Resource · Pharmacology
Pharmacokinetics (PK) Simulator of IR Stimulant Medication
An interactive visualizer for the rise and fall of stimulant ADHD medications in the bloodstream. Compare onset, peak, and clearing across the immediate-release options, then model your own daily schedule below.
Why we built this
A working conjecture about onset, clearing, and task completion
Most ADHD medication discussions focus on starting and sustaining attention. A less-discussed problem is the completion signal: the brief reward burst that says "this is done, you can move on." When that signal is weak, finishing a task can feel hollow rather than satisfying, and the brain quietly avoids it.
The conjecture this resource is built around is simple. Immediate-release stimulants have predictable, sharp peaks. If you time a dose so its peak coincides with the end of a defined task, the pharmacological signal and the completion signal arrive at the same moment. Repeated, that pairing may help train a reward response to finishing, the way classical conditioning trains any other response.
The simulator below makes the timing tangible. Different IR formulations have different rise-and-fall shapes; the right one for a 60-minute task is not the right one for a 4-hour work block. Compare the curves first, then model a real dosing schedule against your own day in the second section.
How to read the chart
- X axis is hours since you took the dose. Y axis is plasma concentration, normalized to 100% of each drug's own peak. The chart is for shape comparison, not dose comparison.
- A narrow, tall peak means the drug arrives quickly and clears quickly. Best for short, well-defined tasks where you want a clear "done."
- A broad, flat plateau covers a longer window but blurs the moment of peak effect. Better for sustained focus, worse for timing.
IR Stimulant Pharmacokinetic Profiles
Approximate plasma concentration curves after a single oral dose, normalized to 100%.
dexmethylphenidate
methylphenidate
dextroamphetamine
mixed amphetamine salts
methamphetamine
lisdexamfetamine
Peak concentration data
| Medication | Time to peak (Tmax) | Duration of action | Half-life |
|---|---|---|---|
| Focalin IR dexmethylphenidate | 1 – 1.5 hours | 3 – 4 hours | 2 – 2.5 hours |
| Ritalin IR methylphenidate | 1.5 – 2 hours | 3 – 4 hours | 2 – 3 hours |
| Dexedrine IR dextroamphetamine | 2 – 3 hours | 4 – 6 hours | 10 – 12 hours |
| Adderall IR mixed amphetamine salts | 2.5 – 3 hours | 4 – 6 hours | 10 – 13 hours |
| Desoxyn methamphetamine | 2.5 – 3.5 hours | 6 – 8 hours | 9 – 15 hours |
| Vyvanse lisdexamfetamine | 3.5 – 4.5 hours | 10 – 14 hours | 10 – 12 hours |
Clinical notes for task-completion timing
Focalin IR offers the fastest and sharpest peak. That makes it the most precise option for timing a dose to a specific task endpoint. For a 60 to 75 minute task this gives the most predictable crescendo, and its rapid offset minimizes interference with whatever comes next or with sleep later in the day.
Ritalin IR peaks slightly later but the curve is still narrow, which suits roughly 90 minute task windows. It clears quickly for the same reason Focalin does.
Dexedrine IR peaks later with a longer tail. The extended duration gives less precision for timing, but the sustained curve can support longer work blocks where you don't need a sharp moment of peak effect.
Adderall IR behaves much like Dexedrine, with a slightly broader peak. The mixed salts (75% dextroamphetamine, 25% levoamphetamine) extend the curve modestly because the levo isomer hangs around longer than the dextro.
Desoxyn peaks the latest of the IR options and lasts the longest. It is less useful for precise timing and better suited to extended focus periods.
Vyvanse (shown for contrast) is a prodrug that produces a very flat, extended plateau with no clear peak. That property makes it unsuitable for task-completion timing strategies. You cannot reliably pair its pharmacokinetics with a specific endpoint, and that is by design.
These curves are idealized approximations. Individual variation is significant based on metabolism, food intake, gastric pH, and genetics. Nothing on this page is medical advice.
Dosing simulator Amphetamine class
Model your daily blood concentration based on medication, dose, and timing.
Configure your doses
Concentrations shown in "amphetamine equivalents" for cross-medication comparison. Doses are sorted by time when graphed. Complete all three fields (medication, dose, time) to see a dose on the graph.
Dosing simulator Methylphenidate class
Model your daily blood concentration based on medication, dose, and timing.
Configure your doses
Concentrations shown in "methylphenidate equivalents" for cross-medication comparison. Focalin (dexmethylphenidate) is roughly 2× as potent as racemic methylphenidate per mg. Complete all three fields to see a dose on the graph.
Further reading
Companion essays and videos that go deeper on the conjecture above will live here as they're published. For now, the references at the bottom of this page point to the source pharmacokinetic literature.
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Coming soon
Why your ADHD brain resists finishing things
The neuroscience of task closure, the Zeigarnik inversion, and the "completion signal" gap.
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Coming soon
The timed-dose crescendo, in practice
A walk-through of how to use this simulator alongside short, well-defined task blocks.
References & data sources
FDA prescribing information (package inserts)
- Adderall / Adderall XR: Teva Pharmaceuticals. FDA Label – Adderall, FDA Label – Adderall XR
- Vyvanse (lisdexamfetamine): Takeda Pharmaceuticals. FDA Label
- Dexedrine (dextroamphetamine): FDA Label
- Desoxyn (methamphetamine): Recordati Rare Diseases. FDA Label
- Ritalin / Ritalin LA: Novartis. FDA Label – Ritalin, FDA Label – Ritalin LA
- Focalin / Focalin XR: Novartis. FDA Label – Focalin, FDA Label – Focalin XR
- Concerta (methylphenidate ER): Janssen Pharmaceuticals. FDA Label
Pharmacokinetic studies
- Ermer JC, et al. (2010). "Pharmacokinetics of lisdexamfetamine dimesylate after targeted gastrointestinal release." J Clin Pharmacol. 50(6):713-20.
- Markowitz JS, Patrick KS. (2017). "The Clinical Pharmacokinetics of Amphetamines Utilized in the Treatment of Attention-Deficit/Hyperactivity Disorder." J Child Adolesc Psychopharmacol. 27(8):678-689.
- Heal DJ, et al. (2013). "Amphetamine, past and present: a pharmacological and clinical perspective." J Psychopharmacol. 27(6):479-496.
- Modi NB, et al. (2000). "Dose-proportional and stereospecific pharmacokinetics of methylphenidate delivered using an osmotic, controlled-release oral delivery system." J Clin Pharmacol. 40(10):1141-9.
- Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th Edition. Chapter on Sympathomimetic Drugs.
Half-life & metabolism data
- DrugBank Online Database: Amphetamine, Dextroamphetamine, Methamphetamine, Methylphenidate
- PubChem Compound Database (NIH): pharmacology and toxicology sections for each compound.
Disclaimer
- These visualizations are for educational purposes only and should not be used to make medical decisions. Actual pharmacokinetics vary significantly based on individual metabolism, genetic factors (CYP2D6 polymorphisms), urinary pH, food intake, and drug interactions. Always consult your prescribing physician for dosing decisions.