Most people choose a Medicare plan once and then stop thinking about it. That is understandable: the first choice took effort, and nothing seems to change from one month to the next. But private Medicare plans are allowed to change their terms every year. The monthly premium can go up, a copay can change, a drug can move to a more expensive tier or leave the list, and a doctor or pharmacy can leave the network.

Your health changes too. A new diagnosis, a new prescription or a few more specialist visits can turn a plan that was a good match into an expensive one. None of this is a mistake on your part. It is simply the reason a yearly review exists.

Plans send members an Annual Notice of Change in the fall. It lists what will be different next year. It is easy to set aside with the rest of the mail, and it is the single most useful document for the review.

The Medicare Open Enrollment Period runs from October 15 to December 7 every year. During it you can switch from Original Medicare to a Medicare Advantage plan, switch back from Advantage to Original Medicare, move from one Advantage plan to another, and join, switch or drop a Part D drug plan. Changes made in this window usually start on January 1.

There are other windows that people often confuse with this one. The Initial Enrollment Period is built around your 65th birthday. The Medicare Advantage Open Enrollment Period, from January 1 to March 31, lets people who are already in an Advantage plan make one change. Some life events, like moving, open a Special Enrollment Period. Knowing which window applies to you prevents a surprise later.

Original Medicare is Part A for hospital care and Part B for medical care. It lets you see any doctor in the country who accepts Medicare, but it has no yearly limit on what you pay out of pocket. Many people add a Medigap policy to cover part of those costs and a separate Part D plan for prescriptions.

Medicare Advantage plans are offered by private companies approved by Medicare. They bundle hospital and medical coverage, usually include drug coverage, and have a yearly limit on out-of-pocket costs. In exchange, most of them use a network of doctors and may require referrals or prior approval. Some add extras such as dental or vision.

Neither path is right for everyone. The useful question is not which one is better, but which one fits the doctors you see, the drugs you take and the way you want to pay.

Every drug plan has a list of covered drugs, called a formulary, and places each drug in a tier. The tier decides what you pay at the pharmacy. Plans can change both the list and the tiers from one year to the next, which is why the same prescription can cost more in January than it did in December.

The most reliable way to check is to enter every drug you take, with the dose, in the official Medicare Plan Finder. It shows what each plan in your area would cost for that exact list, including the pharmacy you prefer. Medicare drug plans also have a yearly cap on out-of-pocket drug costs, so a plan with a higher premium can still be the cheaper option for someone with expensive prescriptions.

If you have a Medicare Advantage plan, networks matter as much as price. Before you keep or choose a plan, write down every doctor, specialist, hospital and pharmacy you want to keep, and confirm each one is in the network for next year, not only this year. The plan's provider directory and a quick call to the doctor's office are both worth doing.

If you have Original Medicare, check that your doctors accept Medicare, and look at whether your drug plan still includes your usual pharmacy as a preferred one.

People with limited income and savings may qualify for Extra Help, a program that lowers what you pay for Part D drug coverage. You can apply through the Social Security Administration. States also run Medicare Savings Programs that can help pay Part B premiums and other costs.

Every state has a State Health Insurance Assistance Program, known as SHIP, with trained counselors who give free, unbiased help comparing plans. You can find yours at shiphelp.org. They do not sell plans, which makes them a good second opinion.

Start with the Annual Notice of Change and mark anything that affects you. Then make a list of your prescriptions with doses, and a list of the doctors and pharmacy you want to keep. Enter the drugs in the Plan Finder and compare the estimated yearly total, not only the premium.

If your current plan still comes out ahead, you can keep it and do nothing. If another plan fits better, you can switch before December 7. If something is unclear, call your SHIP before you decide. Be careful with anyone who calls you without being asked and pushes you to switch on the spot.