NO. You cannot use a 2016 medical expense on your 2018 tax return, unless, as noted above, you actually paid the expense in tax year 2018. You can only use a medical expense you paid for the tax year that you paid it in. If it was significant enough to affect your 2016 refund, you could amend your 2016 return. But be aware that for 2016, the only amount of medical expenses deductible on your federal return would be those expenses you paid out of pocket ABOVE 10% of your adjusted gross income (or above 7.5% if you were 65 or older) AND you need enough other itemized deductions to exceed your standard deduction.