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.

You typically receive an ID card in the mail that shows which P.B.M. you have. Or you may need to ask your employer’s human resources department.

If you’re on Medicare, you can choose an insurance plan each year, but the government website that lets you compare plans doesn’t identify which P.B.M. is attached to which plan. You may be mailed an ID card that identifies your P.B.M., or you can try calling your Medicare plan.

What does my P.B.M. do?

Your P.B.M. most directly affects you by picking the drugs that will be covered by your insurance and deciding how much you will pay out of pocket for them.

The P.B.M. comes up with a list of covered drugs, known as a formulary. It also decides which drugs it will encourage you to take by making them cheaper for you. It discourages you from taking other medications by making them more expensive or erecting hurdles like requiring you to try other drugs first. Your insurer or employer signs off on the P.B.M.’s recommendations.

Is my P.B.M. overcharging me?

If the out-of-pocket costs for your medication feel high, it can be challenging to tell which actor in the health care system is to blame. Sometimes, it may be your employer or your insurance company, not your P.B.M., that’s at fault because they’re offering stingy benefits. But there are several common scenarios in which the P.B.M.’s business practices are the main reason you’re paying too much.

P.B.M.s sometimes force patients to take a brand-name drug even though there’s a cheaper generic version available. If you’re paying a lot for a brand-name drug, ask your doctor or pharmacist if there’s a cheaper alternative you could switch to.

Even when you’re taking a generic, your P.B.M. may be forcing you to pay more than it would cost if you paid for it with your own money — not using insurance — at your local drugstore or an online pharmacy. This often happens with generic drugs for chronic conditions like cancer or multiple sclerosis.

Check out options for paying out of pocket for your drug, instead of using your insurance. These include GoodRx, which lists prices at pharmacies near you, or the Mark Cuban Cost Plus Drugs Company, an online pharmacy that sells hundreds of drugs.

Who can help me?

A good rule of thumb: Talk to your pharmacist.

“Get to know them. Find out the pharmacist’s name, and make sure they know your name,” said Stephen Giroux, an independent pharmacist in upstate New York who is a vocal critic of P.B.M.s. “We know what the prices are. We can suggest lower-cost alternatives.”

Pharmacists are among the few professionals in the system with clear lines of sight into how P.B.M.s cause patients to pay more. Often, they can intervene on your behalf if your P.B.M. is asking you to pay more than you should. For example, your pharmacist may suggest that you forgo insurance to get a lower price, or point you to a manufacturer’s coupon that could save you money.

If you have medication sent through the mail from a pharmacy affiliated with your P.B.M. — such as Accredo, which is part of the same company as Express Scripts — it can still be worth visiting your local pharmacy to see what options you have.

If you have a big bill for a drug that you can’t afford, you can also ask your pharmacy if you can pay over time.

Can I change P.B.M.s?

It depends.

If you have insurance through your employer, you can’t switch P.B.M.s. But you can complain to your employer. Start by talking to human resources. In many cases, complaints from workers about the high costs they’re facing for a certain drug are the only way the employer finds out that there’s a problem.

If you’re on Medicare and want to switch P.B.M.s, you can pick a new plan for the next year. You’ll have to do some research to make sure you are actually picking a different P.B.M., since many Medicare plans use the same P.B.M.s.

Because you have so many choices with Medicare plans, the best advice is to focus on what you can find out about a plan’s benefits, the drugs it covers and how much you will pay for specific medications each month. All of that information can be found on Medicare’s website.