A paper published in the New England Journal of Medicine delves deep into the reasons why many of the nearly 29 million Americans living with diabetes cannot afford insulin, a med that’s more than 90 years old, NPR reports.

Today, there’s no cheap generic insulin available to people living with diabetes to help regulate their bodies’ blood sugar levels. This means diabetes treatment can cost up to $400 each month for patients who need insulin.

According to Jeremy Greene, MD, PhD, a professor of medicine at Johns Hopkins University and an author of the report, in the early 1920s, insulin was originally extracted from the pancreas of cattle and given to people who had diabetes. Fifty years later, scientists successfully used DNA technology to produce large quantities of the hormone from a human gene. Greene said that within years, this new technique, called recombinant DNA technology, pushed the cheaper animal insulin off the U.S. market.

Greene said there’s no one reason why companies stopped producing cheaper animal insulin. He believes businesses felt this version of the hormone lacked profitability. Other docs suggested that marketing hype may also have swayed physicians to believe the newer insulin is better than the older version.

Indeed, recombinant insulin does have some benefits. This version of the hormone requires fewer injections and has fewer side effects for many patients. But for some patients, animal insulin is better. This version causes less variability in blood sugar and fewer episodes of hypoglycemia, or low blood sugar.

In the near future, industry experts expect insulin costs to fall. That’s because the most recent insulin patents have expired, which may finally pave the way for companies to create new, inexpensive generic versions.

Did you know that insulin is also available in an inhaler? Click here for more information.