Did Dextroamphetamine and Methamphetamine Change Brain Glutamate Differently? A 26-Volunteer MRS Study

Answer: In a blinded crossover experiment involving 26 healthy volunteers, the analyzed magnetic-resonance-spectroscopy data showed that a single 20-mg oral dose of dextroamphetamine sulfate increased several measured metabolites in the dorsal anterior cingulate cortex compared with placebo. Glutamate, combined glutamate/glutamine (Glx), and total creatine survived the investigators’ correction for seven metabolite comparisons. An equal nominal 20-mg oral dose of methamphetamine hydrochloride did not produce a significant overall increase in those metabolites, although the study reported an exploratory methamphetamine-by-sex interaction for glutamate. These are regional magnetic-resonance signals, not measurements of the amount of either stimulant in a person’s blood or brain, and not tests of driving impairment.

The distinction matters in forensic toxicology. A compound administered to an experimental participant, a brain-metabolite signal, a participant’s subjective feeling of drug effect, and a quantitatively confirmed concentration in a legally collected blood specimen are four different evidentiary propositions. This study supplied valuable evidence for the first three, but did not report the fourth. Its primary source is White and colleagues’ 2018 original investigation in Neuropsychopharmacology, doi:10.1038/s41386-018-0027-7.

What scientific question did the investigators actually test?

The investigators asked whether prescription psychostimulants acutely changed a cluster of small-molecule signals inside a defined brain region and whether within-person changes in those signals accompanied reported drug liking or feeling high. The experiment compared a participant’s own responses to dextroamphetamine, methamphetamine, and placebo. It did not recruit suspected impaired drivers, people evaluated after collisions, or participants with stimulant-use disorder. Nor did it attempt to establish a numerical intoxication threshold.

The targeted region was the dorsal anterior cingulate cortex, abbreviated dACC. Because the dACC is involved in networks supporting attention, salience, control, and emotion, it is a plausible place to study acute drug-associated physiology. Plausibility, however, is not proof that a regional signal explains a particular person’s conduct. A magnetic-resonance finding can support a mechanistic hypothesis without functioning as a courtroom test of behavior at a historical event.

Who participated, and which denominator belongs to each outcome?

Twenty-six independent healthy adults entered the experiment; 13 were women and 13 were men. Their mean age was approximately 22.3 years. A participant underwent three blinded drug conditions, but a person’s three visits do not count as three different people. The paper’s primary metabolite table specifies 24 participants with usable spectroscopy data and between 63 and 66 spectra per reported outcome. In addition, the cardiovascular table identifies 23 participants with complete physiological data; its mood-correlation figure reports approximately 17–18 participants with full paired imaging and rating data.

Those outcome-specific denominators prevent a common error: treating all 78 theoretically possible person-visits, all 66 usable spectra, and all 26 enrolled volunteers as interchangeable independent samples. Some scans were incomplete or excluded, and the denominators varied across analyses. A p-value from an MRS model does not automatically describe the entire enrollment cohort. The investigators used repeated-measures analysis to take advantage of within-person comparisons, not to manufacture new independent subjects.

Were participants given identical drugs or identical nominal milligrams?

Each participant was assigned sessions containing placebo, 20 mg oral dextroamphetamine sulfate, and 20 mg oral methamphetamine hydrochloride in randomized, counterbalanced order. These are two distinct active chemical drugs supplied as different salts; neither is lisdexamfetamine, mixed amphetamine salts, methylphenidate, or a street-drug sample of unverified composition. Equal nominal milligram labels do not prove equal concentrations of parent compound in any biological compartment. The study employed a dose selected partly from prior human pharmacology, not from individually assayed exposure curves.

The capsules were opaque and blinded, and conditions were separated by at least three days, with test sessions generally around a week apart. Subjects ate a standardized meal before testing, had study-entry screening, and were scanned at a corresponding clock time to reduce circadian variability. These procedural safeguards strengthen inference about average within-person drug-condition differences. They cannot retrospectively supply quantitative pharmacokinetic measurements that the investigators did not report.

When was the brain sampled, and by what instrument?

