How would FIDE handle a player with ADHD in tournaments? I recently stumbled upon Why is coffee provided during big chess events when it contains a banned substance? and learned that amphetamines are banned by FIDE.
How would FIDE handle a player with ADHD who is rightfully prescribed amphetamines?
How would FIDE handle a player with ADHD who is rightfully prescribed
amphetamines?

FIDE handles this in the same way as all Olympic sports handle such things. Anybody who follows such sports (particularly cycling) will be familiar with &quot;Therapeutic Use Exemptions&quot; or TUEs.
They are covered in the FIDE Anti Doping Rules:

4.4 Therapeutic Use Exemptions (“TUEs”)
4.4.1 The presence of a Prohibited Substance or its Metabolites or Markers, and/or the Use or Attempted Use, Possession or FIDE Anti-Doping Rules 14 Administration or Attempted Administration of a Prohibited Substance or Prohibited Method, shall not be considered an anti-doping rule violation if it is consistent with the provisions of a TUE granted in accordance with the International Standard for Therapeutic Use Exemptions.

FIDE provides a standard Therapeutic Use Exemptions (TUE) Application Form which can be downloaded from their site, filled in, signed, and submitted.