If you're signing up for Medicare and are considering supplemental insurance, be aware that the clock is ticking for you to get guaranteed coverage. When you first enroll, you get six months to purchase what's known as a Medigap policy — which helps cover the cost of deductibles, co-pays and co-insurance associated with Medicare — without an insurance company nosing through your health history and deciding whether to insure you. "The 'guaranteed-issue' period is the first six months," said Elizabeth Gavino, founder of Lewin & Gavino in New York and an independent broker and general agent for Medicare plans. "After that, it could be a nightmare."

The ability to pay for health-care needs is one of the most critical issues of retirement. Hero Images | Getty Images

Basically, unless your state allows special exceptions, you must go through medical underwriting after the six-month window. And, depending on your health, that process could cause the Medigap insurer to charge you more or deny coverage altogether. While Medigap is no cure for the the rising cost of health care, about 13.1 million Medicare beneficiaries were using such policies to reduce their out-of-pocket outlays in 2016, according to the American Association for Medicare Supplement Insurance. That's about 22.5 percent of the estimated 58.5 million people on Medicare. Keep in mind that Medigap is different from a Medicare Advantage plan. (More on that below.) If you are considering Medigap, it's worth making sure you understand what you're buying. Beyond knowing the deadline for making a decision, here are some key considerations before you sign on the dotted line.

The basics

While a number of companies offer Medigap insurance, they can only offer policies from a list of about 10 standardized plans. Each is simply assigned a letter: A, B, C, D, F, G, K, L, M and N. Some states also offer a high-deductible version of Plan F. This standardization means that, say, Plan A at one insurance company is the same as Plan A at another. However, not every plan is available in all states. Also, Medicare recipients under age 65 who are disabled might not have access to Medigap, depending on where they live. Gavino recommends checking with your state's insurance department to find out if a plan is available.

The plans differ on what is covered. For instance, Plan F pays your Medicare deductibles while Plan A does not. Or, some plans cover 100 percent of your deductibles and co-insurance, while others might only pay a portion of those costs. The Centers for Medicare and Medicaid Services has a chart on its website that shows the differences. You also can use the agency's search tool to find available plans in your ZIP code. Additionally, Medigap policies can only be coupled only with original Medicare (Part A hospital coverage and Part B outpatient coverage). In other words, if you have a Medicare Advantage Plan (Part C), you cannot purchase a Medigap plan.

When it works

Overall, while about a third of Medicare recipients have an Advantage Plan and their ranks have been growing, some people discover that their favorite physician or pharmacy is not part of an Advantage Plan's network. Staying on (or returning to) original Medicare lets them remain with their doctor. Others, like frequent travelers, want to avoid the limitations that sometimes come with Advantage Plans, such as requirements to visit in-network doctors or pay more if they are out of network. Regardless of the reasons for remaining on original Medicare, it's important to understand its limitations. For instance, Medicare has no out-of-pocket maximum. Additionally, Parts A and B come with deductibles and limitations on what services are covered and to what degree. Part A hospital inpatient deductible and coinsurance

For 2018, you pay:

$1,340 deductible for each benefit period

Days 1-60: $0 coinsurance for each benefit period

Days 61-90: $335 coinsurance per day of each benefit period

Days 91 and beyond: $670 coinsurance per each "lifetime reserve day" after day 90 for each benefit period (up to 60 days over your lifetime)

Beyond lifetime reserve days: all costs Part B deductible and coinsurance

$183 per year. After your deductible is met, you typically pay 20 percent of the Medicare-approved amount for most doctor services, outpatient therapy and medical equipment. "If you're sick a lot, or in and out of doctor's offices, you can end up with some big bills," Gavino said. That's where Medigap policies help. Yet they aren't a cure-all: For instance, Medicare does not cover dental and vision expenses, and Medigap policies do not pick up those costs. Advantage Plans, on the other hand, might offer coverage for both. Additionally, many Advantage Plans include prescription drug coverage (Part D). If you are on original Medicare — whether you have a Medigap policy or not — you must sign up for a prescription plan unless you meet certain requirements. Additionally, depending on a combination of factors — the Medigap policy's features, where you live, and sometimes your age — the cost can reach a few hundred dollars a month.

The most popular Medigap plan (Plan F) cost an average of $159 to $236 for a 65-year-old male in 2016, according to the American Association for Medicare Supplement Insurance. Your location alone can make that number vary wildy, however. In New York City, the highest cost of a Plan F premium was $444; and in San Jose, California, the lowest cost was $135. Plans that offer less coverage are generally less expensive.

Premium differences

Where you might see differences between the same plans is in the monthly premiums. For instance, depending on where you live, several insurers could offer Plan F with different premium amounts. Gavino recommends avoiding insurers that have little history in the marketplace yet offer the cheapest premiums. "Some people will go for the cheapest option and then end up with a 20 percent to 30 percent increase the next year," she said. Additionally, Gavino said, it's worth finding out whether the insurer you're considering will let you switch between plans.

Some people will go for the cheapest option and then end up with a 20 percent to 30 percent increase the next year. Elizabeth Gavino founder of Lewin & Gavino in New York

If your financial situation changes and you want to move to a less expensive policy, there's a chance the company will require you to go through medical underwriting before letting you switch.

Additional considerations