Short answer: In a 2026 observational comparison of 154 adults, frequent cannabis users tested 45 minutes after smoking scored lower on a task requiring them to reproduce a sequence of spatial locations in reverse order. The difference from same-sex nonusing controls was larger among men than women. Three other span tests did not show significant group differences. But the investigators did not assay THC in blood, breath, oral fluid, or urine, did not test the same participants before and after cannabis, and did not measure driving. The results describe group differences in a particular cognitive task; they do not prove the pharmacologic dose received, establish an individual’s impairment, or demonstrate why a particular incident happened.
The source is Ryder and colleagues’ 2026 original study, “Same dose, different impact: acute cannabis intoxication impairs visuospatial working memory in both sexes, with disproportionate male vulnerability”, published in Frontiers in Behavioral Neuroscience. The question for forensic interpretation is not whether a statistically significant effect exists: it does. It is what comparison produced that effect, how exposure was characterized, what was actually measured, and what additional evidence would be necessary to apply the result to one person.
What was actually tested?
The researchers recruited 154 community participants: 77 who reported using cannabis regularly and 77 nonusing comparison participants. Regular use was defined for eligibility as at least five days a week over at least a year; nonusers reported fewer than five lifetime uses and none during the preceding year. The users smoked approximately one gram of their usual medical-grade cannabis product and began testing 45 minutes later. Controls were tested without a corresponding cannabis or placebo administration. This was a between-subjects, cross-sectional comparison, not a randomized trial assigning regular users to THC or placebo, and not a within-person evaluation of change from a sober baseline.
The four tasks crossed two distinctions: auditory-verbal versus visuospatial material, and forward repetition versus backward manipulation. Repeating a series in its original order chiefly measures short-term maintenance. Reversing it adds mental manipulation. The researchers recorded the number of correctly recalled sequences; they did not measure response latency. Thus, an unchanged forward score is not proof that every aspect of attention or reaction speed was unaffected.
Who was in each comparison?
The cannabis group consisted of 46 men and 31 women; controls consisted of 32 men and 45 women. The sex distributions were therefore not identical across exposure groups, even though the total sample comprised 78 men and 76 women. Mean ages in the four groups were similar, and the investigators reported no statistically significant four-group age difference, F(3,150) = 0.45, p = 0.715. Most participants were young adults; the overall reported mean age was 24.58 years (SD 5.73), with eligibility extending from 18 to 50 years.
The study relied on community recruitment through convenience and snowball sampling. That is important for generalizability: people reached through those channels, with frequent cannabis histories and willingness to participate in cognitive testing, cannot be assumed to represent all occasional users, patients, workers, drivers, or people undergoing a forensic investigation. Nor was sex itself randomized or experimentally manipulated.
Was “the same dose” a measured blood THC exposure?
No. Participants were asked to smoke one approximately one-gram cigarette from a product reported to contain 16.1% delta-9 THC and less than 1% CBD. This identifies product mass and labeled/laboratory-tested composition, not each participant’s measured absorbed THC dose. The protocol did not standardize inhalation depth, puff duration, breath-holding, smoke loss, or individual systemic bioavailability. Testing at a common 45-minute interval likewise does not make internal exposures equal.
Most importantly, the authors expressly state that no biological samples were collected to verify drug exposure or nonuse. There was no venous whole-blood THC concentration, plasma THC curve, oral-fluid THC result, urine screen, or breath THC analysis. Consequently the paper contains no concentration-performance regression, no blood-THC threshold, no analytic limit of detection or quantitation for THC, and no specimen-based individual timing estimate. Referring to its results as proof of a particular blood THC level would add measurements the investigators never obtained.
In toxicology, the distinction is basic but consequential: the THC percentage of plant material is a product characteristic; smoked mass is an administration-related estimate; circulating THC is an analytical result from a named specimen; and cognitive performance is a separate functional outcome. None can be silently substituted for another.
Which of the four memory tasks differed?
