CVS Health Corp.’s bid for Aetna Inc., which could be valued at more than $66 billion, would — if successful — combine two businesses that have traditionally been separate, health insurance and pharmacy-benefit management.

The move has a clear precedent: UnitedHealth Group Inc.’s UNH, -1.51% pharmacy-benefit manager unit, Optum, which has driven drug savings for consumers and growth for the company for years.

But in the case of CVS CVS, -0.78%  , “cost savings for the consumer are unlikely to improve,” said Leerink Partners analyst Ana Gupte, in large part because AetnaAET, -1.09%  insures far fewer people than CVS pharmacy-benefit manager services cover.

“Savings could even deteriorate with AET already under the CVS fold, and plans such as ANTM ANTM, -0.89%  potentially leaving,” Gupte said, since the Aetna deal could overshadow Anthem’s recent and high-profile agreement with CVS.

And, although savings on medical costs are a major premise of the deal, Gupte was skeptical: “Medical cost savings through repurposing of the retail front stores is a tall order, financially challenging and a show-me story at best given the lack of alignment with the delivery access model by the American consumer.”

CVS has been pushing to expand the types of medical services available at its pharmacies, something the deal will likely accelerate, The Wall Street Journal reported on Friday.

Easy access to care, especially if it’s early in a disease or condition, is thought to keep health plan costs down, and some observers believe the deal will allow for medical cost savings.

To that, Gupte has another argument: UnitedHealth’s Optum is just, well, better.

“CVS is trying with the AET deal to emulate the Optum Rx/UNH Captive PBM business model with the aim of moving away from spread pricing on drug revenue, toward a business model that is tied to medical cost savings and insurance underwriting,” which determines how much a person’s health insurance should cost, Gupte said.

But because Aetna covers fewer individuals than even CVS’s PBM business, it should take a long time, Gupte said, and Optum is a “formidable competitor.”

Regulatory scrutiny of the mammoth deal is already widely expected. A dearth of pharmacy or medical cost savings is likely to put the merger at risk with antitrust regulators, Gupte said.

Consolidation has already been rampant among pharmacy-benefit managers (PBMs), with just a handful of players — including CVS Health — dominating the industry.

Some have argued that the deal making has benefited consumers, since more powerful PBMs can negotiate better drug prices, while others have argued that PBMs, which negotiate discounts off list prices, have a perverse incentive to drive list prices up. The same types of arguments are likely to pop up again with the CVS-Aetna deal.

But even if the acquisition doesn’t go through, PBMs will likely continue to expand their footprints. A potential entry by online retailer Amazon into pharmacy spurred CVS’s merger proposal, according to the WSJ.

And even outside of Amazon, health insurers have been questioning whether the PBM business model is still worth it. Anthem recently announced that it will start an in-house PBM, with CVS carrying out some pharmacy services.

For PBMs, taking on more looks like a way to prove their value and evade competitive risks. Express Scripts Holding Co. ESRX, -1.35%  , the largest stand-alone PBM, announced plans to buy the privately held medical benefit management company eviCore in early October.

CVS Health was not immediately available for comment. CVS Health shares dropped nearly 6% on Friday, after its interest in an Aetna deal was reported. Company shares have dropped 13.5% over the last three months, compared with a 2.3% rise in the S&P 500 SPX, -0.32%  .