You can only purchase medigap supplemental insurance if you're enrolled in traditional Medicare Part A and Part B, which pay for hospitalization, doctor visits, and other outpatient care. If, on the other hand, you've enrolled in a Part C Medicare Advantage managed care plan, you don't need, and aren't eligible for, a medigap insurance policy.

Who needs medigap? It's difficult for anyone over age 65 to get private healthcare coverage that isn't exorbitantly expensive, so for most people of that age, Medicare is the only health insurance option that makes sense. But out-of-pocket costs under Medicare can easily add up. Between copayments, deductibles, premiums, and coverage gaps, you can easily spend several thousand dollars each year on healthcare, even with Medicare. If an older adult currently spends a lot on items that a medigap plan would cover, particularly if those costs are rising each year, it makes sense to compare different plans to see if one of them could save him money.

If someone qualifies for Medicaid (government health insurance for low-income people), he doesn't need to buy a medigap policy, because Medicaid pays for most expenses that Medicare doesn't pay.