ST. LOUIS — In a blockbuster combination of pharmacy benefits managers, Express Scripts Inc. has agreed to acquire Medco Health Solutions Inc. in a cash and stock transaction valued at $29.1 billion.

In a blockbuster combination of pharmacy benefits managers, Express Scripts Inc. has agreed to acquire Medco Health Solutions Inc. in a cash and stock transaction valued at $29.1 billion.

Executives from the two firms said the merger would generate economies of scale that could lower the cost of prescription drugs and improve the quality of health care for Americans.

However, the deal drew a thumbs-down from the retail pharmacy industry. The National Association of Chain Drug Stores and the National Community Pharmacists Association (NCPA) issued a joint statement opposing the merger.

"Today’s announcement that Express Scripts will buy Medco creates a middleman that is too big to play fair and will have immense power to unfairly dominate the market," the statement read. "This combination will monopolize control of the supply line for brand and generic prescription drugs, threaten access to pharmacy patient care and is a bad deal for America, for health care plans, for pharmacies and — most notably — for patients."

Both trade groups are consulting with regulatory agencies and state attorneys general.