A number of studies have linked regular drinking to a higher risk of breast cancer; it's thought that the risk reflects the effects of alcohol on women's levels of estrogen and other hormones. Similarly, hormone replacement therapy (HRT) after menopause has been shown to raise the risk of breast cancer.
The new findings, reported in the International Journal of Cancer, suggest that alcohol and HRT may combine to further boost the odds of developing the disease.
Among the more than 5,000 Danish women researchers followed, those who were on HRT and averaged one or two drinks per day had a three-fold higher risk of breast cancer than women who neither drank nor took hormones.
Women who had more than two drinks per day had a nearly five-times higher risk of the disease.
In contrast, drinking habits were not related to breast cancer risk among women who did not use HRT, according to Drs. Naja Rod Nielsen and Morten Gronbaek of the Danish National Institute of Public Health, Copenhagen.
The findings, according to the researchers, raise the possibility that drinking affects postmenopausal breast cancer risk differently depending on women's hormone use. However, more studies are needed to confirm the interaction between alcohol and hormones, they write.
The findings come from two decades' worth of data on 5,035 postmenopausal women who were surveyed about their drinking habits and hormone use at the beginning of the study. Over the next 20 years, 267 women developed breast cancer.
Overall, regular drinking was linked to a slightly higher risk of the disease. But when the researchers considered hormone use, drinking affected breast cancer risk only among women who were on HRT at the outset.In one previous study of postmenopausal women taking oral estrogen, those who drank were found to have estrogen levels that were three times higher than those of non-drinkers, Nielsen and Gronbaek note.
This may be one explanation for the higher breast cancer risk seen in this study, they write.
If further research confirms the findings, they conclude, there should be an impact on both HRT use and recommendations for "sensible drinking limits" among postmenopausal women.
