Does an AME's advertisement on the back of the medical certificate affect the document's validity under the FAA?
It's an interesting question...
The AME Guide gives the following &quot;advice&quot;:


Medical Certificates – AME Completion
(Updated 07-26-2017)


Date the medical certificate to reflect the date the medical examination
was performed, NOT the date of import, issuance, or transmission.
Limitations must be selected from the list in the Aerospace Medical Certification
System (AMCS). Additional limitations may NOT be typed/written in.
Signatures: Each medical certificate must be fully completed prior to being
signed.

Both the AME and applicant must sign the medical certificate in ink.
The applicant must sign before leaving the AME’s office.


Give only ONE certificate to the airman
Use AMCS generated certificates only.
Transmit the exam electronically to the FAA using AMCS within 14 days.
The following are NOT valid:

Copies of medical Certificates;
Typewriter or handwritten certificates;
Obviously corrected certificates;
Paper 8500-8 certificates (any remaining paper forms should be destroyed
by the AME).


Replacement medical certificates must be issued by the FAA.


(Remember, the guide, like the Pilot Handbook, is a guide, not regulatory)
The AMCS also gives:

Certificates will be printed on standard printer paper as a single-sided document. Both the front and back sides will be printed on a single sheet of paper that can be folded.

But as far as I can tell in the FAR, the back side (inside when folded) does not have to be blank. As long as the medical certificate follows the 8500-9 and is in a &quot;form prescribed by the Administrator&quot;, then it is a valid certificate.
Technically, even if it was against the rules, the medical certificate wouldn't be invalid, just the paper evidence of it would be. As long as the data was entered into AMCS, which it is required to be, the certificate can be reprinted in the proper format for showing anybody who asked.