A study found that when employers switch to a three-tier prescription drug plan that charges a lot extra for brand-name medications, a disturbing number of people simply stop taking their blood pressure and cholesterol pills, instead of switching to cheaper varieties.

More and more employers have been adopting three-level pricing plans, which are designed to encourage employees to use generics or other less expensive drugs. Those who want to use pricier drugs have to pay the highest out-of-pocket expenses.

In a study of two such plans, as many as half of the employees changed from an expensive drug to a cheaper one when their benefits switched to three tiers. But many others quit getting their prescriptions for chronic conditions.

While researchers said they did not know exactly why the employees gave up their drugs and what effect that had on their health, “it is worrisome to see that people have stopped taking their medication,” said one of the researchers, Dr. Patricia Deverka of Medco Health Solutions, the nation’s largest pharmacy benefit-management company.

The study was conducted by researchers at Harvard Medical School and Medco and was reported in today’s New England Journal of Medicine.

Nearly all employer-provided insurance plans offer workers some kind of prescription drug benefit, and 63 percent of those employees are now in three-tier plans, up from 27 percent in 2000, according to the nonprofit Kaiser Family Foundation. The newly passed Medicare legislation allows for the use of tiered pricing for its prescription drug benefit.

Under the plans, employees pay the lowest copayment for generic drugs, a higher amount for brand-name drugs on a preferred list, and the highest amount for other brand-name medicines.

The study examined how the move to a three-tier plan by two large companies in 2000 affected spending and the use of three commonly prescribed drugs: ACE inhibitors for high blood pressure and heart failure; proton pump inhibitors for acid reflux; and statins for high cholesterol.

The first employer, with 55,500 hourly workers, went from a $7 copayment for all prescriptions to $8 for generics, $15 for preferred brand names and $30 for third-tier brand names. The second employer, with 11,600 salaried workers, had a less dramatic change, adding a third level of $24 for brand names to its two-level plan of $6 for generics and $12 for preferred brand names.

The researchers also looked at similar plans at other companies to see what happened when no changes in copayments occurred.

Only in the first employer group were workers more likely to quit taking their top-tier drugs when the copayment jumped. About 16 percent stopped taking their ACE inhibitors, compared with 6 percent in the comparison group, 21 percent stopped statins compared with 11 percent, and 32 percent dropped acid reflux drugs compared with 19 percent.

Cindy Parks Thomas of Brandeis University, who wrote an accompanying commentary, said more research was needed to find out more about who was quitting their drugs — sick people, or perhaps those who really didn’t need the medication.

“The actual impact on health is a big question mark,” she said.