Previous studies have shown that women who have more than a drink a day raise their risk of breast cancer, and that hormone replacement therapy also increases the cancer risk.
The Nurses' Health Study assessed the risk of the two factors combined. Researchers said the good news is that alcohol and estrogen together do not greatly magnify the danger through interaction. Some scientists were concerned that might be the case.
Instead, what they found is that a postmenopausal woman who has a lifetime breast cancer risk of 4 percent could increase the risk to 8 percent if she drinks and takes hormones.
"The public health message is that the two will substantially increase your risk of breast cancer and you might want to be particularly vigilant about having both of these risk factors," said study co-author Dr. JoAnn Manson of Brigham and Women's Hospital.
The research, published in Tuesday's Annals of Internal Medicine, was based on 44,187 participants in the Nurses' Health Study from 1976-96 and tracked more than 120,000 female nurses overall for a variety of research studies.
During the study years of 1980-94, 1,722 women developed breast cancer. Women who took hormones for at least five years but drank no alcohol increased their risk of breast cancer by about 30 percent, as did women who did not take hormones but consumed more than one drink a day.
For women who took hormones and drank alcohol, the risk nearly doubled.
Even so, the increased risk is "not big enough to say it'll kill you if you drink," said Dr. Norman Lasser of the University of Medicine and Dentistry of New Jersey, who is not affiliated with the Nurses' Health Study.
Study co-author Dr. Wendy Chen, of Brigham and Women's Hospital and the Dana-Farber Cancer Institute, said it's not necessary to stop drinking altogether. For those women who still take hormones, a good compromise would be to consume no more than one drink a day, she said. That way, women can still get the cardiovascular benefits of moderate alcohol use while eliminating the increased breast-cancer risk.
Government scientists raised the red flag on hormone-replacement therapy in July after they announced that long-term use of estrogen-progestin pills significantly increased the risk of breast cancer, strokes and heart attacks. Scores of women have since quit taking the hormone combination.
Researchers with the Women's Health Initiative study are still assessing whether the use of estrogen by itself - prescribed for women who have had hysterectomies - is safer than combination therapy. Most of the women in the Nurses' Health Study had hysterectomies and took estrogen-only pills.
"People are always having to assess how much risk they are willing to tolerate and whether the short-term gain you get (from hormones) is worth it," said Lasser, a co-author of the Women's Health Initiative study.