The health insurance giant Cigna said on Thursday that it had agreed to buy Express Scripts, the nation’s largest pharmacy benefit manager, in a $52 billion deal that could further reshape the roiling health care landscape.

The health insurance giant Cigna said on Thursday that it had agreed to buy Express Scripts, the nation’s largest pharmacy benefit manager, in a $52 billion deal that could further reshape the roiling health care landscape.

The deal is the latest in a wave of consolidations that is sweeping through the health care industry. Companies are reacting to concerns over rising health care costs and the possibility of powerful new rivals entering the fray. In particular, Amazon’s move into the health care business has forced established companies to rethink how they can compete.

Cigna and Express Scripts said the acquisition would benefit consumers by allowing the two companies to bring together patients’ medical and pharmacy histories to improve treatments and lower costs.

“This step furthers our strategy to improve the affordability and value to the consumer in a more personalized way,” said Cigna’s chief executive, David Cordani, who will serve as chief executive for the combined company.

The completion of the deal would mark the end of Express Scripts as the last major independent pharmacy benefit manager, one that has focused on striking deals with drug companies to lower costs for insurers and employers. The company is responsible for the prescription plans of more than 80 million Americans.

“This is the future of the stand-alone” pharmacy benefit manager, said Tim Wentworth, the chief executive of Express Scripts, who would serve as president of the Express Scripts business under the deal. The two companies said that they would continue to offer pharmacy services to other insurers and to employers that do not use Cigna.

The deal would ensure that all of the major pharmacy benefit managers would have ties to big insurers. CVS Health, which also owns pharmacies, recently announced a merger agreement with the health insurer Aetna. OptumRx is owned by the insurance giant UnitedHealth Group. Anthem, which operates for-profit Blue Cross plans in several states, said it had plans to create its own pharmacy business.

With no other large independent pharmacy managers left for smaller insurers, federal and state officials could be reluctant to approve the Cigna-Express Scripts deal, said David A. Balto, an antitrust lawyer who worked at the Federal Trade Commission and the Justice Department who is a fierce critic of mergers of insurers and pharmacy managers. The CVS-Aetna deal could face the same problem, he said. 

“History has shown where there are multiple mergers going on in a single industry, the Justice Department goes on high alert,” Mr. Balto said.

Other antitrust experts said it was unclear how the combination of Cigna and Express Scripts would reshape the market. “It’s not nearly as obvious an antitrust risk as other deals,” said Leemore S. Dafny, a business professor at Harvard. The deal could make the insurance industry more competitive by giving Cigna, which is not a major player in some markets, a way to challenge the dominant companies, she said.

Health insurers, thwarted by federal regulators in their attempts to combine with one another, have been hunting for possible acquisitions. Thursday’s deal came little over a year after a judge blocked a proposed $48 billion merger of Cigna and Anthem. A judge also blocked a $37 billion deal between Aetna and another health insurer, Humana, last year.

The insurance companies now say they need to integrate the delivery of care and pharmacy benefits into their own operations in order to tackle high medical costs. UnitedHealth, for example, has been acquiring doctors’ practices and surgery centers.

“We’re seeing a lot of vertical integration,” said Brian Marcotte, chief executive of the National Business Group on Health, which represents large employers. With insurers looking to operate their own pharmacy managers, “it seemed like this was the next logical shoe to drop.”

Frustration over the rising costs of health care is mounting among employers — who insure most working Americans — and consumers, who are being asked to pay more out of pocket. 

In January, Amazon, JPMorgan Chase and Berkshire Hathaway announced plans to team up to address spiraling health care costs. Berkshire’s founder, the billionaire investor Warren Buffett, described those costs as a “growing tapeworm on the American economy.”

Adam J. Fein, chief executive of the Drug Channels Institute, which studies the drug industry, said the recent mergers may lead to better coordination because the companies will be looking at health care from multiple angles. 

“When you have a hammer, everything looks like a nail,” he said. “If everything you are thinking about is pharmacy costs, everything looks like a drug pricing problem.”

Some experts, though, see the combinations as potentially harmful. George Slover, senior policy counsel for Consumers Union, which also raised concerns about the Aetna-CVS merger, said in a statement that these deals “could result in restricted choices throughout the marketplace, ultimately leading to higher costs and potentially poorer coverage and care for consumers.”

Craig Garthwaite, a health economist with Northwestern University’s Kellogg School of Management, said that whether consumers ultimately benefit “will hinge entirely on the amount of competition in the health insurance market.”

In recent years, the model of a pharmacy benefit manager has also come under scrutiny. The pharmaceutical industry, among others, argues that benefit managers are profiting from higher drug prices by keeping a percentage for themselves. The Trump administration made a similar argument in February, noting that the top three benefits managers controlled 85 percent of the market.

On Wednesday, the commissioner of the Food and Drug Administration, Dr. Scott Gottlieb, said in a speech that the situation had created “misaligned incentives,” as the discounts that manufacturers negotiate “may not always be passed along to employers or consumers.”

Mr. Wentworth, the Express Scripts executive, said the company was “fully transparent about how the money flows and the contracts we do.”

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