The paper obtained proton magnetic resonance spectroscopy, written ¹H-MRS, approximately 140–150 minutes after capsule ingestion. Imaging used a 3-tesla Siemens system and a single localized PRESS spectroscopy acquisition. The methods describe a rectangular region-of-interest voxel measuring 15 × 15 × 10 mm, or 2,250 cubic millimeters, placed in the dACC with individualized anatomical guidance. The study checked tissue fractions and voxel placement across sessions.

MRS is a noninvasive spectroscopy method that estimates signals attributable to endogenous metabolites within an imaging volume. It is not gas chromatography–mass spectrometry or liquid chromatography–tandem mass spectrometry performed on a biological specimen. The voxel incorporates cells and extracellular spaces and its observable glutamate pool is dominated by much larger intracellular quantities. A shift in the voxel’s glutamate signal therefore should not be paraphrased as direct measurement of individual dopamine release, synaptic glutamate release, or the drug concentration at a neuronal receptor.

What does an MRS “concentration” mean here?

The published numerical values are CSF-corrected MRS metabolite estimates in institutional units, rather than independently calibrated blood concentrations expressed in mg/L or ng/mL. The investigators modeled spectra, corrected for cerebrospinal-fluid dilution, and reported means and standard deviations across acceptable data. They distinguished Glu (glutamate), Gln (glutamine), and the broader combined Glx signal, along with total creatine (tCr), total N-acetyl compounds (tNAA), choline-containing compounds (Cho), and myo-inositol (Ins).

These abbreviations are not interchangeable chemical species. Glx can change because a component changes while another does not. Likewise, tCr encompasses creatine and phosphocreatine rather than a drug or its metabolite. A forensic specialist should preserve the original reported units and assay modality; relabeling an MRS institutional unit as a whole-blood drug concentration would be a category error.

What were the numerical findings for placebo, dextroamphetamine, and methamphetamine?

The following table independently reorganizes the authors’ original CSF-corrected regional estimates. All entries are mean ± standard deviation, institutional units. These are not measured concentrations of dextroamphetamine, methamphetamine, or their metabolites. The contrast significance is assessed separately below because a descriptive difference in two means does not, by itself, define a statistically supported drug effect.

The d-amphetamine-versus-placebo means rose most visibly for Glu, Glx, and tCr. The methamphetamine means were generally much closer to placebo. Importantly, the original article does not provide an individual blood concentration to explain the difference, so the numerical pattern should not be used to infer greater absorbed parent-drug exposure after one capsule than the other.

Which dextroamphetamine effects survived multiple-comparison correction?

The authors anticipated testing several metabolites, so they applied a Bonferroni correction across seven metabolite comparisons, producing a family-wise threshold of approximately α = 0.0071. After that correction, the reported d-amphetamine main effects remained significant for glutamate (p = .0001), Glx (p = .003), and total creatine (p = .0067). The authors described within-voxel relative increases of approximately 8% Glu, 9% Glx, and 5% tCr for the drug-versus-placebo condition.

Two nominal p-values that might appear encouraging at a casual reading did not survive this prespecified correction: choline and myo-inositol each had a nominal p = .022. Their descriptive estimates should remain in the evidence summary, but neither result is a corrected-positive primary finding. Similarly, a visual upward movement in glutamine is not proof of a statistically reliable glutamine response, and tNAA did not display a supported d-amphetamine effect.

What did methamphetamine change in the overall MRS model?

There was no statistically significant overall methamphetamine main effect on any measured metabolite; the report states p-values greater than .47 for the main comparisons. This is a genuine tested negative result within the study’s sensitivity and sampled time window. It does not demonstrate that methamphetamine has no biological effect on glutamate generally, no effect outside the dACC, or no effect at other times or doses. It means that the defined average contrast did not meet the reported criterion in these experimental data.

The original figure also shows methamphetamine-versus-placebo estimates of Glx, Glu, and tCr with nominal p-values around .078, .060, and .067 for some illustrated comparisons. These should not be silently promoted into positive effects. The broader statement of no overall significant methamphetamine main effect remains the defensible conclusion. A between-drug visual difference cannot establish equivalent brain exposures because the investigators did not chemically measure those exposures.

Was the methamphetamine result different in women and men?

