By looking at what people and their insurers paid for diabetes medications from 2002 through 2013, the researchers found that the average price of insulin has nearly tripled, from $4.34 per milliliter in 2002 to $12.92 in 2013. Since 2010, the cost of spending on insulin per person is higher than spending on all other diabetes drugs. The study is published in the journal JAMA.

“The dramatic increase in the cost of insulin in the United States may deny some people access to a life saving therapy,” says study author Dr. William H. Herman of the University of Michigan. “The price increases have had the greatest impact on individual patients without pharmacy benefits who pay out-of-pocket for their insulin, not on pharmacy benefit managers for large insurance companies who receive discounts.”

Over the study period, the cost of insulin per person more than tripled, from $231 a year to $736 a year. Simultaneously, insulin use and dosing also went up, which the researchers say may be due to the rise of overweight and obesity nationwide. Obesity can contribute to diabetes, and weight gain interferes with how much insulin a person needs to maintain blood sugar control.

“Although the newer, more expensive insulins have some advantages over the older, less expensive insulins, physicians and patients must ask themselves if those advantages justify their use, especially for patients with type 2 diabetes,” says Herman.