Newer drugs such as Merck's Januvia cost five to 11 times more than older, generic drugs such as metformin and glipizide, Alexander said. Januvia's third-quarter sales more than doubled to $379 million, the Whitehouse Station, New Jersey-based company said on Oct. 22.

Merck spokeswoman Amy Rose said developing and introducing new medicines for type 2 diabetes is "an important health priority."

"Not all diabetes treatments work for every patient, and it is important to research and bring new treatment options to market to help patients to control their blood-sugar levels," she said in an Oct. 24 e-mail. "The public health cost of diabetes is very high, but more than 80 percent of the cost is due to hospitalizations and outpatient care related to the management of diabetes-related complications. Drug cost represents less than one-fifth of the total cost."

Lilly spokeswoman Kindra Strupp said in an e-mail today that the company, along with marketing partner Amylin Pharmaceuticals Inc., temporarily assists low-income patients without public or private prescription drug coverage in obtaining Byetta.

The study found benefits from diabetes drugs including an improvement in the national average of a blood test from 1999 to 2004. That blood test, called hemoglobin A1c, is a three-month average of blood sugar and indicates how well a person's diabetes is being managed. Still the researchers said that doesn't prove patients are benefiting from the newer drugs.

"This near-doubling of diabetes costs may partly reflect better care, but we need to step back and examine the value of newer and more costly medications that may be overused," said Randall S. Stafford, a study author and an associate professor at Stanford University in Palo Alto, California.

"Just because a drug is new or exploits a new mechanism does not mean that it adds clinically to treating particular diseases," he said in a statement from the university. "And even if a new drug does have a benefit, it's important to consider whether that benefit is in proportion to the increased cost of new therapies."

The research was funded by the Robert Wood Johnson Foundation, the Agency for Healthcare Research and Quality and the National Institutes of Health.

In a separate study in Archives, an analysis of 40 previous trials found that the generic diabetes treatment metformin is associated with lower risks of death from cardiovascular disease. Among other diabetes treatments, the researchers found no beneficial or harmful cardiovascular effects mostly because of insufficient data.

The authors of the cost study recommended doctors in many cases prescribe older, cheaper drugs before giving patients the newer, more expensive ones.

Alexander compared the diabetes treatments to those for other conditions such as cholesterol in which new, more expensive medicines haven't proven to be more effective.

"There's a familiar pattern here," he said.