Here's some not-so-great news for America's aging population: For people with Medicare Part D coverage, a subtle change could result in them paying significantly more for their prescription drug coverage. It all comes down to one single word: coinsurance.

The difference between coinsurance and copayment is significant. You've probably had experience with copays already. When you go to the doctor's office, you might pay $10 or $20 (or more) as a flat fee for the appointment. Then the office and the insurance company negotiate the rest of it. Generally, you won't be charged for anything else.

Copays work the same way with prescription drugs, with bottom-tier drugs – the lowest priced – having a certain copay while higher-tier drugs have larger ones or, in some cases, aren't covered at all, leaving you responsible for the whole price tag.

Coinsurance is different. It's based on percentage. If the cost of the prescription is $100 and you have a 25% coinsurance requirement, you're on the hook for $25. That level of coinsurance may seem reasonable, but what about drugs that are much more expensive?

How everything works together depends on your plan. In some cases you might have a coinsurance requirement rather than a copay. In other instances you might have to meet a deductible before coinsurance kicks in, and in most cases coinsurance applies to some tiers but not to others. Your deductible can only be a maximum of $360 in 2016.

If you're not already affected, it's highly likely that you will be very soon, or the amount of coinsurance you're already paying will rise. A study by the Avalere Health consultancy found that a majority of drugs covered by stand-alone Medicare Part D plans have a coinsurance requirement. According to the group at least 25 million enrollees pay some amount of coinsurance. Further, the percentage of Medicare Part D drugs with a coinsurance requirement went from 35% in 2014 to 58% this year. In total, 96% of Part D customers are in plans that have a coinsurance requirement on more than one tier of drugs.

If there's any consolation in all of this, Medicare puts rules in place that prevent private insurers offering coverage under Medicare from charging outlandish amounts. For example, Part D insurers are required by the federal government to cover a fixed percentage of the enrollee's total drug costs, but that doesn't mean that they can't charge different amounts for different drugs.

Because of this, anybody in the market for Medicare Part D coverage has to do some serious shopping. Make a list of all of your brand-name and generic medications, go to the Medicare Plan Finder and check to see if your drugs are covered and how much you'll have to pay to get them. If you're just looking for Part D coverage, the monthly premium isn't as important as the level of coverage you receive for your medications. Sometimes the coverage with the lowest premium is the best buy, but sometimes it's worth it to pay a little more monthly in exchange for better coverage of the drugs you take. (For more, see Apps That Find the Cheapest Drug Prices.)

The study found one piece of positive news: For customers enrolled in Medicare Advantage plans, only 26% had a coinsurance requirement. There's no absence of criticism for many of the Medicare Advantage plans, but in this case they're looking pretty good.

Coinsurance has been around for a long time, but as the price of prescription medication continues to rise and the population ages, insurers are looking for ways to cut costs. One of those is to make policyholders pay more for their drugs.

Never stay with the same coverage you had last year without comparing other plans. Research shows that you could save a significant amount by doing a little shopping. (For more, see Keeping Drug Prices Down When You're Retired.)