Mason pointed out that the vast majority of the women in the study did not develop a problem and researchers did not find an increased risk of "all-cause" mortality.
"It does demonstrate a small increase in the risk of breast cancer," she said. "It's a small magnitude, 20 percent increase. And with longer duration of treatment, a 38 percent increase."
About 16 million women in the United States and about 140 million worldwide use some type of hormonal contraception, according to the AP.
A 20 percent increase does not mean that 20 percent of women who use a hormonal contraceptive will get breast cancer, Manson said. The increase amounts to about one extra case per 8,000 users per year, she said.
While researchers had access to information like how many children the woman had, her education level, her family history of breast cancer, they did not have access to other information on mammogram screening practices or clinical breast exams.
"Women who are taking a hormonal medication and are in the medical system having regular medical follow up visits may be having more regular breast exams by the clinician," Mason said. "And also they may be starting mammography earlier or having more regular mammograms."
"If there are differences in breast cancer screening, it can lead to these small increases in risk or contribute to these small increases in risk," she said.
Manson said women who already have an increased baseline risk of breast cancer due to family history or if they're BRCA positive, for example, may want to heed the study's findings.
Women in their 40s or older, who suffer from increased risk of breast cancer, may want to consider hormone-free contraceptive options, including IUDs that do not release hormones, having their tubes tied, or having their partner undergo a vasectomy, she said.
