A new study says one in three breast cancer patients identified in public screening programs may be treated unnecessarily.

Researchers in Denmark analyzed breast cancer trends for a period of at least seven years before and after government-run screening programs for breast cancer started in parts of Australia, Britain, Canada, Norway and Sweden.

The study says once screening programs began, more cases of breast cancer were inevitably reported. If a screening program is working, there should also be a drop in the number of advanced cancer cases in older women. But the study found little decline. Instead, the screenings simply reported thousands of more cases than previously identified.

Overall, the researchers found that one third of the women identified as having breast cancer didn't actually need to be treated.

The research is published in the BMJ, formerly known as the British Medical Journal.