Women who consume two to five alcoholic drinks per day have a 40 percent greater chance of developing breast cancer than their non-drinking counterparts, according to a study of more than 300,000 women conducted by the Harvard School of Public Health (HSPH) and 11 institutions in four countries.

Each additional drink per day was shown to increase a woman's risk of breast cancer by about nine percent.

Stephanie A. Smith-Warner, a lead author on the study and a research fellow in nutrition at HSPH, said the study's results were widely applicable.

"We didn't see that the association was limited to a particular subgroup like smokers or postmenopausal women, so we believe that the association applies to most women," she said.

The research teams selected more than 300,000 female volunteers who did not have breast cancer and followed them over a period of five to 11 years, depending on the specific study.

According to the study, 4,335 participants developed breast cancer. Participants were then asked to describe their diet and alcohol intake, and those who consumed a significant amount of alcohol on a regular basis showed higher instances of breast cancer.

Researchers did not record information on the specific drinking patterns of respondents. According to Smith-Warner, the finding that one drink per day increases the risk of breast cancer by about nine percent does not necessarily mean that the drinks were distributed equally throughout the week.

Smith-Warner said it could also mean seven drinks were consumed on the weekend, a number frequently surpassed by binge drinking college students.

The study showed that the correlation between alcohol consumption and breast cancer remained the same whether the alcohol came from beer, wine or liquor.

"The source of alcohol did not influence the risk," Smith-Warner said.

The cumulative lifetime risk for breast cancer in American women is about 13 percent, Smith-Warner said.

According to a press release from HSPH, the 40 percent increased risk associated with alcohol consumption is about the same as the risk associated with a family history of breast cancer, menstruation before the age of 12, or never having children.

An earlier period or the lack of children both contribute to higher lifetime levels of estrogen, which has been postulated as a possible cause of breast cancer.

Smith-Warner noted that alcohol in moderation can even have positive effects on one's health.

"One to two alcohol-containing drinks has been consistently shown to offer a protective benefit against heart disease," Smith-Warner said. "Women should weigh the risks of alcohol consumption against other benefits."

"Now we are looking at the relationship between other dietary factors and breast cancer, like vitamin C, vitamin E, beta carotene and types of fat," she said.