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We usually assume that new medical procedures and drugs are adopted because they are better. But a new analysis has found that many new techniques and medicines are either no more effective than the old ones, or worse. Moreover, many doctors persist in using practices that have been shown to be useless or harmful.

Scientists reviewed each issue of The New England Journal of Medicine from 2001 through 2010 and found 363 studies examining an established clinical practice. In 146 of them, the currently used drug or procedure was found to be either no better, or even worse, than the one previously used. The report appears in the August issue of Mayo Clinic Proceedings.

More than 40 percent of established practices studied were found to be ineffective or harmful, 38 percent beneficial, and the remaining 22 percent unknown. Among the practices found to be ineffective or harmful were the routine use of hormone therapy in postmenopausal women; high-dose chemotherapy and stem cell transplant, a complex and expensive treatment for breast cancer that was found to be no better than conventional chemotherapy; and intensive glucose lowering in Type 2 diabetes patients in intensive care, which not only failed to reduce cardiovascular events but actually increased mortality.

In some instances, doctors routinely refused to give beneficial therapies despite a lack of evidence that they were harmful. Vaccines were unnecessarily withheld from multiple sclerosis patients in the belief that they increased flare-ups; women with lupus were denied oral contraceptives for fear they increased the severity of the disease; and epidural anesthesia was delayed during childbirth on the theory it increased the rate of Caesarean sections. Yet good studies showed that none of these fears was justified..

“Contradicted practices don’t disappear immediately,” said the lead author, Dr. Vinay Prasad. “There’s an inertia, a 10-year period of time when the contradicted procedure continues to be practiced.”

Dr. William E. Boden, chief of medicine at the Stratton VA Medical Center in Albany, who was not involved in the work, found the study useful and provocative. “It’s challenging us to look at things we’ve done and attempt to find whether there’s evidence to support their use,” he said. “There’s going to be increasing pressure to come forward with making sure that the health care dollars we’re allocating are being well utilized.”

Dr. Prasad, chief fellow in medical oncology at the National Cancer Institute, said that new medical appliances present a special problem. “Devices are particularly bad because they can be approved if they’re similar to ones already on the market,” he said. He cited as an example the Swan-Ganz catheter, a device threaded into the heart to monitor heart function and blood flow. It gives accurate information, Dr. Prasad said, “but that information doesn’t help. We continue to introduce new catheters all the time, lacking good evidence that they work. This is a tremendous waste of resources.”

Often doctors persist with procedures that lack evidence because they seem to make sense, Dr. Prasad said. “They all sound good if you talk about the mechanisms,” he said. “You have cholesterol-clogged arteries, it makes sense that if you open them up it will help. But when that was studied, it didn’t improve survival.”

Patients, too, like to talk about mechanisms, Dr. Prasad added. “They tend to gravitate toward the nuts and bolts — what does it do, how does it work?” he said. “But the real question is: Does it work? What evidence is there that it does what you say it does? What trials show that it actually works? You shouldn’t ask how does it work, but whether it works at all.”