It's not inflation or even market performance that presents the biggest risk to your retirement plan. It's unexpected medical expenses.
Indeed, health-care costs are often overlooked—or underestimated—by pre-retirees who are putting money away.
"As we age and live longer, our health deteriorates pretty heavily in the last five to seven years of life, and that's when we spend a ton of money," said Bob FitzSimmons, certified financial planner and president of Bob FitzSimmons Inc., a wealth management firm.
"I have quite a few clients who have burned through their capital in assisted-living facilities, spending $200,000 to $300,000. Generally, it's the adult children who have to come to the rescue."
(Read more: Lower increases in premiums won't last)
According to the Employee Benefit Research Institute (EBRI), a 65-year-old couple with median prescription-drug expenses who retire this year will need $295,000 to enjoy a 75 percent chance of being able to pay all their remaining lifetime medical bills, and $360,000 to have a 90 percent chance.
Those figures factor in the premiums for Medigap and Medicare Part D outpatient drug benefits to supplement basic Medicare, along with out-of-pocket expenses for prescription drugs. They do not include the cost of nursing homes or long-term care insurance.