CVS Health shares plunged in the premarket trading session in New York after first-quarter revenue and adjusted earnings missed Wall Street's average expectations. The drugstore chain also slashed its full-year profit outlook, citing rising medical costs in its Medicare insurance unit. 

Here's what CVS reported for the first quarter compared with average Wall Street analysts' expectations tracked by Bloomberg:

Adjusted earnings for the first quarter were $1.31 a share, well below the average estimate of $1.69 and $2.2 in the same period one year ago. Revenue of $88.4 billion missed estimates by nearly $1 billion but up 4% from the year-earlier period, driven mainly through its pharmacy business and insurance unit. 

Due to underwhelming first-quarter results, CVS lowered its earnings outlook for the full year to $7 per share from the previously announced $8.30. It also lowered its forecast for cash flow from operations by $1.5 billion to around $10.5 billion. 

Here's the full-year outlook:

The earnings report noted that rising medical costs were a significant factor in first-quarter results. CVS' Aetna division reported a 90.4% medical loss ratio, up from 84.6% a year earlier. A lower ratio indicates that the company received more premiums than it paid out in benefits, resulting in higher profitability. 

The source of the medical cost spike is a surge in Medicare Advantage patients returning to hospitals and doctor offices for procedures that were delayed during the Covid pandemic. Some of these non-essential medical surgeries include joint and hip replacements. 

The earnings report spooked investors. Shares are down 13% in premarket trading. 

If the losses hold, this will be the largest intraday drop for CVS in years...

And shares are touching Covid lows. 

"The current environment does not diminish our opportunities, enthusiasm, or the long-term earnings power of our company. We are confident we have a pathway to address our near-term Medicare Advantage challenges," CEO Karen Lynch said in the press release. 

Lynch continued: "We remain committed to our strategy and believe that we have the right assets in place to deliver value to our customers, members, patients, and shareholders."