Express Scripts’ proposed $29 billion acquisition of Medco Health Solutions is expected to face intensified scrutiny today when a Senate panel examines potential antitrust concerns raised by the merger of two of the country’s largest pharmacy benefit managers.

Together, the two companies now manage prescription drug benefits for more than 115 million people and handle one of every three prescriptions filled in the United States. With combined revenue of more than $110 billion a year, the merged entity would also become the largest player in the domestic markets for supplying mail-order drugs to patients with chronic conditions and costly specialty drugs for conditions like HIV, hemophilia and rheumatoid arthritis.

Senior executives at the two companies say the merger will significantly reduce the nation’s health care costs and deliver drugs in a safer, more efficient fashion. “A combined Express Scripts and Medco will be well positioned to protect American families from the rising cost of prescription medicines,” George Paz, the chief executive of Express Scripts, told legislators at a House subcommittee hearing in September.

But some lawmakers are concerned that the merger will harm competition, and the Federal Trade Commission has requested additional information from the companies before it decides whether to approve the combination.

Today’s hearing is being held by the antitrust subcommittee of the Senate Judiciary Committee. The same committee has taken a hard look at other mergers, like AT&T’s proposed acquisition of T-Mobile, that have the potential to reduce competition and lead to higher prices.

Regulators are expected to focus on whether the merger of Medco and Express Scripts, which would reduce the number of major competitors to two from three, would also leave customers, particularly large employers, with too few choices and limited bargaining power.

While the two pharmacy benefit managers say they face aggressive competition from other managers for their business, a recent analysis by Morgan Stanley Research indicated that the 50 largest companies in the United States rely heavily on the services of Medco, Express Scripts and the third major benefit manager, CVS Caremark.

The smaller players typically do not have the geographic reach, bargaining power or data-handling capabilities of their larger competitors, said Dan Gustafson, an antitrust lawyer who recently helped write a letter to the FTC objecting to the merger on behalf of the American Antitrust Institute, a Washington organization. “These are customers who require a broad spectrum of services on a national level,” he said.