Are non-prescription drugs deductible if doctor recommends them?

It is worth noting that over the counter medications are eligible to be reimbursed from an FSA or HSA.  You do not need a letter of medical necessity.  The rules for FSA and HSA qualifying expenses are a bit more generous than the rule for the tax deduction. 

This is another weird tax rabbit hole.

The code (law) says that "An amount paid during the taxable year for medicine or a drug shall be taken into account under subsection (a) only if such medicine or drug is a prescribed drug or is insulin."  And "The term “prescribed drug” means a drug or biological which requires a prescription of a physician for its use by an individual."  §213(b).    So a drug that is available OTC is not deductible even if you get a prescription.  (Note that some drugs have both prescription and OTC formulas, and if you get a prescription for the prescription-only formula, that would be deductible.  Also note that getting a prescription for an OTC drug might get it covered under your insurance, and it can be reimbursed from an HSA or FSA.)

I see a lot of web sites that claim that getting a prescription for a non-prescription drug can (or "sometimes") make it deductible, but they never cite a code, regulation, or Tax Court case.  I think they are getting confused between the deduction and FSA/HSA reimbursement.  In 2003, the IRS ruled that OTC medicines were allowable under a pre-tax employee health plan, but in 2011 the ACA changed the rules and said that you could only use an FSA or HSA to pay for OTC medicines if you got a prescription.  In other words, getting a prescription for OTC meds was beneficial because it let you use tax-free FSA and HSA dollars.  However, the CARES Act in 2020 changed things again and allows taxpayers to buy a wide variety of OTC medical items without a prescription and use their HSA and FSA dollars.  

But, this never touched the law regarding the schedule A deduction.  That law still says the medication must be a kind of medication that requires a prescription -- is not available OTC.  

The reason you may need a letter of medical necessity is to allow HSA and FSA reimbursement for items that can be viewed as general lifestyle items -- supplements, for example.  Supplements and other items that are for general health benefits are not deductible OR reimbursable from an HSA/FSA.  If you get a letter of medical necessity for a supplement, for example, that lets you get reimbursement from your HSA or FSA, but still does not allow a schedule A tax deduction.