Drinking just one alcoholic drink a day significantly increases a woman's cancer risk, a large new study finds.

The study found that low to moderate drinking was linked to an increase in cancer risk and may account for nearly 13 per cent of the cancers of the breast, liver, rectum, and upper digestive tract combined.

The findings are in the online issue of the Journal of the National Cancer Institute.

The researchers examined the effect of alcohol consumption on women's cancer risk using the Million Women Study, which included almost 1.3 million middle-aged women in the United Kingdom.

Those women in the study who drank consumed, on average, one drink per day - which the study authors considered typical in most high-income countries such as the U.K. and Canada.

They found the risk of any type of cancer increased with increasing alcohol consumption, as did the risk of some specific types of cancer, including breast cancer, liver cancer and rectal cancer. Women who also smoked had an increased risk of mouth and throat cancers.

And the type of alcohol consumed, whether it was wine, beer or spirits, did not seem to have an effect on the link between alcohol and cancer risk.

Previous recent research has suggested that moderate alcohol consumption may offer benefits to the heart. But the excess cancer risk noted in this study may outweigh that benefit, say two doctors from the National Heart, Lung, and Blood Institute, in an accompanying editorial.

"From a standpoint of cancer risk, the message of this report could not be clearer. There is no level of alcohol consumption that can be considered safe," the editorialists write.

They note that these findings come from such a huge study population and should give readers pause about the effect their drinking is having on their health.

The incidence for these cancers was small for any individual woman, estimated at 118 per 1,000 women in developed countries (about 1.18 per cent). But every extra drink per day added about 15 cancers per 1,000 women.

"Although the magnitude of the excess absolute risk associated with one additional drink per day may appear small for some cancer sites, the high prevalence of moderate alcohol drinking among women in many populations means that the proportion of cancers attributable to alcohol is an important public health issue," the authors write.

The research echoes similar findings made last year by the National Cancer Institute which showed that women who had one to two drinks per day had a 32 per cent greater chance of developing breast cancer. Three or more drinks per day boosted the risk by 51 per cent.

Jasmine Lew, of the NCI, said at the time of that study that researchers aren't yet sure how alcohol influences a woman's breast cancer risk, but it may be that it acts on estrogen levels.

"What we think the alcohol is doing is increasing the amount of estrogen that a woman is exposed to, hence increasing her risk of getting this more common type of breast cancer," said Lew.

The Canadian Cancer Society advises that Canadians who choose to drink should limit the amount to:

• less than one drink a day for women (pregnant women should avoid all alcohol)
• less than two drinks a day for men