A 25-year, 90,000-woman study has found that mammograms play almost no role in whether or not a woman dies from breast cancer. Worse, some women diagnosed with and treated for cancer may have been exposed to harmful cancer drugs unnecessarily.

The study, published today in the British Medical Journal, looked at 89,835 women aged 40-59. Half those women received regular breast exams and mammograms while the other half received just breast exams. Five hundred of the mammogram women died from breast cancer compared to 505 of the exam-only women, an almost imperceptible difference.

Perhaps more damning is the statistic that one in five of the cancers found in the mammograms did not need to be treated at all, exposing women to unnecessary chemotherapy, radiation and/or surgery.

As the New York Times explains:

Many cancers, researchers now recognize, grow slowly, or not at all, and do not require treatment. Some cancers even shrink or disappear on their own. But once cancer is detected, it is impossible to know if it is dangerous, so doctors treat them all.

Other studies have suggested the same. In November 2012, the New England Journal of Medicine published a 30-year study saying that 31 percent of all breast cancers found by mammograms were over-diagnosed and that screening had "at best, only a small effect on the rate of death from breast cancer." A study in Norway found six to 10 women out of 2,500 were over-treated. And a study in the United Kingdom also found many cases of over-diagnosis.

In 2009, the U.S. Preventative Services Task Force changed its guidelines on mammograms. Where it used to recommend mammograms every year or two for all women over 40, it now recommends mammograms every other year for women 50-74 and no routine mammograms for women 40-49. 

The chief of cancer control for the American Cancer Society, Dr. Richard C. Wender, told the New York Times that his data showed that mammograms reduce breast cancer deaths in women in their 40s by 15 percent and women older than that by 20 percent. But, he added, the society is also going to issue "revised guidelines" later this year.