Methylphenidate (MPH) and amphetamine (AMP) are the two first-line stimulant classes for ADHD. Both increase catecholamine availability in the prefrontal cortex and striatum, but through different pharmacological mechanisms. Approximately 70% of patients respond to one stimulant class; 90–95% respond to one or the other. No reliable predictors exist for which class a given patient will respond to.
Methylphenidate: Mechanism of Action
Methylphenidate is a racemic mixture - a 50:50 ratio of d-threo-MPH and l-threo-MPH. The d-threo enantiomer is responsible for the therapeutic effect; l-MPH has lower CNS specificity and binds poorly to the dopamine transporter (DAT).
MPH blocks DAT in the striatum - the brain region with the highest dopamine receptor density - and NET (norepinephrine transporter) in the prefrontal cortex. In the PFC, NET is more abundant than DAT. Both dopamine and norepinephrine compete for binding at NET, so blocking it increases synaptic concentrations of both catecholamines.
The result: more dopamine and norepinephrine remain in the synapse for longer, prolonging their effect on postsynaptic receptors. This is mechanistically distinct from how amphetamine works - MPH does not cause release of stored neurotransmitter. It simply prevents reuptake.
Plasma concentrations of d-threo-MPH correlate with DAT blockade in a dose-dependent manner. Peak DAT blockade occurs 60–90 minutes after oral administration.
Amphetamine: Mechanism of Action
Amphetamine also increases extracellular dopamine in the striatum and norepinephrine in the PFC, but via a broader set of mechanisms:
- Reuptake inhibition. Like MPH, amphetamine blocks DAT and NET, reducing reuptake.
- Vesicular release. At higher doses, amphetamine interacts with the vesicular monoamine transporter-2 (VMAT2), causing dopamine to be released from synaptic vesicles into the cytoplasm.
- Reverse transport. Amphetamine reverses the direction of DAT transport, pushing dopamine out of the presynaptic terminal and into the synapse - effectively turning the reuptake transporter into a release mechanism.
- MAO inhibition. Amphetamine weakly inhibits monoamine oxidase (MAO), the enzyme that breaks down dopamine and norepinephrine inside the neuron, reducing intraneuronal catecholamine metabolism.
An important distinction: the vesicular release and reverse transport mechanisms are primarily associated with higher doses and are implicated in abuse potential and cognitive impairment. At therapeutic doses, the primary mechanism is reuptake inhibition - the same as MPH. This is why therapeutic doses improve cognition while recreational doses impair it.
Formulations
Both drug classes are available in multiple delivery systems designed to control the pharmacokinetic profile - the shape of the blood concentration curve over time.
Methylphenidate Formulations
| Formulation | IR:ER Ratio | Approximate Duration |
|---|---|---|
| Immediate-release MPH | 100:0 | 3–4 hours |
| OROS-MPH (Concerta) | 22:78 | 10–12 hours |
| MPH-CD (Controlled Delivery) | 30:70 | 8 hours |
| SODAS (Spheroidal System) | 50:50 | 8 hours |
| Transdermal Patch (Daytrana) | N/A | 9 hours |
| LiquiXR | 20:80 | 8–12 hours |
| d-Methylphenidate XR (Focalin XR) | N/A | 8–12 hours |
d-Methylphenidate (Focalin) is the enantiomerically pure d-threo form. Its pharmacological profile is nearly identical to racemic MPH but with higher potency per milligram, meaning smaller doses achieve equivalent effects.
Amphetamine Formulations
| Formulation | IR:ER Ratio | Approximate Duration |
|---|---|---|
| Dexamphetamine Sulfate | 50:50 | 6–9 hours |
| Mixed Amphetamine Salts (Adderall) | 50:50 | 8–12 hours |
| Lisdexamfetamine (Vyvanse/Elvanse) | N/A (prodrug) | 10–13 hours |
| Triple-bead MAS-ER | N/A | 16 hours |
The prodrug design substantially reduces abuse potential. Intranasal or intravenous administration does not increase potency because the rate-limiting step is enzymatic cleavage in the bloodstream, not absorption. This makes LDX the preferred amphetamine formulation when diversion or abuse risk is a clinical concern.
Comparative Efficacy
Network meta-analyses (Cortese et al.) provide the best comparative data:
- Amphetamines: SMD -1.02 (most efficacious)
- Methylphenidate: SMD -0.78
- All active drugs superior to placebo
- Amphetamines: SMD -0.79
- Methylphenidate: SMD -0.49
- Bupropion: SMD -0.46
- Atomoxetine: SMD -0.45
Amphetamines show larger effect sizes across age groups, but methylphenidate has better tolerability in paediatric populations. Current clinical guidelines (NICE) recommend methylphenidate as first-line for children and adolescents, and amphetamines (specifically lisdexamfetamine) as first-line for adults.
