Express Scripts sees small risk in Walgreen spat

"We would expect that some small fraction of our business might move away," Hall said at the Morgan Stanley Global Healthcare Conference in New York. "We don't think that's a meaningful amount of our business."

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Hall said there was no cut-off deadline after which no resolution could be reached between the two sides. But he said that after Jan 1, when Walgreen stores would leave the network, some members may not return to the stores should an agreement eventually be struck.

"In general, they don't move back," he said.

Walgreen resolved a similar spat with CVS Caremark Corp last year, but it appears to be digging in its heels more deeply in the dispute with Express Scripts.

Last week, Express Scripts filed an injunction in federal court in Illinois alleging Walgreen published false marketing materials designed to encourage Medicare recipients to leave health plans that are Express Scripts clients.

Walgreen, whose "white paper" to clients outlined its plan to leave its Express Scripts' business, was not immediately available to comment.

The stakes in the dispute with Walgreen rose higher in July when Express Scripts announced its $29 billion deal to acquire rival Medco Health Solutions Inc and create a powerhouse in managing prescription drug benefits.

However, the deal is attracting significant antitrust scrutiny, and Express Scripts said earlier this month that the U.S. Federal Trade Commission had asked for additional information about it.

Some analysts have said it is a coin flip as to whether the deal wins regulatory approval. Medco shares are trading about 24 percent below the offer price, reflecting in large part skepticism the deal will be completed.

Hall said he believes the main regulatory concern is whether the deal would be pro-competitive and reduce costs.