TUESDAY, Aug. 10 (HealthDay News) -- Another analysis of data on hormone therapy use among U.S. women finds that the panorama of risks is even more complex than previously thought: Thinner women taking it showed a higher risk of developing breast cancer than heavier women.

The California researchers also found that the longer a woman used either estrogen-alone therapy or estrogen-plus-progestin therapy, the higher the risk of disease. Similar results were seen for women who used combined therapy continuously rather than taking breaks during the month.

"This gives us a clear correlation between the length of treatment and risk," said Dr. Freya Schnabel, director of breast surgery at NYU Langone Medical Center in New York City. "Ad hoc, people had made the assumption that, for the most part, shorter was better, but this definitely gives you some parameters on that."

Given that current recommendations on the use of HRT to relieve menopausal symptoms are to use as little as possible for as short a time as possible, Schnabel said, "this supports that and gives a framework for that."

The findings appear in the current issue of the journal Cancer Epidemiology, Biomarkers & Prevention.

Previous analyses have verified that taking combined estrogen-plus-progestin ups breast cancer risk. In fact, a dramatic tapering off in HRT use after the results of the landmark Women's Health Initiative (WHI) study were published in 2002 has coincided with a decline in the number of invasive breast cancers doctors are seeing.

"There's no question that [the release of information from the WHI] was a cataclysmic moment for HRT in the U.S.," Schnabel said. "A lot fewer women are using it, and what we've seen as a result is that breast cancer rates have gone down, and epidemiologists seem pretty comfortable that it was because of that."

Since those original results, however, researchers have revealed various nuances in which the risk is highest, often corresponding with age and type of formulation used.

For this paper, researchers from the University of Southern California combed through data on hormone therapy among almost 3,000 women who were followed for about 10 years, roughly from 1995 to 2006.

Using estrogen therapy for 15 years or longer increased the risk of breast cancer by 19 percent, compared with women who had never used HRT.

Using combined therapy for this length of time bumped up the risk by 83 percent.

And those women taking combined therapy without interruption had highest risk.

Meanwhile, women with a lower body-mass index (BMI) were more at risk than those with a BMI above 30, which is considered obese, although it wasn't clear why.

Not surprisingly, only women with estrogen-receptor, progesterone-receptor and Her2-neu positive malignancies carried the higher risk.

"This is another example of why, if you're going to take these medicines, you need to take them for as short a period of time as possible and [it shows] that the estrogen-plus-progestin formulations are worse than estrogen alone," said Dr. Jay Brooks, chairman of hematology/oncology at Ochsner Health System in Baton Rouge, La. "These drugs are still licensed in the U.S., but they have black-box warnings on them for a reason, which are an increased risk of stroke, heart attack and, in the estrogen/progestin combination, an increased risk of breast cancer."

"Women have to make decisions about the quality of their life and also about exposure to potentially dangerous drugs," he added.

One of the study authors reported having served as an expert witness in a lawsuit against the maker of Prempro, used to combat hot flashes and other symptoms of menopause.

More information

Visit the National Heart, Lung, and Blood Institute for more on the accruing results of the Women's Health Initiative.