The cost of healthcare in general is a debilitating, pre-existing condition for Americans. But the high prices of prescription drugs usually stand out as a pain point. While there are many insidious reasons why Americans pay more—often far more—for their medicines than people in peer countries, exploitation of the US patent system is an obvious one. A study published Monday in JAMA highlights just how much patent exploitation has grown since 1990. In that time, researchers found that the number of patents on small-molecule drugs has more than tripled, going from an average of 2.1 patents per drug approved in 1990 to 6.9 for those approved in 2019.

Most of the growth was in “nonprimary” patents—patents that generally aren’t related to a drug’s active ingredient, but are instead for things like minor tweaks to a drug’s nonactive ingredients, updates to the way the drug is used, or the design of specialty delivery devices. Together, those extra patents can create a “patent thicket,” which delays the release of affordable generics. The study—led by S. Sean Tu at the University of Alabama—found that the increase in patents per drug extended the time a drug was patented from an average of two years in 1990 to 6.1 years in 2019.

The researchers focused on drugs granted FDA approval between 1990 to 2019, giving a five-year follow-up period. This likely underestimates current patent thicket sizes, as patent activity now extends up to nine years after FDA approval.