For glutamate, the study reported a methamphetamine-by-sex interaction, p = .020. Under methamphetamine versus placebo, the within-female contrast was reported as p = .029; the within-male contrast was not significant, p = .230. The slope contrast was approximately −0.99 ± 0.40 institutional units in the model. There was no comparable supported interaction for Glx (reported p greater than .15), nor for the other measured metabolites (reported interaction p-values greater than .55).

This sex interaction is intriguing but should be identified as an exploratory subgroup finding. It was obtained in a small imaging analysis with 13 women and 13 men enrolled, and individual accepted spectra varied by metabolite. The interaction p = .020 would not meet a blanket seven-test threshold of .0071 if treated as belonging to the same family; the source’s corrected primary-main-effect inference should not automatically be extended to exploratory sex contrasts. Neither the participants’ sex nor their MRS glutamate value establishes a reliable individual prediction of intoxication or conduct.

What were the genuine null results beyond methamphetamine?

Several controls and secondary results were expressly nonsignificant. Under dextroamphetamine, the authors reported no effect on total N-acetyl compounds, p greater than .40, and no supported glutamine difference, p greater than .40. The dextroamphetamine-by-sex interaction for glutamate was p = .34; the analogous Glx interaction was p = .08. Across conditions, glutamate and glutamine were not directly correlated in the manner that Glx was correlated with each of its component signals.

Quality-control measures also did not display systematic condition differences: example reported p-values were .51 for Glx fit uncertainty, .18 for glutamate fit uncertainty, .62 for spectral line width, and .68 for signal-to-noise ratio. These nondifferences are relevant to the investigators’ argument that the corrected-positive metabolite patterns were not merely accompanied by gross measured degradation of spectral acquisition. Such null checks do not prove the absence of every source of imaging error or every unmeasured physiological confound.

How do the statistically tested positives and nulls compare?

This second original evidence table separates effect tests from descriptive means. A dash is not substituted for an invented p-value. The general cutoff below is the investigators’ adjusted criterion for the seven main metabolite tests, not a legal drug threshold and not necessarily the standard applied to every exploratory interaction or mood correlation.

Did blood pressure and heart rate confirm pharmacological activity?

Yes, the investigators reported expected sympathomimetic changes in cardiovascular variables under both active drugs. Repeated blood-pressure and heart-rate measurements showed significant drug-condition effects, with drug-related effects beginning around 90 minutes after dosing and persisting during the 140–150-minute spectroscopy window. This supports that oral capsule administration had physiological consequences even when the methamphetamine overall MRS metabolite contrast was nonsignificant.

But cardiovascular monitoring is not a quantitative parent-drug assay. A measured systolic-pressure change or elevated heart rate cannot simply be translated into an amphetamine concentration in legally collected human whole blood. Nor do these monitored effects prove the participant was unable to operate a vehicle. The experiment did not administer a standardized field sobriety battery, an actual on-road route, or a forensic impairment opinion protocol.

What did the investigators learn from self-reported liking and feeling high?

The participants provided ratings during the 5.5-hour test sessions, allowing the researchers to relate changes in dACC metabolites to changes in two self-reported states. Dextroamphetamine-related glutamate change correlated with the duration of peak drug liking, r = +.49, p = .02; total-creatine change correlated with that duration at r = +.41, p = .047. Methamphetamine-related glutamate change correlated with the peak rating of feeling high, r = +.52, p = .016, and Glx change with that high rating at r = +.42, p = .049.

Those analyses used approximately 17–18 participants with sufficiently complete matched ratings and spectra, a smaller denominator than enrollment or the primary MRS table. Correlation does not determine causal direction between neural metabolites and feelings, and subjective ratings are not equivalent to a validated driving-impairment standard. Multiple exploratory associations were considered; without independent confirmation, borderline p-values such as .047 or .049 are particularly vulnerable to model and multiple-testing choices. No claim should be made that a single MRS value diagnoses intoxication, deceptive intent, or a specific person’s experienced high.

Does an MRS glutamate increase demonstrate dopamine release?

No. The source discusses dopamine transporters, norepinephrine, glutamatergic metabolism, and plausible links among neural systems. However, it did not directly measure extracellular dopamine in humans, perform microdialysis, or chemically assay dopamine transporter occupancy. Its MRS measurement sampled a regional pool of metabolites—principally intracellular glutamate—at one postdose imaging window. It cannot identify the molecular steps that created the change from the MRS peak alone.