Dosing and Titration
Both stimulant classes require careful titration. The therapeutic window - the dose range that produces benefit without intolerable side effects - varies substantially between individuals.
General titration principles:
- Start low. Begin at the lowest available dose.
- Increase gradually. Weekly dose increases are typical, adjusted based on symptom response and side effect burden.
- Tolerability is worst in the first weeks. Side effects often attenuate with continued use as the body adapts. Premature dose changes based on early side effects can lead to suboptimal dosing.
- No reliable dose prediction. Body weight does not reliably predict optimal dose. Titration is empirical - guided by clinical response.
The MTA (Multimodal Treatment of ADHD) study demonstrated that outcomes with medication management were comparable to controlled trial results when proper titration and monitoring protocols were followed, and significantly worse when they were not. Protocol adherence matters more than drug choice.
Side Effects
- Decreased appetite (often the most reported side effect)
- Insomnia (particularly with afternoon dosing or long-acting formulations)
- Headache
- Dry mouth
- Weight loss
- Increases in blood pressure (systolic and diastolic)
- Increases in heart rate
Amphetamines produce more appetite suppression and insomnia than methylphenidate on average. This is consistent with amphetamine's broader mechanism of action (vesicular release and reverse transport contribute additional catecholaminergic stimulation beyond reuptake inhibition).
Cardiovascular Monitoring
Both stimulant classes produce modest increases in blood pressure and heart rate. Monitoring requirements include:
- Baseline blood pressure and pulse before initiation
- Regular monitoring during titration and at each dose change
- Ongoing periodic monitoring during maintenance treatment
- Height and weight tracking in children and adolescents (growth velocity can be affected)
Serious cardiovascular events are rare at therapeutic doses in patients without pre-existing cardiac conditions. However, screening for structural heart disease and arrhythmia risk factors is recommended before initiation, particularly in patients with family history of sudden cardiac death.
Abuse Potential and Scheduling
Both methylphenidate and amphetamine are Schedule II (C-II) controlled substances, reflecting their potential for misuse. The abuse risk varies significantly by formulation:
- Highest risk: Immediate-release formulations (rapid onset, short duration, easy to crush and insufflate).
- Lower risk: Extended-release formulations (slower onset blunts the euphorigenic peak).
- Lowest risk: Prodrugs (lisdexamfetamine) - rate-limited activation prevents dose-dumping regardless of route of administration.
Extended-release and prodrug formulations also eliminate the need for midday dosing in school or work settings, reducing diversion opportunities.
Choosing Between Stimulant Classes
Given that 70% of patients respond to their first stimulant trial and 90–95% respond to one or the other, the initial choice is less critical than proper titration. Pragmatic factors in the decision:
- Age. Methylphenidate first-line for children (tolerability); amphetamine first-line for adults (efficacy).
- Abuse risk. Lisdexamfetamine or extended-release MPH preferred when diversion is a concern.
- Duration needed. Match formulation duration to the patient's day - short-acting for flexibility, long-acting for coverage without redosing.
- Prior response. Family member response to a specific stimulant is weakly predictive.
- Non-response. If the first stimulant class fails at adequate doses, switch to the other class before considering non-stimulant options.
References
- Cortese, S., et al. (2018). Comparative efficacy and tolerability of medications for attention-deficit hyperactivity disorder in children, adolescents, and adults: a systematic review and network meta-analysis. The Lancet Psychiatry, 5(9), 727–738.
- Faraone, S. V. (2018). The pharmacology of amphetamine and methylphenidate: relevance to the neurobiology of attention-deficit/hyperactivity disorder and other psychiatric comorbidities. Neuroscience & Biobehavioral Reviews, 87, 255–270.
- MTA Cooperative Group. (1999). A 14-month randomized clinical trial of treatment strategies for attention-deficit/hyperactivity disorder. Archives of General Psychiatry, 56(12), 1073–1086.
- Heal, D. J., et al. (2013). Amphetamine, past and present - a pharmacological and clinical perspective. Journal of Psychopharmacology, 27(6), 479–496.
- Coghill, D. R., et al. (2014). A systematic review of the evidence on the pharmacokinetics of medications for the treatment of ADHD. Journal of Attention Disorders, 18(1), 13–30.