When enrollees’ drug costs are relatively low, plans pay a large share, typically about 75 percent. But when enrollees’ drug spending surpasses a certain catastrophic threshold — set at $5,000 in out-of-pocket spending in 2018 — 80 percent of drug costs shifts to a government program called reinsurance. This gives people in charge of private insurance plans an incentive to find ways to push enrollees into the catastrophic range, shifting the vast majority of drug costs off their books. For example, they could be less motivated to negotiate for lower drug prices for certain types of drugs if doing so would tend to keep more enrollees out of the catastrophic range.

Reinsurance spending, which is not reflected in premiums, has been rising rapidly.

“This harms the very competition that Part D was supposed to establish,” said Roger Feldman, an economist at the University of Minnesota. Consumers are naturally attracted to lower-premium plans, but choosing them increasingly shifts higher costs onto taxpayers if plans achieve those lower premiums in part by shifting more drug expenses onto the government’s books.

Documenting this is a recent study by Mr. Feldman and Jeah Jung of Penn State University that was published in Health Services Research. The study found that the disconnect between premiums and reinsurance costs has increased over time. Additionally, insurance company plans exhibiting less of an effort to manage the use of high-cost drugs had higher reinsurance costs. This is consistent with incentives to encourage enrollees into the catastrophic range of spending.

The Medicare Payment Advisory Commission has been warning about this problem for several years in its annual reports to Congress. According to MedPAC, between 2010 and 2015, the number of enrollees entering the catastrophic drug cost range grew 50 percent, from 2.4 million to 3.6 million, now accounting for 8 percent of enrollees.

“It’s ironic for a program supposedly built on market principles,” said Mark Miller, a former MedPAC director. “You wouldn’t see this kind of thing in the commercial market.” For commercial market insurance products — such as those offered by employers or in the health insurance marketplaces — only about 1 percent of policyholders reach a catastrophic level of expenditures at which reinsurance kicks in. (Mr. Miller and I are co-authors of an editorial about Ms. Jung’s and Mr. Feldman’s study, which also appears in Health Services Research.)