Looking at wholesale data for around 27,000 prescription drugs, Hernandez and her colleagues found that, between 2008 and 2016, brand-name oral prescription drugs rose 9 percent annually, while injectable drugs increased 15 percent per year. Inflation, meanwhile, is only about 2 percent.
These percentage changes reflect list prices, which are set by manufacturers and don't fully match what consumers pay, since they don't account for rebates or insurance discounts.
Some increased prices, such as those for specialty and generic drugs, seem to be driven by the entry of new medications into the market, the researchers found. Otherwise, price increases don't necessarily reflect improvement or even change.
In 2014, the list price for a Sanofi brand of insulin called Lantus increased nearly 50 percent, for example, even though it had already been on the market for more than a decade. (The average net price, or what consumers pay, for Lantus has actually decreased in recent years, though, a spokesperson for Sanofi tells CNBC Make It.)
Pharmaceutical companies kicked off the new year with significant price hikes. Dozens of drugmakers raised the list prices of hundreds of medications an average of 6.3 percent on Jan. 1, according to a recent analysis from the health software company Rx Savings Solution.
"Those increases are examples of what we mean by year-over-year inflation in existing products," Hernandez says.
Gerard Anderson, professor of health policy and management at Johns Hopkins University, tells NPR that raising the cost of existing drugs benefits drugmakers and insurers.
"Research and development is only about 17 percent of total spending in most large drug companies," he says. "Once a drug has been approved by the FDA, there are minimal additional research and development costs so drug companies cannot justify price increases by claiming research and development costs."
The study did not examine why prices of existing drugs have gone up, but the researchers say a lack of competition and the regulatory environment in the U.S. allow "for price increases much higher than in other countries."