The American Society of Clinical Oncology issued new guidelines on Monday for the use of hormone-based breast cancer drugs called aromatase inhibitors.
"Women can take up to five years of an aromatase inhibitor therapy," ASCO said in a statement to be published in the group's Journal of Clinical Oncology.
Aromatase inhibitors include anastrozole, made by AstraZeneca under the brand name Arimidex, exemestane, made by Pfizer Inc, under the brand name Aromasin and Novartis's Femara or letrozole.
About 75 percent of these cancers are estrogen-receptor positive, meaning they are driven by hormones. Tamoxifen was the first drug to block the effects of estrogen and the aromatase inhibitors are the next generation.
Studies have shown that women who took tamoxifen for 5 years were 50 percent less likely to have their cancer return and the aromatase inhibitors have similar effects. But if a breast cancer patient has not gone through menopause, tamoxifen is the only safe hormone-based drug to take.
"One of the most important treatments for women with postmenopausal breast cancer is anti-estrogen therapy," ASCO's Dr. Harold Burstein, an oncologist at Harvard Medical School in Boston, said in a statement.
"Our panel carefully reviewed the explosion of research that has emerged in the past five years on anti-estrogen drugs, and filled in gaps in our understanding of how best to use these newer treatments, and what the trade-offs and side effects of therapy would be."
Some studies suggest it is safe to take tamoxifen and an aromatase inhibitor — usually a pill taken daily — for as long as 10 years in total, the panel said.
"While the two drug classes work differently, overall, most women have relatively mild side effects on either drug," ASCO added.