Women can increase their risk of breast cancer by drinking as little as three small glasses of wine a week, a study has found.
Alcohol is a known risk factor for breast cancer, but few studies have looked at its effect at low levels.
The new research showed that even five to 9.9 grams of alcohol a day - the equivalent of three to six small glasses of wine per week - is associated with a 15% increase in risk.
Women who consumed at least 30 grams of alcohol daily, or at least two drinks a day, were 51% more likely to develop breast cancer than those who never drank.
US scientists analysed data on 105,986 participants in the Nurses' Health Study, a major public health investigation focusing on women.
Over a period of almost 30 years, more than 7,600 of the women were diagnosed with invasive breast cancer.
The research, published in the Journal of the American Medical Association, showed that alcohol consumption between the ages of 18 and 40, and after 40, was strongly associated with breast cancer risk.
"Our results highlight the importance of considering lifetime exposure when evaluating the effect of alcohol, and probably other dietary factors, on the carcinogenesis process," wrote the scientists led by Dr Wendy Chen, from Brigham and Women's Hospital in Boston.
"However, an individual will need to weigh the modest risks of light to moderate alcohol use on breast cancer development against the beneficial effects on cardiovascular disease to make the best personal choice regarding alcohol consumption."
The reason why alcohol raises breast cancer risk is not clear, but may involve its effect on circulating oestrogen levels.