Novartis AG’s breast cancer pill Femara can significantly reduce the risks that a woman’s cancer will return if it is used anywhere between one year and seven years after finishing tamoxifen therapy.

U.S. researchers said post-menopausal women who took Femara reduced the risk that cancer would come back by 63 percent. Moreover, the risk of cancer spreading to other areas of the body was reduced by 61 percent.

“What our results have shown for the first time in breast cancer treatment history is that taking an anti-estrogen anywhere along that line appears to have a dramatic reduction in the risk of recurrence,” said Dr. Paul Goss of Massachusetts General Hospital and colleagues in the online issue of the Journal of Clinical Oncology.

The drug known generically as letrozole, belongs to a new class of breast cancer drugs known as aromatase inhibitors, which block the production of estrogen that can lead to cancer. It is recommended for use in women past menopause.

Dr. Goss recommended women who had surgery and completed adjuvant therapy years ago to consider starting on letrozole now.

“There are probably hundreds of thousands of breast cancer patients around the world today who are in exactly this situation,” he said.

Dr. Goss underlined the fact that breast cancer could come back after five-year therapy with tamoxifen.

“What is important for doctors and patients to recognize unfortunately is that the risk of recurrence of this type of breast cancer does not end at five years," Goss said in a telephone interview, according to Reuters.

Dr. Goss’s findings are based on an analysis of 1,579 women who decided to switch to letrozole from a placebo after the trial was halted. Their results were compared to about 800 women who stayed on the placebo. Those who began letrozole had only a 2 percent risk of tumor recurrence almost three years after the study ended compared with almost 5 percent in those choosing no treatment.

Dr. Goss’s findings were sustained by a second study conducted by Hyman Muss of the University of Vermont also published yesterday in the Journal of Clinical Oncology. Muss found that letrozole was effective in cutting the recurrence risk for women of all ages, including those older than 70.

Dr’ Goss said his results would likely apply to all aromatase inhibitors: Pfizer Inc’s exemestane, sold under the brand Aromasin and AstraZeneca Plc’s anastrozole, sold under the brand Arimidex.

A study, involving 1,598 patients, found that exemestane reduced the risk of recurrence by 56 percent if taken soon after completing tamoxifen treatment.

The studies were supported by the Canadian Cancer Society, the National Cancer Institute of Canada, the US National Cancer Institute, and Novartis Pharmaceutical.

Women in the United States have a 1 in 8-lifetime chance of developing invasive breast cancer and a 1 in 35 chance of dying from breast cancer. It is the second most common type of cancer after lung cancer and the fifth most common cause of cancer death.