While you're still working, your employer generally helps cover at least part of your health care insurance/costs (hopefully!). But the moment that you stop working, you'll be responsible for the entire cost.
If you retire before age 65, you will pay full market rates in your area for all insurance and related costs. The total cost of this coverage could be a far greater than you expect, and you would be wise to research those costs before making the decision of when to retire.
Once you are age 65, you will be eligible for Medicare, which will help with some basic health care costs, but it is not totally free. In general, an otherwise healthy individual may pay ~ $110 per month for basic coverage, plus an additional $80 per month if they include additional doctors services and prescription coverage (both of those are optional, but highly recommended). Add in some basic co-pay amounts and deductibles, and the total cost could easily be a minimum of $300 - $400 per month. And that's for a perfectly healthy person. $400 per month for health care costs ($800 per month for couples) is no small amount, and must be included in any retirement plan. Over the course of 25 years, that amounts to $120,000 ($240,000 for couples) before any inflation adjustment. Many would argue that that's on the low end of potential costs.
But wait, there's more (unfortunately). Perhaps the worst part of health care costs is that the U.S. Department Of The Actuary is forecasting that they will increase at a rate of 6% per year, for at least the next decade. That's more than 2.5x the historical inflation rate over the past 20 years (2.13%).
Assuming that 6% inflation for health care costs is correct, that means that it will add ~$65,000 (~$130,000 for couples) to your retirement needs, as opposed to if the costs rose at the regular inflation rate. If you're an individual who is saving $1,000 a month for retirement, it will take you an additional 5 years of saving just to account for the higher inflation rate for health care costs.
The only good news - if you know about this now, and plan accordingly, at least you'll be prepared. It also may inspire you to write to your Congressman/Congresswoman to complain!