For the millions of Americans that take one or more prescription drugs, having a health insurance plan that covers their medications is crucial. The list of covered medications — called a drug formulary — can mean the difference between a ten-dollar copay at the pharmacy and paying thousands out of pocket for a vital medication.

Yet unbeknownst to many patients, insurers can change their drug coverage throughout the year, thereby removing medications that enrollees were promised. When this happens, those who lose access to their medicines are usually barred from immediately moving to a different insurance plan. The problem is widespread and growing: in the last nine years, the number of medications being eliminated from many insurance plans skyrocketed by around 1,584 percent.

Drug formularies are dictated by pharmacy benefit managers, or PBMs — powerful companies that determine drug benefits on behalf of health insurers, Medicaid Part D drug plans, employers, and other health care payers. These companies negotiate with pharmaceutical manufacturers to determine what drugs health plans will cover and how much the drugs will cost. The three largest PBMs are owned by major health insurance companies, a form of vertical integration that some health care experts worry may be leading to higher drug prices and lower quality care for patients.