The same caution applies to the term “excitatory neurotransmission.” Glutamate is an excitatory neurotransmitter, but total MRS-visible Glu is not synonymous with neurotransmitter release at a synapse. A plausible biological mechanism can be discussed as a hypothesis, not testified to as if directly quantified in each subject. The investigators’ mechanistic discussion itself considers precursor availability, synthesis, and catabolism as possible contributors.

Did the study chemically analyze whole blood, plasma, serum, or urine for stimulant concentrations?

No quantitative postdose parent-drug or metabolite measurements are reported in any of those forensic matrices. Eligibility screening included breath alcohol testing and urine toxicology screening to exclude recent extraneous consumption, but that is not the same as collecting timed venous whole-blood specimens and performing a validated quantitative GC–MS or LC–MS/MS stimulant method after the experimental doses. There is no reported whole-blood amphetamine concentration in mg/L, no separated-serum or plasma amphetamine result, and no methamphetamine-to-amphetamine metabolite ratio. No enantiomer-specific d/l quantitative toxicology is presented.

The paper likewise supplies no time series of assayed parent stimulant in oral fluid, sweat, hair, vitreous humor, breast milk, exhaled breath, or tissue specimens. Its analytes were brain-region MRS metabolites, not the ingested drug in circulation. Thus a lawyer or forensic analyst cannot substitute the MRS table for a toxicology laboratory result. Even a known experimental 20-mg capsule does not permit reconstruction of an individual’s incident-time whole-blood stimulant level without measured specimens and an appropriately supported pharmacokinetic model.

Can the null methamphetamine main effect be interpreted as “no methamphetamine effect”?

Only if the conclusion is framed narrowly: no statistically significant overall change in the seven reported dACC MRS metabolite outcomes was detected at approximately 2.5 hours in the analyzed cohort under this dose. Both drugs changed cardiovascular measurements. There was exploratory between-sex variability for glutamate. A study restricted to one region, one imaging window, one dose, a sample of 24 analyzed persons, and MRS-visible metabolites cannot test the absence of effects in other brain regions or under other experimental circumstances.

Moreover, “amphetamine affected glutamate and methamphetamine did not” does not establish that the drugs have chemically equal pharmacokinetic exposure or are generally opposite in function. Equal capsule milligrams are not proof of equal free-base doses, plasma areas under the curve, central nervous-system uptake, or receptor occupancy. The narrow non-significance is an evidentiary limit, not an excuse to make a broad comparative pharmacological declaration.

How strong is the within-person crossover design?

It is a genuine strength: participants served as their own placebo controls, and order counterbalancing reduced confounding by fixed traits. Sessions at similar times of day and reasonable washouts further controlled some variability. In principle, a subject’s baseline brain anatomy and several stable characteristics are held relatively constant across drug conditions. The original authors also reported that voxel overlap and spectral-fit quality were broadly similar across conditions.

Nonetheless, a crossover does not eliminate scan-registration error, learning, expectancy, circadian deviations, within-person day-to-day changes, incomplete outcomes, or time-dependent exposure differences between drugs. The investigators studied healthy screened adults, not a representative impaired-driver population. The final MRS denominator was 24; correlations used fewer participants. These limits are substantive because group mean changes and small-sample interactions can coexist with considerable individual variation.

What is the defensible forensic or courtroom application?

The defensible use is as background scientific evidence that single clinically relevant psychostimulant doses can alter regional human MRS metabolite estimates and subjective states, while changes vary by compound and analytical endpoint. It also demonstrates that different assays answer different questions: proton MRS can study tissue metabolites, cardiovascular monitoring can show physiological activity, subjective rating scales can describe volunteered experience, and validated forensic mass spectrometry can identify and quantify specified drugs in specified specimens.

The study cannot establish a blood-drug threshold for impairment, an individual’s actual use time, tolerance, motor ability, mental state, memory accuracy, degree of intoxication, criminal intent, or causation of any collision or injury. A case-specific evaluation would require the original laboratory’s matrix and sampling records, validated method and uncertainty, actual individual toxicology, contemporaneous behavior, clinical context, and applicable legal standards. None of those external case facts is supplied by this experiment.

