In the workplace, I saw the same folks struggle to work consistently without abusive dosages of such drugs. A close friend eventually went into in-patient care for psychosis due to his interaction with Adderall.
Like any drug, the effect wears off - Cognitive Behavioral Therapy matches prescription drugs at treating ADHD after 5 years. As I recall, the standard dosages of Adderall cease to be effective after 7-10 years due to changes in tolerance. Individuals trying to maintain the same therapeutic effect will either escalate their usage beyond "safe" levels or revert to their unmedicated habits.