At a time when Americans are being forced to pay a growing share of their medical bills and when access to medical care has become a major political issue for states, Congress and the presidential candidates, health care experts say it will be far harder to hold down premiums and expand insurance coverage unless money is spent more wisely.

The problem is not that newer treatments never work. It is that once they become available, they are often used indiscriminately, in the absence of studies to determine which patients they will benefit.

Some new treatments, like the cancer drug Gleevec and implantable heart defibrillators, undoubtedly save lives, contributing to the United States’ reputation for medical breakthroughs. But others  like artificial spinal disks, which can cost tens of thousands of dollars to implant but have not been shown to reduce back pain in many patients, and Vytorin, a new cholesterol drug that costs 20 times as much as older medicines but has not been proved superior  have been criticized for not justifying their costs.

And sometimes, the new technologies prove harmful. Physicians were stunned, for example, when clinical trials showed last year that expensive anemia medicines might actually hasten death in kidney and cancer patients. Such drugs are used more widely in the United States than elsewhere.

“We have too many situations where we thought we knew what the answer was and it didn’t turn out like everyone thought,” said Dr. Mark Hlatky, a cardiologist and professor of health research and policy at Stanford University.

A Tool of Dubious Value

The problem of inadequate study is especially serious for medical devices and imaging equipment like scanners, which typically are not as strictly regulated as prescription drugs. Under Food and Drug Administration regulations, the makers of CT scanners  CT is short for computed tomography  do not have to conduct studies to prove that their products benefit patients, as drug makers do. The manufacturers must certify only that the scanners are safe and provide accurate images.

Once the F.D.A. approves a test or device, Medicare rarely demands evidence that it benefits patients before agreeing to pay for it. But last year, Medicare officials raised questions about the benefits of CT heart scans and said it would demand more studies before paying for them. But after heavy lobbying by cardiologists, Medicare backed down. Private insurers, while initially reluctant to pay for the tests, are also covering them.