If you’ve ever had trouble getting a prescription drug, chances are you’ve run into a pharmacy benefit manager.

These companies, known as P.B.M.s, play a crucial but often hidden role in deciding which drugs you can get and how much you will personally have to pay. They are middlemen in the maddeningly complex American health care system, working on behalf of your employer or government insurance programs like Medicare, which cover most of the costs of prescription drugs.

The job of the P.B.M. is to save money on your medications. But The New York Times found that the three biggest P.B.M.s are often making you pay more than you should.

Here’s what to know about your P.B.M. and how to find out if you are being overcharged.

How do I know which P.B.M. I have?

Most Americans rely on one of the big three P.B.M.s: CVS Caremark, Express Scripts or Optum Rx. Even if you have a smaller P.B.M. like Prime Therapeutics, you may be affected by the business practices of the three giants. (That’s because many smaller P.B.M.s delegate some of their dealings with drug companies and pharmacies to their larger competitors.)

This is different from the health insurance that covers your doctors’ visits or a hospital stay. While you can typically pick your health plan during your employer’s open enrollment period every year, your P.B.M. is picked for you.