A woman's risk of developing breast cancer increases by six percent for every extra alcohol-containing drink she consumes on average daily but is not affected by smoking, according to a report in the British Journal of Cancer from Cancer Research UK.

"There has been a great deal of research on whether smoking or alcohol contribute to breast cancer but until now results have been confused," said Sir Richard Doll, co-author of the study. "For the first time we have undertaken a study large enough and detailed enough to look at the separate effects of tobacco and alcohol reliably. When we did this we found that drinking, but not smoking, increases the risk of breast cancer."

Researchers believe that alcohol accounts for approximately four percent of breast cancers among developed countries.

Previous research on the risk of breast cancer from smoking and drinking has yielded conflicting results because many women who drink alcohol also smoke cigarettes and vice versa. Because of the large size of this new study, researchers were able to separate the two factors and make much more accurate estimates on the risks linked to smoking and drinking. Results were combined from 53 studies and included data on 153,582 women from around the world.

"This research tells us there is a definite link between alcohol and breast cancer and the evidence suggests that the more a woman drinks the greater her risk," said Professor Valerie Beral, co-author of the study. "The impact of drinking on breast cancer is small compared to childbearing factors, but women are drinking more now than they used to and if this pattern continues it is bound to have an impact on the rates of breast cancer in the future."

Although women who drink alcohol are at a higher risk for developing certain cancers including cancers of the mouth, larynx, esophagus and liver, they are at a lower risk of heart disease and stroke than non-drinking women.

"The balance between the harmful effects of alcohol on breast cancer and its beneficial effects on heart disease depend on a woman's age," said Dr. Gillian Reeves, co-author of the study. "Before about 60, breast cancer is a more important cause of death than heart disease. After the age of 65 or so, when the risk of heart disease becomes much greater than the risk of breast cancer, the benefits of moderate drinking are more apparent."

"Large studies of this kind are very important for dissecting the complex causes of cancer," said Sir Paul Nurse, Chief Executive of Cancer Research UK. "This research doesn't alter our advice on smoking because we already know that it's dangerous but it does reinforce our advice that excessive drinking can also be hazardous."

"It seems that women's attitudes to alcohol are changing and this can only have a negative impact on their health," Nurse continued. "It's important that we get the message out to young women that drinking too much is dangerous."

Other sources: British Journal of Cancer