"You can keep your policy’s drug coverage if you choose,
but you cannot also have Medicare drug coverage at the same time.
Most medigap policies (also known as Medicare supplement insurance) are not “creditable” - meaning they do not offer drug coverage as good as Medicare -
so you’d pay a late penalty if you decide to join Part D in the future."
Do I need Medicare Part D if I have supplemental insurance?
How to join a drug plan 1-800-MEDICARE (1-800-633-4227)
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