A blocked artery is not a good thing. Public health campaigns have drilled that message into the national psyche. Surely, then, whenever doctors find a closed artery, especially in the heart, they should open it.

Maybe not. A major study, presented Tuesday at a medical conference in Chicago, challenged the widespread use of tiny balloons and metal stents in people who had suffered heart attacks days or weeks before.

Although such treatment can be lifesaving in the early stages of a heart attack, the study found that opening the artery later did no good at all. It merely exposed patients to the discomfort, risk and $10,000 expense of an invasive procedure.

The new report is the latest example of a rigorous experiment turning medical practice on its head by proving that a widely accepted treatment is not the great boon it was thought to be (except maybe to the bank accounts of doctors, drug companies and makers of medical devices).

Ideally, treatments, operations and diagnostic procedures should be thoroughly tested before they come into routine use. But that is not always the case. Drugs and medical devices have to be approved by the Food and Drug Administration, but once they are on the market, doctors can prescribe them in almost any way they see fit, a practice called off-label use.

Migraine drugs are prescribed for weight loss, and heart pills for stage fright; nobody is breaking the law. At least one in five drug prescriptions are for unapproved uses, studies show, with some popular medicines getting more than 90 percent of their use as treatments for which they were never approved. Ideas for such uses may be suggested to doctors by drug companies.

The approval rules for devices are looser than those for drugs, and while there is little data measuring unapproved uses of medical devices, there are hints that off-label use there is even greater. The F.D.A. does not regulate surgery at all.

Some treatments -- like opening a closed artery -- appeal so strongly to common sense that it becomes irresistible to go ahead and use them without waiting for scientific proof that they are effective. That is especially true if patients are desperate and have few or no other options.

As the treatments start to catch on, people assume they must work, and it becomes difficult or impossible to study them in the most definitive way -- by comparing treated patients with an untreated control group. If most people think a therapy works, who wants to be the control? Doctors may balk at controlled studies, too, calling it unethical to withhold the treatment from patients in the control group.

Dr. Judith S. Hochman, a cardiologist at New York University who directed the recent study on stents, said she encountered exactly that attitude when she was trying to recruit other researchers for her study: some refused to participate, saying it was unethical to leave some patients without stents.

But the counterargument is that it is also unethical to subject people to medicines, operations and invasive tests and treatment without proof that they are safe and effective.

Medical history is strewn with well-intended treatments that rose and then fell when someone finally had the backbone to test them, and the scientific method trumped what doctors thought they knew.

Hormone treatment after menopause, which works for symptoms like hot flashes, was widely believed to prevent heart disease and urinary incontinence. But carefully done studies in recent years have shown that hormones can actually make those conditions worse.

Stomach ulcers were once attributed to emotional stress and too much stomach acid, and were treated with surgery, acid-blocking drugs and patronizing advice to calm down. Then, in the 1980s, two doctors who were initially ridiculed for proposing an outlandish theory proved that most ulcers are caused by bacteria and can be cured with antibiotics.

For decades, women with early-stage breast cancer were told that mastectomies offered them the best chance of survival. But in 1985, a large nationwide study showed that for many, a lumpectomy combined with radiation worked just as well.