The Federal Trade Commission is starting an inquiry into the operations of pharmacy benefit managers who control access to prescription drugs for millions of Americans.

The consumer protection agency said Tuesday that that it will order the nation’s largest PBMs to provide a range of information and records detailing how they do business.

Pharmacy benefit managers run prescription drug coverage for big clients that include health insurers and employers that provide coverage. They help decide which drugs make a plan’s formulary, or list of covered medications. They also can determine where patients go to fill their prescriptions.

These businesses, the biggest of which are run by companies that also operate health insurers, have been criticized by doctors and patients over their formularies and other concerns about drug access.

FTC Chair Lina M. Khan said in a statement that PBMs have “enormous influence” over the country’s prescription drug system.

“This study will shine a light on these companies’ practices and their impact on pharmacies, payers, doctors, and patients,” Khan said.

The FTC said it will seek to learn more from PBMs about drug manufacturer rebates and how they affect formulary design and the cost of drugs.

The agency said it also will look at “complicated and opaque methods to determine pharmacy reimbursement” and how patients are steered toward PBM-owned pharmacies.

The agency will seek information from OptumRX, which is part of UnitedHealth Group; CVS Health’s Caremark business and the insurer Humana, among other companies.

A CVS Health spokesman said the company was looking forward “to working cooperatively with the Federal Trade Commission.”

The federal agency said the FTC Act authorizes it to conduct studies without a specific law enforcement purpose, and the companies will have 90 days from the date they receive an order to respond.