The FAA has broad authority when it comes to medical certification matters. They have the authority to request medical records/information to aid them in determining a person’s eligibility for medical certification under 14 CFR Part 67. In essence, 14 CFR 67.413 says a person must provide requested medical information or authorize its release to the FAA, and failing to do so may result in a suspension, modification, revocation, or denial of a medical certificate.
Over the years, a number of pilots have challenged the FAA’s authority. The legal precedent is well established that the FAA administrator has the authority to request, or demand, medical information so long as there is a reasonable basis. And who establishes reasonableness? It’s mostly the FAA with a little help/affirmation from the National Safety Transportation Board. Note that the FAA authority extends beyond historical medical records and includes requests for current testing. For example, if a pilot reports a DUI event and the FAA requests a current evaluation performed by a certified substance abuse specialist or an addictionologist, it must be provided. And, absent some especially unique circumstances, if an evaluation is not performed and submitted, the FAA has grounds to suspend, modify, revoke, or deny medical certification.
Despite the FAA’s noted authority, if you receive a request that doesn’t seem to make sense, consult with a knowledgeable medical or legal advisor before blindly complying. The FAA sometimes makes mistakes. I once read a letter issued by a regional flight surgeon to an airman that said, "Due to your history of in (sic) violation of low-flying aircraft, you must provide...an evaluation by a specialist that specifically addresses the incident." Huh?