Can I deduct medical bills that I paid in 2018 that were incurred in 2017? I am itemizing.

[Carl]

Yes. They're deductible in the tax year paid - not the tax year incurred. Just understand this:
 - Until the total of all itemized deductions on SCH A exceed your standard deduction, they make no difference in your tax liability.
 - Before itemized medical expenses count for anything, they must exceed 7% of your AGI. If they do not, then they will not be included on the SCH A, even if your other itemized deductions do exceed your standard deduction.
 - The standard deduction for 2018 has been doubled. $12K for single and MFS, and $24K for MFJ. So unless the total of all your itemized deductions exceed the standard deduction, it's a waste of time to itemize.

[VolvoGirl]

Do you think you will have enough to itemize this year?  This year the Standard Deduction will be doubling so many people will be switching to the Standard Deduction.  And there is a max 10,000 limit (5,000 MFS) of property tax and state taxes "SALT".

FAQ on 2018 changes <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/4482394-how-will-tax-reform-affect-my-2018-federal-tax-return">htt...>

For 2018 the standard deduction amounts are:
Single 12,000 + 1,600 for 65 and over or blind
HOH 18,000 + 1,600 for 65 and over or blind
Joint 24,000+ 1,300 for each 65 and over or blind
Married filing Separate 12,000 + 1,300 for 65 and over or blind

[Opus 17]

 Medical expenses are deductible in the year they are actually paid to the provider, not when the procedure occurred. For example, if you pay with a credit card or take out some other loan arrangement, the expense counts as paid when the provider is actually paid, not when you later pay off the loan. 

Of course, the deduction is subject to the limits discussed above in comments.