Millions of seniors rely on Medicare in retirement, but if you're not careful, you could end up paying more for your health coverage under it and miss out on key benefits the program has to offer. Here are a few Medicare mistakes you're sure to end up regretting.

1. Signing up late

Medicare eligibility starts at age 65, but you get a seven-month window to sign up. Your initial enrollment period begins three months before the month in which you turn 65 and ends three months after the month of your 65th birthday. If you neglect to enroll during those seven months, you can always sign up during the general enrollment period of January 1 through March 31 of each year. But not signing up on time could cost you.

Older woman in bed while another woman sits with her, holding her hand

IMAGE SOURCE: GETTY IMAGES.

While Medicare Part A, which covers hospital care, is free for most enrollees, seniors are charged a premium for Part B, which covers doctor visits and diagnostics. Wait too long to sign up for Medicare, and you'll be hit with a 10% surcharge for your Part B premiums for each 12-month period you were eligible for coverage but didn't enroll. Furthermore, if you wait too long to get yourself a Part D drug plan, you'll face a penalty there, as well. Signing up on time will help you avoid paying more for Medicare than necessary, all the while ensuring that you have the coverage you need when you need it.

2. Not reviewing your Part D plan from year to year

Once you choose a Part D prescription plan, you may be inclined to hang onto it for the long haul, as doing so will help you avoid the legwork you put in initially. But if you don't reassess your Part D coverage from year to year, you could end up overpaying for your prescriptions.

Part D plan formularies can change from year to year so that the medications you once got great coverage for are no longer treated the same way. That's why you must re-examine your Part D options annually, even if you're happy with your plan at present.

3. Not considering an Advantage plan

Traditional Medicare's scope of coverage is somewhat limited, and there are many key services, like dental care, vision, and hearing, that Medicare won't pay for. Covering those costs on your own could get quite expensive, which is why Medicare Advantage might be a better solution.

An alternative to traditional Medicare, Advantage plans are required to offer at least the same level of coverage as original Medicare -- but generally, they do a lot more. Advantage plans generally pay for dental, vision, and hearing, and they're often just as affordable as traditional Medicare, if not more so.

It pays to look into an Advantage plan if your out-of-pocket costs under original Medicare have been eating up too much of your budget for comfort. This especially holds true if you tend to travel overseas a lot, since original Medicare won't pay for health services abroad, while many Advantage plans will.

4. Passing up your free wellness visit

As a Medicare enrollee, you're entitled to a free wellness checkup every year, and passing it up is a move you might regret. It's not uncommon for health issues to be discovered during these routine visits, and if you don't schedule one, a brewing medical condition might escalate, thereby compromising your health and costing you more money to boot. You're better off taking some time out of your schedule than running that risk.

The better you manage your Medicare, the more the program can do for you. Be sure to avoid these brutal mistakes that could hurt you financially and, worse yet, put your health at risk.