NEW YORK (Reuters) - CVS Health Corp's CVS.N proposed purchase of Aetna Inc AET.N will change the way many major U.S. corporations buy health coverage for employees and raise new questions over the cost of those benefits, benefit consultants said.

CVS on Sunday said it planned to buy Aetna for $69 billion.

Most national companies employing more than 20,000 people keep their prescription drug benefits separate from medical coverage. They believe they are paying less by shopping those contracts around to competitors within each industry.

CVS and Aetna argue that their deal will lower healthcare costs for employees of their large corporate customers, giving the company greater clout to negotiate down drug prices and better manage the use of those medicines.

“It’s the lower overall cost of therapy. It’s not just the drugs. It’s not just the PBM (prescription benefit manager). It’s the overall outcome for the patient,” Aetna CEO Mark Bertolini told Reuters in an interview.

But employers are expected to scrutinize that kind of claim closely, according to benefit consultants in touch with hundreds of large employers.

So far, they are taking a "wait-and-see attitude as to whether there is a direct favorable impact on their pricing" of a CVS-Aetna combination, said Jim Winkler, senior vice president for health at Aon, part of Aon Plc AON.N.

Last year, large employers' concerns over two proposed mergers between health insurers Aetna and Humana Inc HUM.N and between Anthem Inc ANTM.N and Cigna Corp CI.N were a major factor in U.S. antitrust regulators blocking the deals.

Industry experts say that is less likely to happen with CVS-Aetna because of their minimal direct overlap, historically the main concern for customers. “By and large they are in separate places in the value chain, so it’s more of a vertical integration,” said Winkler.

FILE PHOTO - A customer waits at the counter of a CVS Pharmacy store in Pasadena, U.S., May 2, 2016. REUTERS/Mario Anzuoni/File Photo

CVS is the No. 2 U.S. provider of prescription drug benefits and competes with larger rival Express Scripts Holding Co ESRX.O. Aetna is the nation's No. 3 health insurer, competing against UnitedHealth Group Inc UNH.N, Anthem and Cigna to provide coverage for doctor and hospital visits.

A NEW LANDSCAPE

A combined CVS-Aetna will reorder those options, leaving Express Scripts as the only standalone company big enough to easily provide pharmacy benefits to top employers.

Aon’s Winkler expects this will lead large companies to turn to their insurer for pharmacy benefits the same way mid-sized companies have. About 63 percent of large corporations use a separate pharmacy benefit company, consultant Mercer’s 2016 employer survey found.

In scale, CVS and Aetna offer a much bigger pharmacy benefits manager than UnitedHealth, which expanded its OptumRx business with the $13 billion purchase of Catamaran in 2015.

Anthem in October said it would expand its own pharmacy benefits business, and hired CVS to help do so. That partnership could be thrown into jeopardy by the new Aetna agreement, health industry analysts said.

Insurers say combining the two benefits saves money. Large employers who combine the benefits under one insurer have begun to demand insurers hit a specific dollar figure of medical cost savings per member, per year, said David Dross, national pharmacy practice leader at Mercer, part of Marsh & McLennan Co. MMC.N.

Insurers may also try to make it more pricey for large companies to keep the benefits separate.

Some have started charging mid-size corporate customers, who employ 2,000 to 10,000 people, additional fees for integrating medical claims and pharmaceutical claims when they are managed by different companies, Dross said, and could require those fees for larger clients as well.

“The plans have sort of decided this is the model we need to move to now,” Dross said. He expects that next year, as large companies negotiate their benefits contracts, they will be asking much more than in the past “will a carve-out option look better, or does the carve-in option look better?”