First of all, ADHD is hardly the only disease or disorder with an unclear cause. Alzheimer's, autism, and fibromyalgia come to mind.
Second of all, here is a comprehensive list of peer-reviewed studies defending the existence of ADHD and the efficacy of amphetamines for ADHD (much better than some random anecdote of an improperly diagnosed taxi passenger):
Second of all, here is a comprehensive list of peer-reviewed studies defending the existence of ADHD and the efficacy of amphetamines for ADHD (much better than some random anecdote of an improperly diagnosed taxi passenger):
https://www.reddit.com/r/ADHD/comments/3at2u2/psa_the_existe...
Admittedly, psychologists seem to have a problem properly diagnosing it, but let's not diminish a disorder suffered by millions every day.