NEW YORK (AP) — Pharmacy benefits manager Express Scripts Inc. said Wednesday that it is suing Walgreen Co., arguing the drugstore chain is trying to lure away its customers.

The lawsuit alleges that Walgreen is telling Express Scripts plan members, and especially Medicare Part D beneficiaries, that they will not be able to fill their prescriptions at Walgreen pharmacies unless they leave Express Scripts and switch to a new pharmacy benefits management plan. Express Scripts said this violates the contract between the two companies.

Express Scripts administers prescription drug benefits for health plan sponsors and members, and it pays drugstores like Walgreen to fill prescriptions. Walgreen has said it will stop doing business with Express Scripts Jan. 1 because the prescription drug manager is not paying it enough. The drugstore chain has said it will lose $5.3 billion in revenue as a result, but that it would rather do that than conduct business on unacceptable terms.