What is the difference between dextroamphetamine sulfate, dextroamphetamine (the d‑isomer), and racemic amphetamine?

Dextroamphetamine Sulfate vs. Dextroamphetamine vs. Racemic Amphetamine

Dextroamphetamine sulfate is simply the salt form of dextroamphetamine (the d-isomer), while racemic amphetamine contains both d-amphetamine and l-amphetamine in equal proportions—the key clinical difference being that the d-isomer is approximately 3-4 times more potent than the l-isomer for ADHD symptoms, though the l-isomer may contribute unique pharmacokinetic properties.

Chemical Composition

Dextroamphetamine sulfate is the dextro (d-) isomer of amphetamine in its sulfate salt form, chemically designated as d-alpha-methylphenethylamine sulfate with molecular formula (C9H13N)2·H2SO4 1, 2, 3. This is the active pharmaceutical ingredient in medications like Dexedrine.

Racemic amphetamine (also called d,l-amphetamine) contains equal amounts of both the d-isomer and l-isomer 4. Mixed amphetamine salts (Adderall) contain approximately 75% d-amphetamine and 25% l-amphetamine 5.

Pharmacological Differences

Potency and Receptor Activity

The d-isomer is markedly more potent than the l-isomer at inhibiting norepinephrine reuptake in the brain, being approximately 10 times more effective for locomotor stimulation 6. However, both isomers are equally active at dopamine neurons in the corpus striatum 6.

Clinical dosing reflects this potency difference: starting doses are 2.5 mg for dextroamphetamine (DEX) versus 5 mg for methylphenidate 4. Maximum daily doses are 40 mg for amphetamines versus 60 mg for methylphenidate 4.

Pharmacokinetics

When racemic amphetamine is administered, the d-enantiomer achieves significantly higher concentrations in brain and plasma compared to the l-enantiomer, despite equal dosing 7. This occurs because the l-isomer may interfere with hydroxylation of d-amphetamine 7.

The l-isomer in mixed amphetamine salts changes dopamine release kinetics, producing significantly faster rise times and shorter signal decay times compared to pure d-amphetamine 5. This may explain why some patients respond better to mixed salts than to pure dextroamphetamine.

Brain Activity Patterns

Neuroimaging studies demonstrate that l-amphetamine produces greater global BOLD responses than d-amphetamine, particularly in somatosensory cortex, motor cortex, orbitofrontal cortices, prelimbic and infralimbic cortex—regions not significantly activated by the d-isomer alone 8. This suggests the isomers have distinct actions on brain regions affected in ADHD.

Clinical Efficacy

In a double-blind trial of 48 children with ADHD, dextroamphetamine was significantly superior to racemic amphetamine on average, with similar side effect profiles 9. However, in 7 of 48 cases (15%), racemic amphetamine produced superior clinical improvement 9. This supports the guideline recommendation that approximately 70% respond to either dextroamphetamine or methylphenidate alone, but nearly 90% respond when both stimulant classes are tried 4, 10.

Side Effect Profile

Both formulations cause similar adverse effects, but dextroamphetamine and mixed amphetamine salts produce greater problematic effects on appetite and sleep compared to methylphenidate, consistent with their longer excretion half-lives 4.

Common side effects include 4, 11:

• Decreased appetite and weight loss
• Insomnia (dose early in day to minimize)
• Increased blood pressure (mean increase 1.93/1.84 mmHg) 12
• Increased heart rate (mean increase 3.71 bpm) 12
• Headache, stomachache, irritability

Prescribing Considerations

Start with either formulation based on clinician and patient preference 4. The choice between pure dextroamphetamine and mixed amphetamine salts should consider:

• Pure dextroamphetamine may have slightly less impact on appetite/sleep due to absence of l-isomer
• Mixed amphetamine salts may provide benefit in the 15% of patients who respond better to racemic formulations 9
• Titration schedule is identical: start 2.5 mg once or twice daily, increase by 2.5-5 mg weekly 4

Monitoring Requirements

• Baseline and quarterly blood pressure and pulse 4
• Height and weight monitoring (stimulants reduce growth velocity) 11
• Cardiovascular screening before initiation 11

Contraindications

Both formulations are contraindicated in 4:

• Concomitant MAO inhibitor use (risk of hypertensive crisis)
• Active psychosis
• Uncontrolled hypertension or symptomatic cardiovascular disease
• Recent stimulant abuse (relative contraindication requiring close monitoring)

Important caveat: Package insert warnings about motor tics and Tourette's are not supported by recent controlled trials showing methylphenidate does not worsen tics 4. Amphetamine may worsen tics more than methylphenidate 4.

Practical Algorithm

1. First-line choice: Either pure dextroamphetamine or mixed amphetamine salts (clinician/patient preference)
2. If inadequate response after optimal titration: Switch to the alternative amphetamine formulation or to methylphenidate class
3. If side effects problematic: Consider pure dextroamphetamine if using mixed salts, or switch to methylphenidate
4. Special populations: In pregnancy, amphetamines show no increased risk of major malformations but possible small increased risk of preeclampsia and preterm birth if continued in second half of pregnancy 13