The statistically significant pattern centered on Spatial Span Backward, the task of reproducing a series of locations in reverse order. Digit Span Forward, Digit Span Backward, and Spatial Span Forward did not show significant main effects or group-by-sex interactions in the reported primary tests. These three null findings should be preserved, rather than presenting the paper as evidence of generalized memory failure after cannabis.
The following table reproduces the reported descriptive means and standard deviations from the original Table 2. Values are task scores, not percentages of persons impaired and not THC concentrations. Higher values mean more correct sequences.
| Task | Men using cannabis (n=46) | Male controls (n=32) | Women using cannabis (n=31) | Female controls (n=45) |
|---|---|---|---|---|
| Digit Span Forward | 8.80 (2.05) | 9.16 (2.55) | 8.71 (2.33) | 8.07 (2.12) |
| Digit Span Backward | 5.80 (2.21) | 6.75 (2.52) | 5.65 (2.47) | 5.53 (2.44) |
| Spatial Span Forward | 6.76 (1.73) | 7.41 (1.95) | 7.52 (1.67) | 7.67 (1.81) |
| Spatial Span Backward | 5.60 (1.77) | 7.15 (1.76) | 6.32 (1.40) | 7.10 (1.95) |
It would also be incorrect to call the three null tasks proven unaffected. A nonsignificant result says the study did not detect a difference under the particular test, sample, and analysis. It does not establish equivalence at every effect size, in every population, or in unmeasured facets such as processing speed. These tests scored accuracy rather than the time needed to respond.
What did the reported group-by-sex interaction mean?
The primary Spatial Span Backward analysis yielded a group main effect, F(1,150) = 18.84, p < 0.001, partial eta-squared 0.112, and a group-by-sex interaction, F(1,150) = 9.74, p < 0.01, partial eta-squared 0.061. The reported main sex effect was F(1,150) = 4.13, p < 0.05, partial eta-squared 0.027. The authors applied a Bonferroni-adjusted threshold of 0.0125 across the four primary ANOVAs; they report that the group effect and group-by-sex interaction remained significant under that threshold.
Compared with male controls, the men in the cannabis group averaged 5.60 rather than 7.15, a 1.55-sequence descriptive gap. Compared with female controls, women in the cannabis group averaged 6.32 rather than 7.10, a 0.78-sequence descriptive gap. The published standardized between-group effects were Cohen’s d = −0.87 for men (95% CI −1.34 to −0.41) and d = −0.48 for women (95% CI −0.94 to −0.02). The male and female comparisons were both reported statistically significant, with the male comparison p < 0.001 and the female comparison p < 0.05.
This interaction addresses the difference between user and nonuser groups within each sex. It does not mean every man had a larger deficit than every woman. It does not establish what the same man or woman would have scored before smoking. And it does not show that being male biologically caused susceptibility: the comparison is observational, with exposure history and other potential differences tied to group membership.
What did adjustment for alcohol and nicotine change?
The investigators also modeled Spatial Span Backward with self-reported weekly alcohol and nicotine use frequency as covariates. Neither frequency was a statistically significant predictor in that reported sensitivity analysis: alcohol F(1,148) = 1.12, p = 0.291; nicotine F(1,148) = 0.85, p = 0.358. The group effect remained F(1,148) = 17.56, p < 0.001, and the group-by-sex interaction F(1,148) = 8.23, p = 0.005, partial eta-squared 0.053.
These adjusted findings make it harder to attribute the association solely to the two recorded frequency variables. They do not remove the principal design limitations. Frequency of alcohol and nicotine use is self-report, not a contemporaneous blood or breath toxicology test. The researchers did not directly measure THC dose, confirm control-group abstinence, assess pre-exposure cognitive scores, or comprehensively adjust for unmeasured baseline intelligence, education-related performance, sleep, motivation, psychiatric screening error, or other correlates of habitual use.
What did the study measure directly—and what did it not?
