One of the nation’s biggest pharmacy benefit managers owes nearly $16 million in prescription overcharges, according to Ohio’s Attorney General Dave Yost.

Yost claims OptumRx, the PBM arm of UnitedHealthGroup, owes the state Bureau of Workers’ Compensation $15.8 million in overcharges connected to its dealings with state agencies. Yost claims the PBM failed to provide contractually agreed-upon drugs as part of its arrangement with the state, leading to the overcharges.

The bureau’s contract with OptumRx expired in October 2018 and it has since entered a contract with a different PBM.