Anything that diagnoses, treats, or prevents a medical condition — defined by IRS Publication 502. General health and cosmetic spending is excluded.
The governing definition comes from IRC Section 213(d), expanded in IRS Publication 502: expenses for the diagnosis, cure, mitigation, treatment, or prevention of disease, and for treatments affecting any structure or function of the body.
The test is treatment, not health. Gym memberships make you healthier and don't qualify. Crutches don't make you healthier and do.
Commonly eligible and commonly forgotten:
Commonly assumed eligible and not:
Borderline items can often be made eligible with a Letter of Medical Necessity from a provider. Store that letter with the receipt — a decade later, the receipt alone won't explain itself.