Post-menopausal women who take combined hormone replacement therapy for at least five years double their risk of developing breast cancer, according to a new study.
However, once they stop taking the combination of oestrogen and progestin their risk of cancer falls by at least 28% within one year, said the researchers at Stanford University in California.
"This is very strong evidence that oestrogen plus progestin causes breast cancer," said Marcia Stefanick, co-author of the study that appears in the New England Journal of
Risk reduces when treatment is stopped
"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 oestrogen only, she said.
Safe for 2 years
However, another new study by American Cancer Society epidemiologists identified a potential two-year "safe period" for HRT.
Their research showed that for the first two years of use HRT does not increase the risk of breast cancer.
The researchers said that this suggests a window of two to three years for the risks associated with the oestrogen-plus-progesterone combination to become apparent after initial use, and to diminish after the therapy is halted.
Oestrogen-only therapy safe
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 oestrogen-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% 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% decline in hormone use. This "coincided with a 43% reduction in their breast cancer rates between 2002 and 2003," the researchers said. – (Sapa)
HRT safe for 2 years: study