In 1988, alcohol was declared a class 1 carcinogen by the World Health Organization. The National Cancer Institute says alcohol raises breast cancer risk even at low levels of drinking. And in the United States, an estimated 15 percent of breast cancer cases are related to alcohol.
http://www.wbur.org/hereandnow/2018/05/10/alcohol-breast-cancer-risk
Alcohol Is A Major Risk Factor For Breast Cancer. Why Don't More Women Know?