Switching to Pfizer Inc.‘s drug Aromasin after a few years on another breast cancer treatment cut some women’s risk of dying from the disease by 17 percent in a study released on Saturday.

Researchers looked at more than 4,700 women past menopause who had been treated for early stage breast cancer and were disease-free after two to three years of taking the drug tamoxifen to prevent a recurrence.

About half were randomly chosen to substitute Aromasin, while the other half continued on tamoxifen. Total treatment lasted five years.

Scientists tracked their progress for about 2.5 years after treatment stopped.

They found that women who switched to Aromasin had a 17 percent lower risk of dying from the disease than others who took only tamoxifen. There were 210 deaths in the Aromasin group and 251 in the tamoxifen group.

The findings, presented at the annual meeting of the American Society of Clinical Oncology, applied to women with what is known as hormone-sensitive breast cancer. It is the latest study to compare the benefits of the two types of drugs.

“Worldwide there are still thousands of women taking tamoxifen, and you could immediately affect their chances of survival by switching to Aromasin,” Dr. Stephen Jones, one of the study researchers and medical director for U.S. Oncology Research, said in an interview.

Tamoxifen - long the drug of choice for preventing breast cancer - blocks estrogen, which can help fuel the growth of tumors in some cases.

Aromasin, known generically as exemestane, and similar drugs inhibit the enzyme aromatase, which is needed to produce estrogen. The aromatase inhibitors are now being used just after breast cancer surgery instead of tamoxifen in many women to keep the disease from returning. Both are taken orally.

In the new study, side effects from the drugs differed. Women who took only tamoxifen had higher rates of blood clots, uterine cancer and vaginal bleeding. Aromasin patients had slightly more fractures, a known risk from aromatase inhibitors.

A separate study showed women treated for five years with AstraZeneca Plc’s aromatase inhibitor Arimidex lost about 6 percent of bone in the spine and 7 percent in the hips. That was about twice what would be expected in women in the study, who were age 64 on average.

None of the patients with normal bone density at the start of therapy lost enough bone mass to be diagnosed with osteoporosis, which can increase fracture risk, said Dr. Robert Coleman, a medical oncologist at Weston Park Hospital in Sheffield, England.

Women taking Arimidex should have their bone density monitored every one to two years and take calcium and vitamin D supplements, Coleman said.

Tamoxifen was sold by AstraZeneca Plc under the name Nolvadex but now is marketed by several generic drug makers.