The reasons to go on hormones are different for every patient, but new information can help women figure out what's best for them.
Many women enrolled in the estrogen-only trial as part of the 15-year-long Women's Health Initiative.
But in 2004, the study was stopped early because researchers found an increased risk of stroke.
However, a recent report in the Journal of the American Medical Association found estrogen had a protective effect against heart disease overall, but women in their 50s had far fewer heart attacks than women in their 70s.
"The effect of estrogen on heart disease differed significantly by age," said Dr. Andrea LaCroix of the Fred Hutchinson Cancer Research Center.
Hip fractures also went down while on the medication, but it started to rise once women stopped.
As expected, the risk of stroke and blood clots went down once women went off estrogen.
But the risk of breast cancer was reduced regardless of age.
Good Samaritan Hospital's Dr. Joanna Davies said other studies showed similar results, but estrogen alone may slightly increase a woman's risk for another cancer.
"Estrogen may be potentially carcinogen for the uterus," Davies said.
So should a woman with hot flashes and mood swings take estrogen only based on this study?
Dr. Davies said estrogen isn't a good thing or a bad thing. It's different for each individual.
