Drinking as few as three to six glasses of wine per week may increase a woman’s lifetime risk of breast cancer by 15%, according to an analysis by Harvard researchers.

The study, published Tuesday in the Journal of the American Medical Assn., reaffirms that heavy alcohol use raises breast cancer risk, and it adds that light drinking matters too.

“Alcohol is a real risk factor, and the more you drink the higher your risk,” said Dr. Steven A. Narod, the Canada research chair in breast cancer at the University of Toronto, who wrote a commentary accompanying the study.