Per IRS Publication 502, Medical and Dental Expenses, page 2:
You can include only the medical and dental expenses you paid this year, but generally not payments for medical or dental care you will receive in a future year. (But see Decedent under Whose Medical Expenses Can You Include, later, for an exception.) This is not the rule for determining whether an expense can be reimbursed by a flexible spending arrangement (FSA). If you pay medical expenses by check, the day you mail or deliver the check is generally the date of payment. If you use a “pay-by-phone” or “online” account to pay your medical expenses, the date reported on the statement of the financial institution showing when payment was made is the date of payment. If you use a credit card, include medical expenses you charge to your credit card in the year the charge is made, not when you actually pay the amount charged.
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