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 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.
"You start women on hormones and within five years, their risk for breast cancer is clearly elevated. You stop the hormones and within one year, their risk is essentially back to normal. It's reasonably convincing cause-and-effect data."
The results do not apply to women who take estrogen only, she said. A previous large-scale study by the Women's Health Initiative in 2002 did not find a rise in breast cancer for most women on estrogen-only therapy.
The Stanford University researchers looked at data from two major study groups: more than 15,000 women from one landmark study that was halted in 2002 after initial findings showed an increase in breast cancer for those on combined therapy versus those on a placebo; and a second group of 41,449 women who joined a 1994 study and were free to choose hormone therapy or not.
"The results from the two groups of women were quite similar," the findings said.
"In the clinical trial, the incidence of breast cancer was much higher in the hormone group in the five years leading up to 2002. But after they stopped taking the hormones, breast cancer rates dropped very rapidly. The number of breast cancer diagnoses fell 28 percent within the year."
As initial findings began to emerge pointing to increased breast cancer risk for women on combined hormone therapy, women in the observational study reflected what women were doing in society as a whole and showed a 50 percent decline in hormone use.
This "coincided with a 43 percent reduction in their breast cancer rates between 2002 and 2003," the researchers said.