the journal Lancet Oncology.