Tens of thousands of Americans will soon be forced by their health insurance to switch from one popular obesity drug to another that produces less weight loss.

It is the latest example of the consequences of secret deals between drugmakers and middlemen, known as pharmacy benefit managers, that are hired by employers to oversee prescription coverage for Americans. Employers pay lower drug prices but their workers are blocked from getting competing treatments, a type of insurance denial that has grown much more common in the past decade.

One of the largest benefit managers, CVS Health’s Caremark, made the decision to exclude Zepbound in spite of research that found that it resulted in more weight loss than Wegovy, which will continue to be covered.

Those research findings, first announced in December, were confirmed in an article published on Sunday in The New England Journal of Medicine. The study involved a large clinical trial comparing the drugs that was funded by Eli Lilly, the maker of Zepbound. Earlier research not financed by Eli Lilly reached similar conclusions.

Ellen Davis, 63, of Huntington, Mass., is one of the patients affected by Caremark’s decision. “It feels like the rug is getting pulled out from under my feet,” she said.

After taking Zepbound for a year, she has lost 85 pounds and her health has improved, she said. She retired after working for 34 years at Verizon, which hired Caremark for her drug coverage.