Deductions & credits

https://www.irs.gov/pub/irs-pdf/p502.pdf

From that publication

What Expenses Can You Include This Year? 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. If you didn't claim a medical or dental expense that would have been deductible in an earlier year, you can file Form 1040-X, Amended U.S. Individual Income Tax Return, to claim a refund for the year in which you overlooked the expense. Don't claim the expense on this year's return. Generally, a claim for refund must be filed within 3 years from the date the original return was filed or within 2 years from the time the tax was paid, whichever is later. You can't include medical expenses that were paid by insurance companies or other sources. This is true whether the payments were made directly to you, to the patient, or to the provider of the medical services.