Per IRS Publication 17, Your Federal Income Tax (For Individuals), page 158:
Medical and Dental Expenses. Beginning January 1, 2017, and ending before January 1, 2019, you can deduct only the part of your medical and dental expenses that exceeds 7.5% of your adjusted gross income (AGI).
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. This is not the rule for determining whether an expense can be reimbursed by a flexible spending arrangement (FSA).
What Expenses Can You Include This Year?
If you pay medical expenses by check, the day you mail or deliver the check generally is 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.
See IRS Publication 17 Your Federal Income Tax (For Individuals) for more information.