The cost of insulin in the United States has almost tripled for certain diabetes patients during the last five years, according to a new research letter published in the Journal of the American Medical Association. Doctors say the troubling trend is forcing many low-income people across the country to either forgo life-saving treatment or “spread out” taking the blood-sugar-lowering drug, which often leads to disastrous health effects, the Hartford Courant reports.

According to a recent study by researchers at the University of Melbourne in Australia, the price of insulin worldwide tripled between 2002 and 2013. In the United States, just three pharmaceutical companies currently hold patents that allow them to manufacture insulin—Eli Lilly, Sanofi and Novo Nordisk. What’s more, the companies hiked the price of the drugs by 168 to 325 percent during the last five years.

Today, this means people living with diabetes often have to shell out up to $600 a month, depending on their insurance coverage, for regular doses of insulin, a drug used to treat diabetes since 1923. The issue, said industry experts, is that there isn’t enough competition among insulin manufacturers in this country. In addition, people living with diabetes who cannot afford treatment need generic insulin products made available to them. (Currently, there are none.)

The American Diabetes Association (ADA) commented that no person living with diabetes should go without insulin because of “prohibitive costs of accessibility issues.” But the ADA also said the high price of insulin is a complex issue affecting almost every stage of the U.S. healthcare industry.