Wall Street analysts are a bit confused by pharmacy CVS Health's possible purchase of health insurance provider Aetna, the largest proposed deal ever for the insurance industry.

Calling the idea "insane," Jefferies health-care desk analyst Jared Holz said CVS' move to acquire Aetna came much sooner than he expected.

"My view is [that there is] too much debt here and CVS winds up being levered nearly 5x to get this done," said Holz in a note to clients. "The entire supply chain is a structural short until Amazon [is] proven to be [an] anemic player. Which won't happen."

If CVS bought the health insurer at the proposed price of $200 per share, the $66 billion deal would be the largest ever in the health insurance history, based on analysis of Thomson Reuters data.

Shares of CVS have fallen nearly 9 percent since the news of the deal broke, with the stock closing down 6 percent Friday.

Many Wall Street analysts feel that such a costly move by CVS would be in direct response to e-commerce giant Amazon.com's moves into the pharmaceutical space. CNBC reported in May that Amazon has been recruiting health insurance experts to develop an internal pharmacy benefits manager for Amazon employees.

"I hear smart people tell me it's a slam dunk and other people say it could have a lot of problems," Cowen analyst Charles Rhyee told CNBC. "It's a lot to spend, and it's not that by doing so CVS will automatically transform its retail footprint."

"We see the move into healthcare services as a natural defense against the potential threat of Amazon," he added in a note. "Our question, however, is why can't CVS affect the transformation into a healthcare platform on its own?"

Based on Rhyee's estimates, CVS could end up paying out more than $75 billion for Aetna, exceeding the company's current $70.9 billion market cap.

CVS did not immediately respond to CNBC's request for comment.

"Couldn't CVS effect the changes to its retail footprint and then partner with multiple health plans, including Aetna, and achieve similar results with less capital?" asked Rhyee, suggesting that CVS could "roll up" the entire $16 billion urgent-care market if it wanted.

To be sure, CVS has been trying to establish greater control in its corner of the health-care industry. In 2007, the company acquired Caremark pharmacy benefits manager, which has more than 75 million plan members.

Following the Caremark acquisition, the company changed its name to CVS Health from CVS and permanently halted cigarette and cigar sales in 2014.

— With reporting by Meg Tirrell.