Women using a certain type of hormone therapy to treat menopause symptoms could be at higher risk for breast cancer, according to a new study. 

The study, published Tuesday in The Lancet Oncology, found that estrogen plus progesterone increases the risk of breast cancer in women under 55 with a uterus, while estrogen alone reduces the risk.

Dr. Céline Gounder, CBS News medical contributor and editor-at-large for public health at KFF Health News, told "CBS Mornings" Wednesday the study participants, all under age 55, were divided into two groups — women who still have their uterus and those who do not, since they require different types of treatment. 

A hysterectomy is the surgical removal of the uterus, which may be done for a number of reasons, including abnormal vaginal bleeding, severe pelvic pain, uterine fibroids, severe endometriosis and certain types of cancer. Approximately 600,000 females get hysterectomies in the U.S. each year, according to the Cleveland Clinic. 

"For the women who do still have a uterus, you have to give estrogen plus progesterone together. Estrogen alone will cause a thickening of the uterine lining, which can cause a different kind of cancer, what we call endometrial cancer. So you have to give the combination," Gounder said. "In women who no longer have a uterus, you can give estrogen alone."

And the difference in treatment mattered. "What they found is estrogen alone reduced the risk of breast cancer," she said. "There seems to be a slight increase in risk in breast cancer if you also took progesterone."

This research is a "great example of (how) science evolves," Gounder added. 

It comes after an early 2000s study called the Women's Health Initiative that focused on older women who may not have started hormone replacement therapy until 10 or so years after going through menopause. The study, which raised safety concerns and led to a longtime public misunderstanding around hormone therapy, has since been found problematic in terms of how it was designed. Now healthcare professionals largely consider the treatment option safe and effective.  

"The newer research is focused on what actually happens in real life, which is women who go through menopause in their 40s or 50s, (and) start treatment around that age," Gounder said. 

As with most medications, hormone replacement therapy comes with certain risks for some people. The Mayo Clinic notes it can lead to increased risk of heart disease, stroke, blood clots and some cancers. In other cases, however, it can help reduce risk of cancer, prevent bone loss and reduce menopausal symptoms like hot flashes.

As the latest study shows, timing and type of hormone therapy can be crucial to potential outcomes, making it important that people consult their doctor and consider family history and other factors when considering treatment options.

Gounder said hormone replacement therapy is the most effective treatment for a lot of perimenopausal symptoms, and added, "You really need to sit down and talk to your doctor (to determine) what is the appropriate thing for you."