February 13, 2014 11:07 AM EST

The Cancer-Prevention Mantra “the more screening, the better” just took another hit. After following nearly 90,000 women for 25 years, a group of researchers in Canada found that women who got mammograms every year for five years were no less likely to die of breast cancer than those who did not. The study supports the controversial recommendation in 2009 of the U.S. Preventive Services Task Force (USPSTF) that women cut back on yearly mammograms and start at age 50 instead of age 40.

The main reasons: overdiagnosis and overtreatment. When a mammogram detects an irregularity, there’s a natural urge to correct it. But 22% of the breast cancers detected in the Canadian study may not have been malignant. That means those patients could have been subjected to unnecessary procedures, such as surgery, chemotherapy and radiation, which could actually do more harm than good, given their complications.

So why did doctors push for yearly mammograms in the first place? Before the more detailed analysis of risks and benefits in 2009, doctors believed that more cancer screenings meant better chances of detection and greater opportunity for lifesaving treatments. This message was driven home by advocacy groups that turned screening into practically an obligation for anyone health-minded and led to insurers’ covering routine testing.

With more big-picture data from groups like USPSTF, however, it’s clear that for most women under 50, annual screenings don’t lead to fewer deaths from the disease. Better treatments for breast cancer have also improved survival odds, so diagnoses don’t necessarily mean a higher chance of early mortality. (These findings do not apply to women with a family history of breast cancer, who have a higher risk of disease.)

While some doctors have already started advising women to get fewer mammograms, many major cancer organizations, including the American Cancer Society, continue to recommend annual screenings beginning at age 40. Studies like this one, however, could change that and lead to screening practices that are more grounded in solid scientific evidence.

This appears in the February 24, 2014 issue of TIME.

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