You probably think you have a good handle on your retirement expenses.

You understand your current spending patterns. You can reasonably estimate how much money you'll need for groceries, gasoline, vacations, utilities, and other regular bills. If you still have a mortgage, you know exactly when it will be paid in full. You're confident you can cover your bills during retirement.

You're probably not worried about health-related costs. After all, Medicare will cover medical expenses, so why worry about it, right?

Unfortunately, reality isn't that simple. The rising cost of healthcare has the potential to add incredible financial stress to what otherwise would have been a happy retirement.

Here are four reasons healthcare costs can derail your retirement, and what you can do about it:

1. Medicare won't stretch as far as you think
According to data from the Employee Benefit Research Institute, or EBRI, Medicare will only cover about 60% of an individual's medical expenses during retirement. The other 40% needs to come out of your pocket.

How much money are we talking about? As of 2014, a 65-year-old married couple with median prescription drug expenses is projected to need $241,000 in retirement savings to have a 90% chance of covering their healthcare expenses during retirement. A single male needs $116,000 to have a 90% chance of covering his healthcare expenses. A single female, because of her longer life expectancy, needs $131,000 in retirement savings.

Think these are inflated projections? Think again. This data is recent and comes from authoritative sources -- and it only gets worse.

Medicare will cover a declining percentage of healthcare expenses in the future, EBRI projects. Retiree out-of-pocket healthcare costs are rising, and this trend is expected to continue.

2. Medicare won't cover this long list of expenses
What are some of the out-of-pocket healthcare costs burdening retirees?

Here are just a few examples of expenses that are unlikely to be covered: dental care, hearing aids, routine foot care, dentures, eye exams for prescription eyeglasses or contact lenses, long-term care, and any type of medical care required outside the United States.

Does that sound like an exhaustive list? We're just getting started.

Medicare will cover some of the most critical expenses, such as surgeries, physician and specialist appointments, and routine preventative checkups. But don't expect it to cover every dime of health-related costs.

Also don't forget that even if Medicare does cover a service, you're still responsible for paying deductibles, co-insurance, and co-payments.

3. You may need to cover ancillary costs
That list doesn't even cover the crucial -- and often overlooked -- ancillary costs associated with health problems.

During retirement, you may need to hire people to take care of day-to-day tasks and errands you currently manage on your own. For example, you may need to hire workers to clean your home, mow your lawn, and take care of other repair and maintenance tasks you typically handle. If you have a severe health condition, you may even need to hire people to handle your grocery shopping and food preparation.

And this doesn't even broach the subject of intensive long-term care. If you have severe medical problems, you may need help with a category called "activities of daily living," which include taking showers, getting dressed, brushing your teeth, and other day-to-day activities.

These unexpected costs can put a strain on your budget if you're not prepared.

4. You may need individual health insurance if you retire early
Let's also not forget that Medicare doesn't necessarily start on the day you retire.

Many people are choosing to retire before 65, which is when they become eligible for Medicare. If you aspire to join the ranks of other early retirees, you'll be responsible for finding an individual health insurance policy until you reach 65.

Premiums for an individual health insurance policy vary based on the level of coverage received. Generally, you can expect the monthly premiums for a person in his or her 50s or 60s to be on the pricier side. If you're hoping to retire early, budget adequately for this expense and review your options.

The Patient Protection and Affordable Care Act categorizes plans into four coverage levels: Platinum, Gold, Silver, and Bronze. Platinum plans have the highest monthly premiums, but they pay (on average) 90% of health-related expenses. Bronze plans have the lowest monthly premiums, but they pay out only 60% of health-related expenses, on average.

Key takeaway
Medical expenses will occupy a large and growing portion of an individual's retirement budget. In fact, healthcare costs are the second-largest expense for older Americans (after housing), and medical bills are the No. 1 cause of bankruptcy in the United States. Nearly half of all Americans cite healthcare as their biggest financial concern.

If you haven't started planning for healthcare costs in your retirement budget, it's time to begin. To learn more, check out the 5 Ways to Manage Healthcare Costs e-book I sourced for this article, review the wealth of helpful retirement-related articles here on The Motley Fool, and, most importantly, make the time to talk with a financial advisor.