(CNN) — A class-action lawsuit has been filed against CVS Caremark over its decision to stop covering Zepbound, Eli Lilly’s blockbuster GLP-1 drug to treat obesity.

When the major pharmacy benefit manager informed patients about the change this year, it suggested that there was “another covered medication that’s safe and effective for your condition and may cost less”: namely Wegovy, from Novo Nordisk, with which CVS Caremark announced a partnership just before the change took effect.

But many patients and their doctors say the treatments aren’t medically interchangeable.

John Cole, who lives in the Midwest, says he lost more than 60 pounds over the course of about five months while using Zepbound. Letters from his family medicine doctor, cardiologist and rheumatologist all indicate that he’s benefited from this treatment. But he’s gained some weight back after losing access to Zepbound through CVS Caremark, and his care team has submitted dozens of letters to the company over the past two months amid repeated appeal denials.

“It is my professional opinion that the patient’s health problems all seem to be improving on Zepbound and we would be hesitant to switch medications when things are going well,” one of Cole’s doctors wrote to CVS Caremark.

Zepbound and Wegovy are both effective at treating obesity and approved by the US Food and Drug Administration to do so, but there are differences.

Studies have found that people who used tirzepatide injections such as Zepbound lost more weight and were more likely to reach specific weight loss targets than those using semaglutide medications such as Wegovy. The two drugs have a different set of broader indications, with Zepbound also approved to treat sleep apnea in people with obesity, for example. Side effects can vary too, sometimes making one medication more tolerable than the another.

The lawsuit, filed Wednesday in the Southern District of New York, alleges that the change from CVS Caremark – and subsequent denials of patient and doctor appeals – violates standards for evaluating the medical necessity of treatment options under the Employee Retirement Income Security Act, which is applicable to employer-sponsored health plans.

“Because of their different methods of action, different clinical outcomes, and different side effects for individual patients, Zepbound and Wegovy are not clinically interchangeable,” the complaint reads. “And as studies have proven, Zepbound is more effective than Wegovy in achieving weight loss, and only Zepbound is has been proven effective and approved by the FDA for treating sleep apnea in patients with obesity.”

The lawsuit seeks to restore coverage of Zepbound for people on CVS Caremark plans and award other “appropriate equitable relief.”

CVS Caremark says the suit is “without merit.”

“We’ll defend ourselves vigorously against those claims,” David Whitrap, vice president of external affairs for CVS Health, said in an email.

CVS maintains that Wegovy and Zepbound are “two clinically similar products” and that “forcing the drug manufacturers to compete with one another on price” will encourage both Eli Lilly and Novo Nordisk to lower prices for their products in the US and broaden access overall.

“Our formulary strategy maintains clinically appropriate coverage while using competition to drive lower costs. By drawing upon our decades of expertise in making prescription drugs more affordable and accessible, we are confident that our formulary move means lower costs and better outcomes for consumers and our customers,” Whitrap wrote in the email, also noting that a medical exceptions process is available for the “rare cases” in which “on-formulary options prove ineffective or cause harmful side effects.”

The coverage change made by CVS Caremark in July applies to the most common formulary template, which represents between 25 million and 30 million individuals overall.

An exclusive analysis of electronic health records for CNN by Truveta, a health-care data and analytics company, suggests that it has had a significant impact nationwide.

About 1 in 10 people using Zepbound switched to a different GLP-1 in July, the first month after the change – about 16 times more than typical, according to the Truveta data. The vast majority of those who stopped using Zepbound – more than 8 in 10 – switched to Wegovy.

And overall prescribing rates for Zepbound in July were half of what they had been for the past year and a half, the Truveta analysis found. New prescriptions for Zepbound fell for the first time in nearly a year, while first-time prescriptions for Wegovy – which had been declining – saw the largest monthly increase in more than a year.

Neither drug manufacturer commented on the lawsuit specifically, but they emphasized the differences between their products.

Eli Lilly says that Zepbound is a dual receptor agonist that “targets both GIP and GLP-1 receptors, unlike other incretins approved for obesity management, which target only GLP-1 receptors.” And Novo Nordisk says that Wegovy has the “broadest GLP-1 label,” including indications for reducing risk of major adverse cardiovascular events in some patients and for treatment of a certain liver condition in some adults.

“The notion that all GLP-1s are the same and ‘interchangeable’ is not correct,” Jason Brett, executive medical director for Novo Nordisk, said in a statement.

Some appeals to stay on Zepbound have been approved and other people have been able to switch to Wegovy with only mild side effects, but the change has been disruptive for many patients and their doctors.

The process is burdensome, said Dr. Jody Dushay, an endocrinologist at Beth Israel Deaconess Medical Center and assistant professor of medicine at Harvard Medical School.

“The majority of the switches have been OK; however, ‘OK’ means extra work for the pharmacist with whom I work and for me,” she said.

Also, there’s not a simple one-to-one conversion for Zepbound and Wegovy doses, and it can take a lot of trial and error to make the switch appropriate.

“When the dosing is not right, people may gain weight or plateau for a month and get frustrated,” Dushay said. “I’m deliberately more conservative with dosing so that people don’t get slammed with side effects.”

Cole says he’s spent more 100 hours over the past two months working to regain coverage for Zepbound – and the logistics and medical changes take him away from the things he cares about.

“When I got [Zepbound], I tell everybody, it’s like a gift to be able to lose this weight and to feel so much better and spend time with the grandkids and be able to go on walks and move around the yard,” he said. “Now, it’s something I shouldn’t have to be going through. They should be cared about my health and not take things away from me.”

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