Post-menopausal women who take combined hormone replacement therapy for at least five years double their risk of developing breast cancer, according to a study published on Wednesday.

However, once they stop taking the combination of estrogen and progestin their risk of cancer falls by at least 28 percent within one year, said the researchers at Stanford University in California.

"This is very strong evidence that estrogen plus progestin causes breast cancer," said Marcia Stefanick, co-author of the study that appears in the February 5 edition of the New England Journal of Medicine.