A new prospective study has reported that consuming even as few as three to six drinks per week increases the risk of breast cancer by 15% compared to not drinking. The excess risk increased to 50% in women who averaged more than 30 drinks per week. The study was designed to investigate the association between alcohol consumption during adult life (including quantity, frequency, and age at consumption) and risk of breast cancer. A variety of studies have tied alcohol consumption to breast cancer risk, however the risk associated with modest levels of consumption has not been established. In addition, the role of drinking patterns such as binge drinking and consumption at different times of life are not well understood. The study included 105,986 women enrolled in the Nurses' Health Study. The women, who underwent an early adult alcohol assessment and eight updated alcohol assessments, were followed from 1980 to 2008.

A total of 7,690 cases of invasive breast cancer were diagnosed during follow up. The higher the level of alcohol consumption, the greater the risk of breast cancer. However, the association held for levels as low as 5.0 to 9.9 grams of alcohol per day (equivalent to three to six drinks per week). Binge drinking, but not how frequently drinking took place, was also independently associated with breast cancer risk after controlling for cumulative alcohol intake. Alcohol intake at young age and later in adult life were both independently associated with increased risk. The authors conclude that low levels of alcohol consumption are associated with a small increase in risk of breast cancer, with the most consistent measure being cumulative alcohol intake throughout adult life.

"We observed a 10% increase in risk with each 10 g per day of alcohol intake," study author Wendy Y. Chen and co-authors wrote in their assessment of their findings, adding "Consistent with other studies, we did not find any difference between type of alcohol beverage."