U.S. researchers have found that letrozole, an oral non-steroidal aromatase inhibitor that blocks the production of estrogen, is effective in preventing the relapse of breast cancer even if it's taken years later than previously thought.

The initial FDA approval for letrozole, known by its brand name Femara, targeted the use of the estrogen blocking drug within three months after treatment with another cancer drug, Tamoxifen. It was previously thought that if Femara would be taken later it would have little influence on the cancer relapse rates.

In fact, the new research showed stunning effectiveness of almost 60 percent in cutting the relapse rate if taken beyond the three-month window. There is also a 50 percent reduction of death rate, a 61 percent reduction of metastasis rate and a 82 percent reduction of the risk to develop cancer in the other breast.

Tamoxifen has similar protection capabilities, but its side effects make it unsuited for long-term use. This demands that after a maximum of five years it is replaced or interrupted to prevent risks outweighing the benefits. The research was performed by Paul E. Goss of Massachusetts General Hospital and his colleagues and involved looking at data of some 1,500 women. The study was published in the Journal of Clinical Oncology.

"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 to Reuters.

The same publication has received another related study, by Hyman Muss of the University of Vermont, which shows that the same substance is effective in cutting the recurrence risk for women of all ages, including those older than 70.