Riggs and Greene acknowledge that insulins have gotten better over the years. "But whether each incremental innovation is worth the price we pay, in a world where insulin remains unaffordable to many patients with diabetes, is less certain," they wrote.

On a practical note, Dagogo-Jack pointed out that shipping and distributing insulin might cost as much or more than it does to manufacture a generic insulin.

"Insulin has to be liquid. It has to be stored in heavy glass containers, preserved in cool conditions, and insulin only has a shelf life of three to six months. Shipping may be more than the cost to produce insulin. These entry costs may have discouraged traditional generic companies from competing," he explained.

Also, the insulin market is relatively small, Dagogo-Jack said. Background information in the study said about 6 million people currently take insulin in the United States. Although the market may not be as large as it is for more common drugs, Riggs noted that current synthetic insulins are some of the highest-grossing medicines on the market.

The first patent on a long-acting synthetic insulin expired in June 2014. Several companies have announced plans to develop a biosimilar, or highly similar, version of long-acting synthetic insulin. The first such product was recently approved in Europe. And unregulated versions of these insulins are available in countries such as China, India, Mexico and Peru, according to the study.