Shares of Express Scripts Inc. and Medco Health Solutions Inc. fell Monday on reports that antitrust regulators are deciding whether or not they will try to block Express Scripts' purchase of Medco.

Reuters reported Monday that some FTC officials want to block the $29.1 billion deal, and will decide in late February or early March if they will file a lawsuit to stop Express Scripts from buying Medco. At the same time, a group that represents hundreds of grocery-store chains is lobbying the FTC to stop the companies from combining.

The Food Marketing Institute represents around 1,250 grocery-store operators that run 26,000 stores and 14,000 pharmacies. Members include Wal-Mart Stores Inc., Kroger Co., Safeway Inc., and Supervalu Inc.

In a letter to FTC Chairman Jon Leibowitz, the FMI said the supermarket industry is very competitive and profit margins are low. A combined Express Scripts-Medco would be able to drastically cut reimbursement rates, which would "destroy competition."

"A merger of these two giants would lead to the demise of the reduced price and free generic drug programs that millions of Americans depend on, and reduce access to other key pharmacy services," the group wrote. Drugstore and pharmacist groups have said they oppose the deal because it would reduce access to independent pharmacies and drive up costs.

Express Scripts agreed to buy Medco on July 20 for $29.1 billion in cash and stock, or $71.36 per Medco share. Express Scripts and Medco are two of the three largest pharmacy benefits managers in the U.S., and the combined company would be by far the largest in the industry.

Express Scripts and Medco say that if they merge, they will be able to cut costs and make the health care system more efficient.

Express Scripts is based in St. Louis and Medco's headquarters are in Franklin Lakes, N.J. Shares of Express Scripts lost $2.41, or 4.6 percent, to end at $49.66 on Monday. Medco shares fell $5.14, or 8.1 percent, to $58.47.