The system, often called Tier 4, began in earnest with Medicare
drug plans and spread rapidly. It is now incorporated into 86 percent of those plans. Some have even higher co-payments for certain drugs, a Tier 5.
Now Tier 4 is also showing up in insurance that people buy on their own or acquire through employers, said Dan Mendelson of Avalere Health, a research organization in Washington. It is the fastest-growing segment in private insurance, Mr. Mendelson said.