This, of course, creates a conflict of interest. Benefit managers are supposed to be driving down costs, but the system incentivizes them to choose the products with the largest rebates. It’s not clear whether most of these “savings” are passed along to consumers or simply pocketed. Last month, a large insurer, Anthem, complained publicly that its P.B.M., Express Scripts, was not sharing enough of its savings.

What we do know is that business is booming for P.B.M.s. Together, the three biggest benefit managers — Express Scripts, CVS Health and OptumRx — bring in more than $200 billion a year in revenue. They also control over 80 percent of the P.B.M. market, involving 180 million insured people.

Where does this leave my patient? Mrs. B has Medicare Part D coverage. She is responsible for co-payments on her insulin. But every year, by early fall, she typically reaches a coverage gap (known as the doughnut hole) when she becomes responsible for paying for insulin out of pocket. So Mrs. B skimps on her medicine, allowing her blood sugar to rise to worrisome levels.

There is some hope that the insulin industry is about to become more competitive. The patent on Lantus (Sanofi’s top-selling insulin) recently expired, allowing other companies to start preparing generic forms. The first generic competitor usually sets a price that is only slightly below the branded insulin. Research shows that once there are two manufacturers of a generic drug, the price typically drops by about half; with eight, it drops to about a fifth. But because insulin is a biosimilar, the decline may be more modest. And this will take time; additional testing is needed to ensure the safety and effectiveness of each new generic before it is approved.

IN the meantime, we need a fair and transparent system for setting prices. In much of Europe, insulin costs about a sixth of what it does here. That’s because the governments play the role of pharmacy benefit managers. They negotiate with the manufacturer directly and have been very effective at driving down prices. In the United States, we rely on the private sector and a free market for drug pricing. But in order for this to work, we need to regulate it better and demand greater transparency.

Over the past 10 years, the Federal Trade Commission has brought only a single enforcement action against benefit managers, over an issue of patient privacy violations.