Diabetes drug costs in the U.S. have almost doubled in six years to $12.5 billion as more people are diagnosed with the condition and patients receive newer, more expensive treatments such as Merck & Co.'s Januvia and Eli Lilly & Co.'s Byetta, a study found.

Researchers, writing in this week's issue of the Archives of Internal Medicine, questioned whether these newer drugs are more effective then older, cheaper treatments.

Diabetes, the inability to properly process sugar in the blood, is one of the world's fastest growing diseases, as people eat more and exercise less. About 18 million people in the U.S. were diagnosed with diabetes in 2007, an increase from 8.1 million in 1994, according to the Centers for Disease Control and Prevention, a U.S. agency in Atlanta. Worldwide sales of diabetes drugs may jump to $22 billion by 2016, analysts say.

"Are the newer costs worth it, is really the million dollar question," said lead author G. Caleb Alexander, an associate professor of medicine at the University of Chicago, in an Oct. 24 telephone interview. "The jury is still out if the greater costs we document are worth the money."

The study found the cost for diabetes drugs rose 87 percent to $12.5 billion in 2007 from $6.7 billion in 2001. Type 2 diabetes, which is linked to age, obesity and lack of exercise, accounted for as many as 95 percent of diabetes cases in the U.S. People with the condition don't produce enough insulin, or their cells ignore the insulin needed for the body to convert blood sugar to energy, according to the National Institutes of Health, a U.S. agency. The condition can lead to complications such as blindness, kidney damage and heart disease.

Researchers in the study used two national databases to look at trends in diabetes treatment in patients ages 35 and older with type 2 diabetes who visited doctor's offices from 1994 to 2007. Information on drug costs was available from 2001 to 2007.

The authors attributed the rising number of diabetes cases to Americans' calorie-rich diets and sedentary habits, and because doctors are screening more intensively and diagnosing patients who have milder stages of the disease. The CDC has said an additional 5.7 million Americans have the condition and don't know it.

The researchers also found that more people are being prescribed more than one drug. In 2007, 47 percent of patients received only one diabetes treatment, down from 82 percent in 1994.

The average price of diabetes' prescriptions rose to $76 in 2007 from $56 in 2001, the authors said.