In a report published in the of the Journal of American Medical Association (JAMA), it was concluded that healthy middle-age women can have up to two alcoholic drinks a day without apparently being at risk of irregular heartbeat. However, more than two alcoholic drinks is associated with an increased risk of irregular heartbeat, also known as atrial fibrillation.

The current study is based on 34,715 women aged 45 and older and in good health at the start of the trial. None had atrial fibrillation or any other heart condition. They described their alcohol intake when the study began, and again 48 months later. Over an average follow-up period of 12.4 years, 1.9 percent of the women who had one drink or less a day developed atrial fibrillation, compared to 1.8 percent of those having one to two drinks a day and 2.9 percent of those having two or more drinks a day. Dr. Christine M. Albert, a professor of medicine at Harvard Medical School said, "So there is a 40 to 50 percent increase in the incidence of atrial fibrillation at about three drinks a day."

In atrial fibrillation, the two upper chambers of the heart (atria) beat irregularly and faster than they should. Blood can pool in the atria, leading to the formation of clots that can block a major artery to the brain, causing a stroke.

Alcohol consumption was assessed via questionnaires at the beginning of the trial and at follow-up checks. Irregular heartbeat was self-reported on annual questionnaires and later confirmed through a medical checkup.

David Conen of Brigham and Women's Hospital and Harvard Medical School, one of the study co-authors said "While this finding needs to be interpreted with some caution because of the small number of women in some subgroups, it supports a possible threshold effect in the relationship between alcohol consumption and risk of atrial fibrillation among women."