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.
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