It sounds like a simple idea for improving health care: draw up guidelines on how best to treat a particular illness and then pay doctors to follow them. That strategy, which some insurers and health plans already employ, has been embraced during the health care debate by some lawmakers in Congress who want to extend it more broadly.

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Fabrizio Costantini for The New York Times

Dr. Rodney A. Hayward, center, expressed concern about treating all diabetics with aggressive glucose control.

The goal is to improve treatment and, at the same time, save money. But setting guidelines that are good for every patient, it turns out, can get messy, with some experts warning that a big national plan of this sort poses risks. A recent case involving treatment for diabetes, one of the nation’s most pervasive illnesses, illustrates the difficulties.

Last year, a national guideline-setting group abruptly withdrew a controversial diabetes standard it adopted in 2006 that called for aggressive control of blood sugar, or glucose. The change came after a large federal study indicated that lowering glucose too quickly or too much in some patients could harm or even kill them.

In medical journal articles and elsewhere over the last year, some diabetes experts have lashed out at the group’s initial decision to approve the guideline, saying they warned back in 2006 that it was medically ill-advised for some patients.

“This was a case in which the advocates of a disease got caught up in their disease rather than the interests of patients,” said Dr. Rodney A. Hayward, a diabetes expert at the University of Michigan who had opposed the benchmark.

Critics like Dr. Hayward have also suggested that pharmaceutical companies influenced the guideline so they could sell more glucose-lowering drugs like insulin. The group that set the guideline, a Washington organization called the National Committee for Quality Assurance, received about $3 million, or 10 percent of its revenue, last year from drug and medical device makers.

The group’s officials, and several outside experts who advise it, rejected such suggestions of industry influence. Still, some experts question why the group, which sets standards that are widely used by insurers and private health plans, should take even small amounts of drug-industry money.

“There should not be any industry funding of a group that is involved in working on national guidelines,” said Dr. Jerome E. Groopman, a professor of medicine at Harvard.

To many experts, the diabetes case shows how setting one guideline that works for all patients suffering from the same disease can be tricky.

The main problem is that many guidelines are based not on rigorous studies like clinical trials but on weaker types of medical evidence. And critics like Dr. Groopman have argued that the guideline-setting process is often influenced by industry or by medical ideologues looking to advance their personal agendas.

“These guidelines often come out of specialty societies who tend to want to treat rather than not to treat,” said Arthur Levin, the executive director of the Center for Medical Consumers, a patient advocacy group. Mr. Levin also serves as an adviser to the National Committee for Quality Assurance and took part in the diabetes decision.

The diabetes controversy began after several studies showed that patients with diabetes could significantly benefit from tighter control of their blood sugar. As a result, in 2005, the American Diabetes Association, along with others, began urging the National Committee on Quality Assurance to adopt an aggressive glucose control standard. The group’s guidelines are used by insurers and others to assess the performance of doctors who work for them and, in some cases, to determine whether they qualify for a bonus.

On its face, the glucose proposal made sense. Excess levels of glucose can cause some of the worst complications of diabetes, including blindness and kidney failure. But lowering glucose too much can also create problems for some patients, particularly older, sicker ones, causing them to black out or to have seizures.

Pointing to such risks, some doctors like Dr. Hayward, who also works for the Veterans Administration, opposed extending the guideline too broadly. They said that one significant group of diabetics, those who also had heart problems or cardiovascular disease, had never been studied during the trials that showed the benefits of aggressive glucose control.

But supporters of the move, like Dr. Richard Kahn, then a top official of the American Diabetes Association, rejected such arguments. Dr. Kahn argued that even if the standard were adopted, doctors would still have plenty of discretion about how to treat their patients. “All of us can no longer sit silently thinking that our discipline is unified toward improving diabetes care,” he wrote in a 2006 e-mail message as the argument unfolded.