An alliance of drugstores and community pharmacists has filed a federal lawsuit to stop Express Scripts Inc.'s $29.1 billion takeover of Medco Health Solutions Inc., a deal they fear would create a giant pharmacy benefits manager with too much leverage and market share.

The National Association of Chain Drug Stores and the National Community Pharmacists Association say a combination of two of the biggest U.S. pharmacy benefits managers would have dire consequences for patients and retail community pharmacists. They warn that the deal could reduce competition and service for patients and raise mail-order drug prices, among other consequences.

"This merger would create a huge new middleman that stands between patients and their pharmacies, between patients and the medications they need," said Don Bell, a senior vice president for the chain drugstores association.