It’s the most common financial fear: No matter how much cash you have stashed away for retirement, you’ll run out of money.

You’re right to be worried. That happens too often to ignore…and it’s usually due to surprise expenses that derail the best-laid plans.

Yes, market downturns and low investment returns can threaten your future finances, but the biggest threat comes from spending – specifically costs that didn’t make it into your retirement budget or were underestimated.

And most unexpected expenses fall into one category: medical costs.

The first surprise most people face: what Medicare doesn’t cover. Basic Medicare (called Part B) doesn’t cover most prescription drugs or any routine dental or eye exams – not even checkups, fillings, dentures, or glasses.

Even more surprising to most people: Medicare does not cover long-term care for any reason…and that doesn’t only mean nursing homes. It only picks up the cost for rehab facilities under very specific circumstances: You must have been a hospital in-patient for three days immediately before going into rehab, and even then they only pick up the full tab for the first 20 days (and partial for the next 80 days max).

Another big surprise: hefty deductibles and co-pays. Even with Medicare, you still have to pay a portion of your medical expenses. If you have a chronic health issue, those copays can add up to a huge chunk of your retirement funds.

Armed with that information, you can make some solid educated health care estimates based on your current health and family history.

Equally – and maybe even more important – is your medical care personality. Take an honest look at the way you experience health care now. If you’re the kind of person who goes to the doctor at the first sign of a problem, for example, you’ll need to allow for higher healthcare costs.

That combination of information paints a medical picture than can help you come up with a reasonable estimate for medical expenses. For example, if you have Type 2 diabetes, a family history of heart disease, and go to the doctor more than a few times a year, you’ll need to include a significant cushion for medical expenses in your retirement budget.

But even if you’re in great health, with a family history of long-lived healthy people, and you rarely visit the doctor, it still pays to set aside substantial funds for future health costs.

Bottom line: Medical expenses can chew through your retirement savings – but you can cushion that blow with realistic planning based on your probable health care needs. To get an idea of your future medical expense baseline, check out this calculator.