A forensic report benefits from a measurement inventory before any legal characterization. Here the direct study endpoints were the correct-sequence counts on standardized memory tasks. The product’s nominal THC percentage and the approximate cigarette mass characterized the administered material, while participant questionnaires characterized use history. Those are different evidentiary categories.
| Item | What the study actually supplied | What it does not establish |
|---|---|---|
| Spatial working-memory performance | Number of correct reversed spatial sequences, compared across four sex-by-use groups | Individual driving fitness, workplace failure, crash responsibility, or universal performance cutoff |
| Other memory performance | Three additional forward/backward span score distributions, no significant group effects | Equivalence of all mental abilities or absence of any THC-related behavioral effect |
| Cannabis product | Approximately 1 g smoked material; product tested at 16.1% THC and <1% CBD | Mass of THC absorbed by each participant or concentration at the brain |
| Testing time | Tasks started 45 minutes after reported smoking in the cannabis group | Individual peak blood concentration, time-to-recovery, or earlier-event concentration |
| Biological toxicology | No THC blood, plasma, saliva, or urine samples | A measured THC concentration, validated cutoff, analytic uncertainty, or drug-test interpretation |
| Sex-related association | Group-by-sex interaction and comparisons against separate same-sex controls | Person-specific biologic susceptibility, hormone mechanism, or causal sex-specific dose-response |
Why a statistically significant result is not an individual impairment test
A conventional statistical test asks whether an observed association is unlikely under a specified null model. It does not by itself classify an individual. The original researchers did not derive or externally validate a threshold separating impaired from unimpaired drivers. There is no sensitivity, specificity, false-positive rate, false-negative rate, positive predictive value, or individual prediction interval for a proposed forensic determination.
Two people with the same task score may differ in premorbid ability, education, sleep, task familiarity, underlying illness, medications, use history, anxiety, or effort. In a real case, the relevant question is not merely whether the group mean changed; it is whether the scientific method has been validated to connect a person’s observations to the claimed conclusion with known limitations. A sex-stratified Cohen’s d cannot legitimately be converted into a probability that an identified person drove impaired.
Why the lack of a pre-smoking baseline matters
The study tested frequent users after cannabis and different individuals who did not use cannabis. It did not obtain each user’s sober Spatial Span Backward score on a separate matched day, and it did not use a placebo-controlled crossover. The lower average score among users could include an acute drug component, longstanding differences associated with frequent use, pre-existing group differences, or combinations of these. The authors explicitly acknowledge that acute and chronic effects could not be disentangled.
This problem persists even when a drug-related hypothesis is biologically plausible and a statistical interaction survives adjustment for two self-reported covariates. A controlled within-person design can remove many stable between-person differences; this investigation did not provide that counterfactual. Its positive finding is an observational association under recent smoking, not a clean estimate of the effect of administering THC to the same individual.
Does a 16.1% cannabis product define a comparable dose for everyone?
Not in the pharmacokinetic sense. A product labeled or assayed at 16.1% THC expresses the proportion of THC in the plant material, while approximately one gram describes material selected for smoking. Systemic delivery depends on what actually burned or was lost, the smoke inhaled, smoking technique, absorption, distribution, metabolism, and timing. None of those individual exposure variables was quantified. One cigarette does not yield one uniform venous whole-blood THC result.
The paper’s phrase “same dose” therefore should be read in the narrow procedural sense of using approximately the same amount and product specification. It is not confirmation that men and women absorbed equivalent THC amounts. Nor can the absence of a blood assay be corrected by substituting the product percentage or applying a theoretical blood-THC conversion. Such a calculation would require assumptions unsupported by these individual data.
Do the negative memory tests show tolerance?
The study’s discussion considers whether experienced users might develop different degrees of tolerance across cognitive systems. That is a potential explanation, not a measured physiological outcome. There was no repeated individual drug challenge over time to quantify tolerance, no first-time-user comparison arm, and no receptor imaging. Alternatively, the three null tasks may not have been sensitive enough to detect smaller effects. Both possibilities are consistent with the reported observations.
Likewise, the paper proposes fronto-parietal and hormonal mechanisms to explain the spatial and sex-related findings. No participant underwent neuroimaging or had estradiol, CB1 receptor availability, or relevant enzyme activity assayed for the study’s principal comparison. These mechanisms belong to interpretation and hypotheses for future testing, not direct measured findings from this sample.
What is relevant to DUI and other litigation?