What should a reviewer ask before citing this paper?

A reviewer should identify the claimed fact with precision: is the citation supporting a corrected d-amphetamine-associated regional Glu difference, an exploratory sex interaction to methamphetamine, a correlation with questionnaire ratings, or a forensic drug-blood assertion? Only the first three were examined, and each has a different sample and inferential strength. The reviewer should check the original Table 2 units, the final analyzable denominator, the correction threshold, and whether a null result is being described as equivalence or absence of effect.

The reviewer should also distinguish original experimental dose from a measured chemical concentration, the dACC voxel from all human brain regions, a within-subject comparison from an individual clinical prediction, and an institutional spectroscopy unit from a validated whole-blood mass-concentration unit. These distinctions are more consequential than a confident-sounding headline because they delimit what the source actually supports.

Frequently asked questions about this 26-volunteer trial

Did dextroamphetamine increase brain glutamate in the study?

Yes. The corrected dACC MRS mean increased from 8.62 institutional units under placebo to 9.32 under 20 mg oral d-amphetamine, with a model p = .0001. The estimate was regional and MRS-derived; it was not a blood dextroamphetamine assay or a direct measurement of synaptic glutamate release.

Did methamphetamine cause the same average glutamate increase?

No statistically significant overall MRS metabolite main effect was detected for oral 20 mg methamphetamine hydrochloride. The placebo and methamphetamine Glu means were 8.62 and 8.80 institutional units. The paper additionally reported an exploratory methamphetamine-by-sex interaction, which should not be confused with a positive overall result.

Why did the researchers use both Glu and Glx?

Glu estimates glutamate while Glx reflects a combined glutamate/glutamine signal. The authors used the relationship between the two to determine which component appeared to explain the combined change. They found a supported d-amphetamine Glu effect but no significant isolated glutamine contrast.

How many people were actually included in the spectroscopy results?

Twenty-six healthy adults entered, but the original metabolite table lists 24 with usable MRS and 63–66 accepted spectra depending on outcome. Cardiovascular effects included 23 with complete results and mood-correlation analyses about 17–18. Repeated spectra and three sessions do not create new independent people.

Is the methamphetamine-by-sex interaction a validated female impairment marker?

No. Its p = .020 is an exploratory subgroup finding in a small imaging experiment, and there was no validated individual impairment classification or legal stimulant-blood quantitation. It cannot diagnose a woman’s intoxication, explain a particular action, or establish an individual dose-to-effect relationship.

Were quantitative methamphetamine or amphetamine concentrations measured in whole blood?

No postdose quantitative whole-blood, plasma, serum, oral-fluid, urine, sweat, hair, vitreous, or breath stimulant concentrations are reported. A standardized 20-mg oral dose and a brain MRS metabolite value cannot replace chemically confirmed laboratory drug concentrations from a separately collected legal specimen.

Can this research be used to calculate an individual’s drug use time or accident causation?

No. There was no individual incident, observed road test, validated forensic impairment outcome, or pharmacokinetic blood sampling series. The controlled experiment supports restricted group-level scientific conclusions and does not determine any actual person’s last ingestion, driving ability, intent, or responsibility for a specific event.

Evidence boundaries and final answer

The tested and corrected-positive finding was an increase in dACC Glu, Glx, and tCr after a single 20-mg oral d-amphetamine condition relative to placebo. Genuine tests also produced null findings: methamphetamine had no significant overall MRS-metabolite main effect, several d-amphetamine secondary signals failed correction or were nonsignificant, and imaging-quality metrics did not meaningfully differ by drug condition. An exploratory methamphetamine-by-sex Glu pattern and small mood correlations merit scientific interest without becoming individual forensic conclusions.

For review of the original research, see the complete White et al. report, original article DOI 10.1038/s41386-018-0027-7. For separate, already published educational discussions of specimen specificity and analytical confirmation on this site, consult Biological Matrices in Forensic Toxicology and Confirmatory Drug Testing. Those general guides are contextual; they do not substitute for this experiment’s original methods or for a case-specific laboratory record.