“Brain fog” is a symptom, an outward sign of something going on. The source might be ADHD.  When ADHDers are taking the right meds sometimes they report the “brain fog” or “cobwebs” are cleared.

Brain Fog

Brain fog is not necessarily correlated with ADHD.  Brain fog feelings can be caused by different conditions and these need to be ruled out before an ADHD diagnosis can be made.