A transition refill, also known as a transition fill, is typically a one-time, 30-day supply of a drug that you were already taking. It lets you get temporary coverage for drugs that are not on your plan’s formulary or that have certain coverage restrictions. You can get a transition refill when you switch to a different Part D plan (either stand-alone or through Medicare Advantage) that does not cover your drug, or when your current plan changes its coverage at the start of a new calendar year.