A population-based case-control study on the association of exposure with risk of breast cancer.

Background: Due to a lack of evidence from large-scale epidemiological studies by far on this issue, whether there is a link between exposure and breast cancer risk remained inconclusive.

Methods: We conducted a population-based case-control study using Taiwan's National Health Insurance claim data, in which all breast cancer patients newly diagnosed between 2005 and 2008 were employed as the case group ( = 34,262) and a random sample of non-breast cancer individuals selected from 1-million beneficiaries registered in 2005 was served as the control group. For fair comparability, we employed the time density sampling method to select controls who were matched to case on date of breast cancer diagnosis and age with a case/control ratio of 1/3 ( = 102,786).

Results: We found that the use of presents a weakly but significantly protective effect on breast cancer (adjusted odds ratio (aOR) 0.95, 95% confidence interval (CI) 0.93-0.98), with a significant dose-gradient relationship. We also noted a stronger association with breast cancer with initial use of at a longer time before breast cancer diagnosis, and found that the seemingly protective effect of was more obvious among women who had initial use at 47-55 years (aOR 0.93, 95% CI 0.88-0.98).

Conclusion: This population-based case-control study revealed that exposure to showed a weakly but significantly protective effect on breast cancer risk, which could ease people's concern over the potential carcinogenic effect from exposure to .