I never realized that the Federal Aviation Administration could decline someone from becoming certified if they took certain psychiatric medications – even antidepressants and/or medications for ADHD, or if they had a psychiatric diagnosis such as depression or ADHD.

If you have or have had a psychiatric diagnosis or are taking medication, you are required to get a “Authorization for Special Issuance of a Medical Certificate” from the FAA before you are allowed to fly.

Here’s a list of diagnoses/medications and the FAA’s stance on them. Note that Major Depressive Disorder is not listed. This is not because it is an “acceptable” psychiatric diagnosis for FAA certification.
FAA list of psychiatric diagnoses

One could argue that they would rather be a passenger in a plane flown by a pilot with successful treatment of his depression rather than a pilot that cannot/will not seek treatment due to the ramifications on his career. Likewise, I would rather be a passenger in a plane flown by a pilot that is taking effective medication for ADHD rather than not taking any at all!