U.S. presidential hopeful Amy Klobuchar on Tuesday led a letter by U.S. senators that urged the Federal Trade Commission (FTC) to closely scrutinize pharma mergers, raising concerns about the potential harm to customers.

The U.S. healthcare industry has seen a string of multi-billion dollar deals and consolidation is expected to remain a major theme for the rest of the year.

Bristol-Myers Squibb’s $74 billion buyout of Celgene Corp and AbbVie Inc’s $63 billion bid for troubled smaller rival Allergan Plc were among the largest deals announced this year.

The deals proposed by AbbVie and Bristol-Myers raise significant antitrust issues, the senators wrote, calling on the FTC to “take appropriate action” to protect consumers from mergers that may threaten competition, raise drug prices or reduce patient access to essential medications.

They also raised concerns that the deals may reduce innovation in drug development and give the merged companies increased negotiating leverage to secure positions on drug formularies and greater ability to condition buyers’ access to their multi-billion dollar drugs.

“This industry consolidation is occurring against a backdrop of ever rising prescription drug spending... It is more important than ever that the FTC take appropriate action to protect consumers,” Klobuchar wrote in the letter signed by senators Bernie Sanders, Kamala Harris, Elizabeth Warren and Cory Booker among others.