Blue Cross-Blue Shield health insurer Anthem is suing pharmacy benefits manager Express Scripts over prescription drug prices.

Express Scripts runs prescription drug coverage for Anthem, the nation's second-largest health insurer. Anthem said Monday that it wants to recover damages for prescription drug prices that are higher than competitive benchmarks.

Express Scripts Holding Co. says Anthem's lawsuit has no merit and that it has acted according to the terms of its deal with the insurer.

Anthem Inc. says its federal lawsuit doesn't change the way customers get prescriptions filled.

Pharmacy benefit managers run prescription drug plans for employers, government agencies and insurers, among other clients. They use their large purchasing power to negotiate on price.

Shares of both companies slumped in midday trading while broader indexes edged up.