This answer is both incomplete and incorrect. While you cannot deduct what you are paying back to your mother, you CAN deduct from your taxes a portion of the $85K in medical expense your mother paid on your behalf (so long as she paid the expense directly to the hospital or billing entity). The portion of the $85K that is more than 10% of your Adjusted gross income is deductible.
So for example, if she paid the $85K bill(s) for you in 2015, and you had $100K in income, you could have itemized that medical expense and deducted $75K it from your 2015 taxes (plus the deduction would have carried over to future years if you didn't have $75K in taxes to pay in 2015).
If you paid your mother back the $85K in 2016, you cannot claim it as a deduction because you have already claimed it as an expense in 2015.
NOTE: If you have not claimed a portion of the $85K as a deduction in the year it was paid, you could file an amended tax return for the year the $85K medical expense was incurred.
Please consult with an accountant to confirm the info above and exact numbers.
