Moderate drinking before and after a breast cancer diagnosis was also tied to better heart health and fewer deaths from non-cancer causes, the study team found.
"This is a lifestyle choice," said Dr. Pamela Goodwin from Mount Sinai Hospital in Toronto, who wrote a commentary published with the new study.
"With alcohol, what we're saying is, if you are someone who would like to have the odd drink, it's probably safe," she told Reuters Health. "We're not telling women to go out and start drinking."
Researchers asked close to 23,000 women who were diagnosed with breast cancer in 1985 through 2006 about their drinking habits, exercise and use of hormones before their diagnosis.
About 5,000 of those women were surveyed again about their diet and lifestyle habits a few years later.
The study team found women who reported drinking three to six alcoholic drinks per week before getting cancer were 15 percent less likely to die of the disease over the 11 years post-diagnosis, on average, compared to non-drinkers.
However, there was no link between either occasional drinking or heavier drinking before diagnosis and survival from breast cancer, Polly Newcomb from the Fred Hutchinson Cancer Research Center in Seattle and her colleagues wrote in the Journal of Clinical Oncology.
Research has shown regular drinking raises a woman's risk of developing breast cancer in the first place.
One possible explanation for the new findings, Goodwin said, is that alcohol predisposes women to a less-dangerous form of cancer, making their survival better than the average non-drinker who develops cancer. Or, she added, women who drink moderately may have a healthier lifestyle, in general, than non-drinkers or heavy drinkers.
About one in eight women in the United States will be diagnosed with breast cancer during her lifetime, and one in 36 will die of the disease.
Among women in the study who completed surveys in the years after getting cancer, post-diagnosis drinking did not affect the chance of dying from breast cancer - but it did seem to improve general health.
For example, women who drank 10 drinks per week were about half as likely to die of heart disease and 36 percent less likely to die from all causes combined than non-drinkers. The effect was similar, but not as strong, for women who had three to six or seven to nine drinks per week.
"What this finding does is it sort of frees up a woman to make that choice, whereas in the past we might have cautioned that women not even consider a single drink," Goodwin said.