Related: Senators Warren, Hawley introduce bipartisan bill that would force insurers, pharmacies to sell off PBMs

The PBM critics are the same people who joined to introduce a PBM control bill, the Patients Before Monopolies Act bill, in December. That bill would have blocked any person from owning a PBM and a pharmacy at the same time or an insurance company or a pharmacy at the same time.

The PBM control bill died in committee Jan. 2, when the 118th Congress ended. The new PBM critics' letter may be a sign that the bill will be returning in the 119th Congress.

PBMs help health insurers and employers with self-insured health plans set up prescription benefits programs.

The Pharmaceutical Care Management Association, a group for PBMs, contends that its members help employers and patients save an average of $1,040 per person in a plan served by a PBM, that employers express a high level of satisfaction with their PBMs, and that the pharmaceutical manufacturers are the prescription drug market players with the big profit margins.

Warren, Hawley, Harshbarger and Auchincloss accused PBMs of being "prescription drug middlemen" that "use their market dominance to overcharge taxpayers, hide profits and squash competition."

An interim FTC staff report released in July showed that the six biggest PBMs oversee about 95% of U.S. prescription volume, "employ lopsided and unilateral contracting practices," often "force unaffiliated pharmacies to 'enter into one-sided, non-negotiable contracts with the leading PBMs,'" send "billions in inflated payments to their pharmacy subsidiaries," and sometimes make patients with insurance pay out-of-pocket costs for covered drugs that are much higher than the underlying cost of the drugs, the lawmakers wrote in the letter to the FTC commissioners.

"We expect a second interim report will shine further light on the industry that will be invaluable to legislators as the priorities of the 119th Congress take shape," the lawmakers added.

The big PBMs disagree with the PBM critics about the first interim report.

Cigna's Express Scripts has sued the FTC over publication of the first interim report, alleging that the FTC caused Express Scripts to spend millions of dollars to provide detailed data showing how the prescription drug distribution market really works, then relied mostly on unverified comments from people who work for the entities that PBMs negotiate with in an effort to hold down prescription costs.