I reckon the latter is maybe 70% of all the ‘my pdoc gave me adderall and it destroyed my brain!!!’ posts you see on reddit and drugs dot com. also, benzos
Pharmacological withdrawal: Symptoms from CNS adaptation to the drug... onset tracks pharmacokinetics, profile is stereotyped (dizziness, brain zaps, sensory disturbance), dose-responsive, rapidly relieved by reinstatement, resolves over weeks-months off-drug. Real mechanism, real syndrome.
Functional discontinuation symptoms: Genuinely experienced symptoms (not feigned) whose generation and maintenance involve mechanisms beyond ongoing pharmacological adaptation... nocebo/expectancy, attentional capture, symptom-monitoring loops, identity formation around being "injured." Symptoms can persist long past plausible drug clearance, may not track dose, may not respond to reinstatement in the expected way. (plus, there are many signs of functional impairment like non-physiological behavioural symptoms)