The Federal Trade Commission has begun an inquiry into pharmacy benefit managers and the “prescription drugs middleman industry,” which control millions of Americans’ access to prescription drugs.

Tuesday’s announcement by the consumer protection agency stated that it would require the largest pharmacy benefit managers in the country, or PBMs to give a variety of records and information detailing their business practices.

PBMs provide prescription drug coverage to large clients, including health insurance companies and employers that offer coverage. They determine which drugs are included in a plan’s list of covered medications, or formulary. They can also help patients determine where they will get their prescriptions filled.

Patients and doctors have criticized these businesses, which include the largest, because they also run health insurance companies.

Lina M. Khan, Chair of Federal Trade Commission, stated that although many people don’t know about pharmacy benefit managers but these powerful middlemen have immense influence over the U.S. prescription drugs system.

“This study will shed light on the practices of these companies and their impact on pharmacies and payers, doctors and patients.”