I had an injury in 2016 and my doctor was supportive of me getting physical and massage therapy. I never got a written note for it, though.
Can I get this note retroactively in order to deduct the cost of massage therapy? Not sure if the note has to be dated prior to start of service.
Maybe not. You can only deduct it is the doctor "prescribed" it as a form of medical treatment. You cannot deduct it only because your doctor was supportive of your choice.
For medical expenses to be deducted on the tax return, you must itemize deductions on Schedule A, and your unreimbursed medical expenses must exceed 10 percent of your adjusted gross income (AGI). Then, you can only deduct the amount by which your unreimbursed medical expenses exceed this 10 percent threshold (7.5% of AGI for age 65 and older until 2017). However, some states allow the full deduction, even if you don't have enough for your federal taxes.
Where do I enter my medical expenses?
Please see the following link for a comprehensive list of deductible medical expenses. IRS Medical Expenses