With one bit of bad news, CVS Health has lost more than $12 billion of its stock market value.

The pharmacy benefits manager and drugstore chain’s shares fell 16% on Tuesday morning after it said recent moves by health insurers seeking to cut costs by limiting pharmacy choices would slam CVS Health. Those insurers are bringing their business to arch rival Walgreens, costing CVS a big chunk of its drug-filling business.

CVS warned investors of a likely “loss of more than 40 million retail prescriptions related to new restricted pharmacy networks,” the company said in its third-quarter earnings release. Last year, CVS filled or managed approximately 1 billion prescriptions, giving it 21.7% of the market and making it the biggest player.

CVS’ prescription business has lost a lot of ground to Walgreens WBA of late: In September, CVS lost a contract to fill prescriptions through the Tricare pharmacy network. Tricare, the health care program with 9.7 million members made up of uniformed service members and their families, is owned by large Blue Cross and Blue Shield plans, which are restricting health plan enrollees to Walgreens as of Jan. 1.

CVS also recently lost a contract with the fourth-largest U.S. PBM, Prime Therapeutics, which has 22 million members, to Walgreens.

“Very recent pharmacy network changes in the marketplace are expected to cause some retail prescriptions to begin migrating out of our pharmacies this quarter,” said CVS CEO Larry Merlo.

That combined with slowing growth in the overall U.S. prescriptions market led CVS Health, the No.7 company on the Fortune 500, to lower its profit forecast for 2017.

 

CVS relies on filling prescriptions to bring people into its stores to also buy general merchandise like toothpaste. But dispensing drugs generates about 73% of its retail revenues, which came out to $50 billion in 2015. In the most recent quarter, same-store sales for general merchandise fell 1% as fewer people came into CVS stores. CVS’ Caremark business last year generated $100 billion and is the second largest PBM behind Express Scripts. esrx

The company now expects 2017 adjusted profit in the range of $5.77-$5.93 per share, below analysts’ estimate of $6.52 per share, according to Thomson Reuters.