GOP senators raise concerns with drug benefit merger

The FTC must vet the deal for potential antitrust violations before it can go through. The two companies say they save consumers, government and health insurers billions of dollars every year by negotiating cheaper drug prices. Critics — particularly retail pharmacies — say the merger would give the two companies too much control over people's drug benefits, potentially driving up costs and hurting access to needed medications.

"We understand that a combined entity has the potential to control 60 percent of mail order and 50 percent of the specialty market," the letter says. "With that type of market concentration, it is important that your agency works to see that access to health care is not limited."

Opposition to the proposed deal cuts across party lines. The latest letter brings to at least 23 the number of lawmakers who have raised concerns with the merger.