February 3 (HealthDay News) -- A potential two-year "safe period" for hormone replacement therapy has been identified by American Cancer Society epidemiologists, who also confirmed that using estrogen-plus-progesterone therapy increases the risk for both ductal and lobular breast cancer far more than taking estrogen alone.

The finding follows a report last month from the Society of Obstetricians and Gynaecologists of Canada that women have been needlessly scared away from using hormone therapy during menopause.

Researchers for the U.S. study followed 68,369 postmenopausal women who were cancer-free when the study began in 1992. They were followed through mid-2005.

Along with confirming previous findings of increased breast cancer risk from combination therapy, the research team also found that the cancer risk increases substantially within three years of starting estrogen-plus-progesterone therapy.

The risk for lobular cancer doubled after three years of estrogen-plus-progesterone use, and ductal cancer doubled in risk after 10 years of use, the study found.

With estrogen-only therapy, the likelihood of developing lobular cancer increased 50 percent after 10 years of use, but there was no increase in the risk for ductal cancer, even after 20 years.

However, the study found that women who used an estrogen-progesterone combination for less than two years had no increased risk, possibly representing a safe period for combination therapy, the researchers said.

Nor was added risk found for women who had used the hormone combination but had been off the therapy for two or more years.

The researchers said that this suggests a window of two to three years for the risks associated with the estrogen-plus-progesterone combination to become apparent after initial use and to diminish after the therapy is halted.

The study was expected to appear in the March issue of the journal Cancer and has been published online.

In the Canadian report, the doctors' group said that it was revising its guidelines for use of hormone replacement therapy, because evidence has shown that the drugs offer a safe and viable option for women experiencing troublesome menopause symptoms.

However, it was recommending that hormone therapy should start early in menopause and be used only short-term, according to an account in the Canadian Press.

In 2002, a large U.S. study found that menopausal hormone therapy increased the risk of heart attack and stroke. But, the Canadian experts said, that study incorrectly concluded that the increased risk seen in older women applied to all women who use the drugs.

SOURCES: American Cancer Society, news release, Feb. 2, 2009; Jan. 22, 2009, Canadian Press