This story was co-published with the New York Times' The Upshot. It is not subject to our Creative Commons license.

It's not easy being an educated health care consumer.

I was reminded of this when I went to refill a prescription this month for an asthma and allergy medication for my 9-month-old son, Holden.

The first time I filled his prescription for Montelukast granules — the generic version of Singulair from Merck — my insurance co-payment was $15. A month later, the co-payment had risen to $30 (and my insurance was paying $85.94, rather than $118.53).

Why? My insurance coverage hadn't changed. My son's prescription hadn't changed. Our pharmacy was the same. Why was I now asked to pay twice as much out of pocket?

I asked the CVS pharmacist. This happens all the time, she replied. Call the insurance company to find out why.

Consumers are navigating a health care system in which they pay an increasing share of the cost but often have insufficient information to make the right decisions. They assume that pharmacies are charging them the right co-payments, that insurance companies are paying the correct share. But as health plans' rules for prescription drugs become more complicated, it's harder to tell.

It used to be that generic drugs had one common co-pay and name-brand drugs another. But that's not always the case anymore, as with my plan. Some generic drugs are expensive, and consumers sometimes pay a higher share of their cost, more akin to what they would pay for a name brand.

Pam, the first customer service agent with whom I spoke at my insurer, Oxford Health Plans, a division of UnitedHealthcare, told me that it looked as if there was a mistake with the refill, and that I was entitled to a $15 refund. She gave me a tracking number and told me to call back in two to five business days.

Dutifully, I did so, and talked to another agent, named Mike. He told me that there had been a mistake, but that it was with the first prescription. The co-pay should have been $30, not $15, but as a courtesy because of its error, the plan would not seek to recoup the money. The baby's prescription was on a higher-cost tier because it was for granules of the drug, essentially a powdered version, and not for tablets, which are in the lowest-cost tier.

But a look at Oxford’s website and at its drug list, also known as a formulary, revealed that Montelukast is listed as a Tier 1 drug, with the lowest cost.

Insurance plans with multiple cost tiers have become more prevalent in recent years, as prescription drug costs have increased over all. In 2000, nearly half of workers with private insurance had two price categories — typically, one for generics, the other for name-brand drugs, according to a survey by the Henry J. Kaiser Family Foundation and the Health Research & Educational Trust. An additional 22 percent of covered workers paid the same price for all drugs.

By 2013, though, more than eight in 10 workers had private insurance plans with three or more tiers of drug prices.

But my co-payments may not be the same as yours. Each insurance company — and employer — sets its own list of approved drugs and out-of-pocket costs. Some are fixed amounts and others are percentages of a drug's cost, sometimes called coinsurance. Medicare prescription drug plans also have their own rules.

Another change is that generic drugs aren't as cheap as they used to be. An article in The New York Times in July detailed how the cost of some generic drugs had doubled recently, as suppliers left the market and reduced competition. Other reports have found similar problems.

Some insurance companies, including mine, have increased the share that consumers pay for more expensive generic drugs, placing them on tiers once reserved for name-brand drugs. Your health plan should have a list on its website.

I wasn't satisfied with the conflicting answers about Holden's prescription. I could have asked to speak with a supervisor and then a supervisor's supervisor. Instead, I emailed the health plan's representatives, telling them that I was a reporter and planned to write about this experience.

Pretty soon I received a call from a manager and her supervisor, offering me an apology and telling me that, based on the recordings of my interactions with Oxford staff members, there were "opportunities for improvement."

They said that Mike, the second customer service agent, was correct and that the drug dispensed initially was coded incorrectly by Oxford's pharmacy benefits manager — another division of UnitedHealthcare — and should have been classified as a higher-cost, Tier 2 drug. They also called my son's doctor, and he said my son could switch to the tablet version (with the lower co-pay) and I could crush it myself. Or, they said, I could apply for an exemption to the higher co-payment, citing the confusion.

I'm not sure that every consumer gets such a call from supervisors, so I told them that I didn't want to be treated any differently because I was a reporter. A few days later, my request for an exemption to the higher co-payment was denied. I was given instructions for how I could appeal.

Mary McElrath-Jones, a UnitedHealthcare spokeswoman, said in an email: "Although we strive for perfection when entering hundreds of NDC [drug] codes and testing our system for accuracy, we sometimes find errors like the one you brought to our attention. And just as we have in this instance, we act quickly to resolve the issue and notify our members."

Mistakes can happen in any industry. But what I still can't understand is why Montelukast is listed as Tier 1 in the company's online formulary. Shouldn't consumers get accurate information if they spend the time to research a drug's cost? The answer wasn't exactly encouraging.

"Our online prescription drug list (PDL) — while comprehensive — is not all encompassing for every drug and classification for all manufacturers," McElrath-Jones wrote. "It is published twice per year and includes the top 500 most commonly used drugs. It is a great first stop for our members who wish to know if a particular drug is included in our formulary. For a more customized pharmacy tool, Oxford and UnitedHealthcare members can get specific drug pricing" online.

Many consumers aren't up for this fuss. They either throw up their hands and pay what's asked or turn to experts like Lorie Gardner, a registered nurse and the chief executive of Healthlink Advocates Inc., a paid service that helps patients navigate the health care system. "It's a maze, a complete maze trying to figure out which end is up," Gardner said. "There are errors everywhere, unfortunately."

If you find yourself in such a predicament, what should you do? First, be prepared. Sign up for an account online with your health insurance company, review your benefits and review your claims. You'll be amazed by how much — and sometimes how little — your health insurer pays for various treatments and drugs. Second, if you encounter a problem, ask questions. While you may have to pay the bill at the pharmacy if you want to leave with the prescription, you should follow up with your health plan and ask to speak with a supervisor.

Finally, if the stakes are high enough, consider a health advocate like Gardner. Some advocacy firms are run by former health insurance executives, who help navigate the roadblocks that their former companies have erected.

But, ultimately, you may well end up doing what I did: paying the higher fee with gritted teeth and gaining a new appreciation of how confusing our health care system really is.

Have you experienced price confusion at the pharmacy? Email Charles Ornstein to let him know about what happened.