Now, new findings in a journal of the American Association for Cancer Research (AACR) further clarify that risk.
The study found that, compared with women who had never used hormone therapy, women who used estrogen therapy for more than 15 years had a 19 percent greater risk of developing breast cancer.
Women who used combined therapy with estrogen plus progestin for 15 or more years had an 83 percent greater risk. Breast cancer risk was highest among women who used the combination regimen.
Other important findings :
• Risk varied according to Body Mass Index (BMI), with greater risks for thinner women.
• Longer duration of hormone therapy increased breast cancer risk.
So what should women take away from this new study?
According to CBS News Medical Correspondent and OB-GYN, Dr. Jennifer Ashton, this study looked at women taking hormone replacement therapy, also known as HRT, over a certain period of time and then helped to stratify their risk of breast cancer.
While we already know HRT increases the risk of breast cancer, Ashton points out this study focuses on duration of use and combination of HRT, including estrogen and progesterone.
The findings that women who used combined therapy with estrogen plus progestin for 15 or more years had an 83 percent greater risk raises a lot of concern, "Early Show" co-anchor Erica Hill pointed out.
"If taken for more than 15 years," Ashton reiterates. "Remember, if it was 100 percent risk, that would be equivalent to doubling your risk. This 83 percent is actually very similar to what came out in 2002."
Therefore, length of time is important.
"Length of time, longer risk associated with higher risk. Women who took HRT for about two years really showed a minimal increase in their breast cancer risk, so that's important.
"Women who took both combination, as we just saw, estrogen and progesterone, had a higher risk. And women who took what we call continuously without a break also had a higher risk. So that further delineates what the risks are. So like anything, (it's about) risk versus benefits. You want to be able to stratify those risks as much as possible," she added.
Weight also plays a significant role, but the results are surprising.
"Usually we hear obesity can contribute to the risk of breast cancer. This study found thinner women had higher risk," Ashton explained. "It's important to remember, again, this needs to be individually discussed with your doctor, that WHI study from 2002 actually found hormone replacement therapy can lessen the risk of colon cancer. So, if that's a bigger risk for you versus breast cancer, you want to discuss that with your doctor. It's not a simple equation."