Medicare mails you a 120-page handbook. It is accurate, it is thorough, and it is written by people who are not allowed to have an opinion.

That is the problem. Almost everything that goes wrong for people turning 65 goes wrong not because they did not have the information, but because nobody told them which three sentences in those 120 pages actually matter.

Here are those sentences.

  • One of your first decisions is effectively one-way, and nothing in the handbook flags it as such.
  • Two of the deadlines create penalties you pay every month for the rest of your life.
  • The single most important input is not a plan feature — it is your own list of doctors and prescriptions.

1. One door closes behind you

You can move between and Original Medicare during the open windows. That part is true and it is what the commercials emphasize.

What they leave out: going back to Original Medicare is easy, but buying a policy at that point usually is not. Outside a narrow set of protected situations, a Medigap insurer can put you through — ask about your health history and decline you.

A handful of states require Medigap to be sold with additional guaranteed-issue or birthday protections. Most states do not. Which one you live in changes how reversible your first decision is.

2. Two deadlines have permanent teeth

Most deadlines in life are inconveniences. These two are not.

Part B. If you do not sign up for when you are first eligible and you do not have active employer coverage protecting you, your premium goes up 10% for each full year you went without — and you pay that surcharge for as long as you have Part B. Not for a year. For life.

Part D. Same structure, different math. Go without for 63 days or more and a penalty is added to your premium, permanently, calculated from how many months you went uncovered.

Retiree coverage and COBRA are not the same thing as active employer coverage — and that difference is worth thousands of dollars.

The sentence I say most often on the phone

This is the mistake I see more than any other. Someone retires at 64, keeps their employer plan through COBRA, feels covered, and finds out at 66 that COBRA never protected them from the Part B penalty. It does not. Neither does retiree coverage.

3. The most important input is not a plan feature

Every comparison chart you will find online — including the one on this site — organizes information by plan structure. That is a useful way to understand the system. It is a terrible way to make your decision.

The two things that actually determine whether you are happy with your coverage are:

  • Your doctors. Which ones you want to keep, and whether they participate in the network you are considering.
  • Your prescriptions. The exact drug names and doses, checked against the of the specific plan, in your specific county.

Everything else is downstream of those two lists. A plan that looks excellent on paper is the wrong plan if it does not cover the drug you take every morning.

The one that costs you for life

If you only remember one thing: find out, in writing, whether your current drug coverage is creditable — and find out before your 65th birthday, not after.

Your employer or plan administrator is required to tell you. Ask for it in writing. Keep the letter. It is the single cheapest thing you can do this year, and the penalty it prevents is the one that never goes away.

What to do this week

  • Write down every doctor you want to keep and every prescription you take, with doses.
  • Ask your current plan administrator for written confirmation of whether your drug coverage is creditable.
  • Find out whether your state has special Medigap rules — it changes how reversible your first decision is.
  • Figure out which enrollment window you are actually in before you look at a single plan.

That last one takes about two minutes. Check which window applies to you, then come back and read the rest of these guides in whatever order you like.

And if what actually brought you here was a television or Facebook advertisement, start with what the "$900 Medicare food card" ads really mean — the number in those is almost never the number you would receive.