Stimulant effect follows an inverted-U (too little / too much)

The prefrontal cortex operates on a Goldilocks (inverted-U) principle. Too little dopamine and noradrenaline and you get the familiar ADHD picture — executive dysfunction, restlessness. Too much over-activates the same circuitry and you get the other failure mode: anxiety, hyperarousal, and a shift from cognitive flexibility toward cognitive rigidity, sometimes even repetitive, OCD-like thought loops in adults. So an apparent ‘the medication stopped working’ is often not ‘I need more’ but a sign the dose has overshot the peak, or that something else moved the curve.

Several things effectively raise the dose without changing the prescription. Life stress — positive or negative — activates the amygdala, which raises dopamine and noradrenaline on top of the medication, so a previously-good dose can tip into over-activation during a hard stretch. A baseline emotional-arousal profile (common in women with ADHD, and with co-occurring PTSD/CPTSD) starts further up the curve, so an agent like dexamphetamine can show early benefit and later turn over-activating, bringing anxiety or rigidity. The practical upshot: when a stimulant seems to fail, describe the specific change (sleep, racing thoughts, anxiety, rigidity) so the cause can be targeted — reflexively raising the dose can make the real problem worse.