A legal battle has pitted a major national insurer and its pharmacy benefit manager (PBM) against each other in dueling legal actions in litigation that seeks class action status that could include tens of thousands of claimants.

Anthem is one of the nation’s largest health insurers with more than 38 million members. Express Scripts handled more than 175 million claims for Anthem in 2015 alone, according to the complaint.

The saga starts when Anthem sued Express Scripts last March, accusing it of excessive pricing and operational failures. It also sought the right to terminate its 10-year contract with Express Scripts, which began in 2009.

Express Scripts was contracted as Anthem’s exclusive provider of PBM services for Anthem-administered health insurance plans for a ten year period. Part of the agreement was a “periodic pricing review” to ensure that Anthem was receiving competitive benchmark pricing for drugs. However, when Anthem engaged Health Strategy, LLC. as a private consultant to conduct a comprehensive market analysis Anthem discovered that Express Scripts did not provide competitive benchmark pricing.

Based on the Health Strategy’s analysis, ESI’s current pricing to Anthem exceeded competitive benchmark pricing by more than $3 billion annually, and $13 billion over the remaining term of the Agreement. This was the basis of the March Anthem v Express Scripts litigation.

In its counterclaims against Anthem, Express Scripts said the insurer rejected several proposals to renegotiate prices. In addition, Express Scripts’ legal document says Anthem was offered a choice of “less money up front but lower pricing” or a bigger upfront payment “with higher pricing for Express Scripts’ services.”

It chose the higher prices over the course of the contract in exchange $4.6 billion more in upfront fees, according to the PBM’s counterclaim. That money, Express Scripts’ documents allege, was then used by Anthem to buy back its own stock, rather than passing it along to health plan members. The stock buyback “applied upward pressure to Anthem’s stock price, thereby enriching shareholders and management,” the filing alleges.

Two months later, two health plan participants sued both companies under the Employee Retirement Income Security Act challenging Express Scripts’ alleged overbilling.

Express Scripts Inc. and Anthem Inc. are accused in a proposed class action of breaching their ERISA fiduciary duties by entering into the 10-year, multibillion-dollar prescription-drug agreement that caused plan participants to overpay for benefits ( Burnett v. Express Scripts, Inc. , S.D.N.Y., No. 1:16-cv-04948, complaint filed 6/24/16 ). “This action seeks to recover losses suffered by the plaintiffs…who overpaid and continue to overpay for the portions of the costs of prescription drugs…they are responsible for paying as plan participants,” says the lawsuit.

Express Scripts spokesman David Whitrap said the firm denies “the allegations and will defend ourselves vigorously.” Anthem, too, denied the allegations and said it would fight the charges. However, the “denied” allegations echo those in Anthem’s March lawsuit against Express Scripts, and counterclaims filed shortly thereafter by Express Scripts against Anthem.

The court has not yet decided if the suit will have class action status.

The federal judge has dismissed two of the six counterclaims that Express Scripts raised in Anthem $15 billion lawsuit. In a decision recently made public, U.S. District Judge Edgardo Ramos in Manhattan dismissed Express Scripts’ claim that Anthem breached an implied covenant of good faith and fair dealing, saying it duplicated a breach of contract claim. He also dismissed an unjust enrichment claim filed by Express Scripts.

Express Scripts has contracts with insurers and other administrators of workers’ compensation benefits in California. It is unclear if any of Anthem’s allegations apply to any of the California workers’ compensation pharmacy benefit contracts.