“Medical bills can be a huge expense, so the Internal Revenue Service gives people a break so they can recoup some of that money,” says Lisa Greene-Lewis, a certified public accountant with TurboTax.

Some 10.2 million U.S. households collectively deducted $85.3 billion of medical expenses on their 2012 federal tax returns, according to the latest available Internal Revenue Service statistics. Many Americans can make similar write-offs from their state income taxes as well.

But who can deduct what is pretty complicated, and experts say few taxpayers really understand the rules.

“I think the biggest misconception is that people think that all medical expenses are deductible from dollar one. But for my clients, I’d say that well under 10 percent actually qualify to deduct anything,” says Rob Seltzer, a Los Angeles certified public accountant and chairman of the California Society of CPAs’ Financial Literary Committee.

Here’s a look at the basics of deducting medical expenses from your federal income taxes. Consult your tax advisor for specifics regarding your personal situation.

Who qualifies for medical-expense deductions?
The Internal Revenue Code includes two big rules that can severely limit who truly qualifies for relief from medical expenses:

• You must generally itemize deductions on Form 1040 Schedule A rather than take the “standard deduction” if you want a break on medical expenses. If what you plan to deduct for everything (from medical bills to mortgage interest) adds up to less than the standard deduction ($6,400 for singles and $12,600 for married joint filers in 2015), there’s no point in itemizing.
• Most taxpayers can only deduct allowable medical expenses that exceed 10 percent of “adjusted gross income” (AGI). That’s the amount you earn in a given year from wages, investments and other sources minus what you paid for alimony, student-loan interest and a few other things. So, if a married couple has $100,000 AGI and $10,500 of qualified medical expenses, they can deduct only $500 — $10,500 minus $10,000 (10 percent of their $100,000 AGI). Note: In 2015 and 2016, seniors age 65 or older can deduct any medical expenses above 7.5 percent of AGI.

Seltzer says the only taxpayers who pass both tests are typically those with unusually high medical expenses relative to income. That’s often just the elderly, the unemployed, low-income people or those with big medical bills due to serious illness, in-vitro fertilization or a child’s birth.

Are health insurance premiums deductible?
Yes, in certain circumstances, you can deduct your health insurance premiums as part of your overall medical expenses.

But you can deduct only premiums that you pay with after-tax money from your own pocket. For example:

• If your health insurance premiums are paid entirely by your employer or the government, you cannot deduct the cost.
• If you have health insurance through your employer and your share of the premium is deducted from your paycheck pre-tax, you cannot deduct the cost because the premiums were tax-free already.  If you don’t know whether you pay pre-tax or after-tax, ask your human resources department.
• If you buy health insurance through the state- or federally run health insurance marketplaces, you can deduct only the portion of the premium you pay out of your own pocket. You cannot deduct the amount of any subsidy.
• If you buy an individual or family health insurance plan, either on the open market or through a marketplace, and you pay all of the cost out of pocket, then the whole amount is deductible.

Your total medical expenses, including premiums, must surpass 10 percent of your adjusted gross income to be deductible.

What else is deductible?
Assuming you pass the above tests, the IRS lets you write off pretty much every out-of-pocket medical expense that’s ordered by a doctor or other health care professional. (See IRS Publication 502 for a list.)

Common items you can deduct from taxes include medical appointments, tests, prescription drugs and durable items like wheelchairs and prescription glasses. In fact, you can even write off unusual expenses as long as they’re medically necessary. For instance, one of Seltzer’s clients deducted a home lap pool because a serious injury meant the man could only swim for exercise but couldn’t risk colliding with others in a public pool.

You can also deduct transportation expenses for going to the doctor — parking, tolls, mileage, cab or bus fares — and even air fare and certain lodging costs for out-of-town treatments.

But remember, you can only write off out-of-pocket expenses — copays, deductibles, etc. — not bills that your insurance covers.

What’s not deductible?
There’s a wide list of things you can’t deduct, from medical marijuana to over-the-counter vitamins and drugs (except insulin). Hair transplants and cosmetic surgery are also out, unless procedures correct underlying medical problems (like breast-reconstruction surgery following mastectomies).

As noted above, you also can’t deduct expenses that your insurance covers, nor things you paid for with money from a flexible spending account or health savings account. If you get insurance through work, you typically can’t write off your share of the premiums because your employer won’t normally withhold taxes on the money in the first place.

Writing off health insurance for the self-employed
One big exception to the above rules involves health insurance premiums paid by self-employed people. You can write those off as adjustments to income even if you don’t itemize your deductions. The adjustment to income cannot exceed what you earned, though.

Self-employed people can deduct health insurance premiums directly on Form 1040 (Line 29 on returns for the 2014 tax year). You deduct all other qualified medical expenses on Schedule A, Line 1.

How to maximize your health care deductions
You obviously can’t control when you get sick, but TurboTax’s Greene-Lewis says Americans who are close to meeting the annual AGI threshold should “bunch up” procedures to maximize any deductibility.

For instance, if one family member has a major illness in a given year and rings up big hospital bills, everyone else in the family should get any needed dental work, prescription eyeglasses, etc., during the same year in order to boost the available tax break.

“You should look at anything you were putting off and bump it up [to the current tax year] if that’s going to put you over the AGI threshold,” she says.

You don’t need to attach receipts to your 1040, but it’s a good idea to keep them for three years after filing your return just in case the IRS audits you.

This article original appeared on Insurance.com