Visuospatial working memory has plausible relevance to navigation, maintaining spatial relationships, and mentally reversing or updating a sequence. Yet this study did not ask participants to drive on a road or simulator, avoid hazards, respond under time pressure, perform standardized field sobriety tests, or recreate a collision. An association in one board-based span task is not a calibrated estimate of lane deviation, reaction time, collision risk, intent, capacity to consent, or truthfulness.
A careful analysis of a driving or workplace matter would begin with the person’s actual acts and recorded times, any contemporaneous observations, the specific specimen and confirmed analytical method if a sample exists, reported dose or product evidence, co-exposures, clinical conditions, and alternative explanations. This paper can provide context for why one cognitive domain deserves attention. It cannot supply missing evidence from the event or support a case-specific expert conclusion solely from group membership, product label, or sex.
For a contrasting randomized THC potency and driving-simulator experiment, consult the separate same-site research brief; its controlled design, whole-blood assays, and driving endpoints differ from the current observational memory comparison. Neither study alone creates a universal THC impairment threshold.
How should the four-task pattern be reported fairly?
A scientifically balanced statement names both the positive result and the three negative results, then qualifies the population and design. One defensible summary is that, among 77 frequent cannabis users tested after reported smoking and 77 separate nonusing controls, backward spatial span was significantly lower for the users, with a larger user-versus-control difference in men, while three other span scores were not significantly different. It must also state that no toxicology concentration or driving performance was measured.
Overstatements run in both directions. “Cannabis did not affect memory” suppresses the positive spatial result. “Cannabis impairs every cognitive function, especially in every man” ignores the three null domains, distributions, observational confounding, and variability between individuals. “The same THC dose impaired men more” mistakes standardized material for measured absorbed exposure and a group interaction for a controlled pharmacokinetic comparison.
Frequently asked questions
Did the researchers measure THC in blood?
No. The original article states that biological drug verification was not performed. There are no individual or group venous whole-blood THC concentrations to compare with the memory scores, and no blood concentration cutoff can be inferred from this study.
Were participants randomized to THC or placebo?
No. Frequent users smoked their usual standardized-source product before testing; different people formed the nonusing comparison group. The data therefore combine recent reported smoking with differences associated with the histories of the two groups.
Did men always perform worse than women?
No. The statistically significant interaction describes larger male-user versus male-control differences than female-user versus female-control differences. It does not compare every individual man with every individual woman, or demonstrate a universally higher male susceptibility.
Which cognitive task showed the clearest difference?
Spatial Span Backward, which requires recalling a tapped sequence of locations in reverse order. The investigators did not report significant group effects on Digit Span Forward, Digit Span Backward, or Spatial Span Forward.
Does one gram of 16.1% THC cannabis mean everyone absorbed the same THC dose?
No. Those figures characterize the material and approximate smoking amount. The absorbed dose and blood concentration were not measured, and smoking behaviors were not tightly standardized.
Can the findings prove a defendant or driver was impaired at a specific time?
No. The paper provides no validated individual classification threshold, event-time blood measurement, driving test, or causal reconstruction. It is evidence about average test performance under a selected study design, not a direct finding about an identified person.
Do three nonsignificant span tests prove those skills were unaffected?
No. They indicate that the study did not detect statistically significant differences with those measures, sample sizes, and models. Smaller or different impairments, reaction-time changes, or effects in other populations were not excluded.
Source and further reading
Original primary research: Ryder CH, Gal C, Levy E, et al. Same dose, different impact: acute cannabis intoxication impairs visuospatial working memory in both sexes, with disproportionate male vulnerability. Frontiers in Behavioral Neuroscience. 2026;20:1785335. doi:10.3389/fnbeh.2026.1785335. The study’s Methods, original Tables 1–3, Results, and Discussion govern the numerical interpretation above.
For methods and other research questions, use the Attorney Scientists research library. For information on sourcing, AI-assisted editorial preparation, and review boundaries, consult the editorial and research source policy. This material is an educational appraisal of one original study, not an individual medical assessment, legal opinion, or certification that a particular driver was impaired.