As soon as news surfaced last week about the potential merger of CVS Health and Aetna, all eyes turned to the looming threat from Amazon.

The online retailer’s flirtation with the pharmacy business is a factor, no doubt. But many industry experts say CVS and Aetna have another huge competitor on their minds: UnitedHealth Group.

UnitedHealth is best known as the nation’s largest health insurer, with more than 45 million members in the U.S. But behind the scenes, it has extended its reach deep into America’s medicine cabinets, operating rooms and doctor offices.

Its Optum unit fills more than 100 million prescriptions per month as a pharmacy benefit manager, poaching big customers from rivals CVS and Express Scripts. UnitedHealth owns more than 400 surgery centers and urgent-care clinics and runs medical practices for about 22,000 physicians across the country.

“People have gotten carried away with Amazon,” said Ana Gupte, a health care analyst at Leerink Partners. “CVS and Aetna is an Optum wannabe. UnitedHealth is the winning business model, and Optum is showing the way.”

UnitedHealth’s expansion into dispensing prescription drugs and treating patients has put the company on track to reach $200 billion in annual revenue this year and profits for the first nine months of 2017 already topped $7 billion.

UnitedHeath is admired on Wall Street for its dependable results and diverse stable of businesses, which helps insulate it from rough patches in the insurance sector. However, the prospect of further industry consolidation alarms some consumer advocates and health policy experts. And they say UnitedHealth hasn’t always been a good role model.

In 2009, U.S. Senate investigators said the company built an industrywide database that deliberately understated what insurers should pay for out-of-network care, exposing consumers nationwide to hundreds of millions of dollars in extra charges.

More recently, patients have accused the company’s prescription drug business, OptumRx, of overcharging for routine medications in order to pocket a pharmacy “clawback” that boosts profits. The company has denied any wrongdoing in response to lawsuits over the drug pricing.

Employers, lawmakers and consumer groups accuse the three largest pharmacy middlemen — Express Scripts, CVS and UnitedHealth — of keeping drug prices high and pocketing too many of the discounts they negotiate with pharmaceutical companies.

Consumer advocates also are concerned about the prospect of companies mining a vast supply of consumer data to maximize profits rather than improve care.

“It is hard to find instances where these very large companies used their market power for the good of consumers, rather than for their shareholders,” said Lynn Quincy, a consumer advocate and director of the Healthcare Value Hub at the Altarum Institute, a nonprofit think tank. “The lack of transparency at these really large companies is appalling. That’s why we’re skeptical it will make things better.”

In a statement, UnitedHealth said it’s committed to “helping people live healthier lives” and its Optum unit is trying to make the entire health system work better.

In the past, company executives have said they’re fighting on behalf of employers and consumers against high costs, as well as poor outcomes and mind-boggling complexity. Before the merger news, executives at Aetna and CVS had already hinted at working together to tackle many of the same issues through the retailer’s vast network of stores.

Last week, The Wall Street Journal broke the news about the potential merger between CVS and Aetna, which could be worth more than $66 billion. CVS and Aetna say they won’t comment on market rumors or speculation.

In general, these companies are trying to address problems familiar to most Americans: poor coordination of care. Doctors rarely talk to each other. It’s incredibly hard to share medical records among providers or even with patients. Despite a lot of talk about linking pay to performance, a surprising amount of medical care is still reimbursed under the old-fashioned fee-for-service model that rewards quantity over quality.

For some experts, CVS and Aetna are well-positioned to fix many of those issues and that might make their deal more likely to pass muster with antitrust officials.

UnitedHealth, which has reached into everything from home health care to billing technology, has won praise for some of its efforts. One 2015 study published in Health Affairs found that the company’s use of house calls helped reduce costly hospital admissions for Medicare patients by 14 percent.

“One of the big failures of the U.S. health care system has been fragmentation, and these vertical mergers are trying to cure that problem,” said Thomas Greaney, a former federal antitrust lawyer and now a professor at the University of California’s Hastings College of the Law in San Francisco.

“You want to encourage efficiencies and integration that helps promote better care and lower costs. But you don’t want that to turn into a local monopoly,” he added.

Farzad Mostashari, a former official in the Obama administration who has studied health care competition, said it’s too soon to tell whether a CVS-Aetna deal would be good or bad for consumers. But Mostashari, who now heads Aledade, a tech start-up that works with doctors, said it warrants intense scrutiny of the more subtle ways it could put rivals at a disadvantage.

“These vertical mergers can create competitive challenges where you use your dominant market position to tip the ball to yourself in another area,” he said.

This story was produced by Kaiser Health News, which publishes California Healthline, an editorially independent service of the California Health Care Foundation

KHN’s coverage of prescription drug development, costs and pricing is supported in part by the Laura and John Arnold Foundation.