The study, carried out by Fatma Akyıldız, used the EpiXS software package together with the EPICS2017 photon cross-section library, which is based on ENDF/B-VIII.0, to calculate a comprehensive set of photon interaction parameters for each compound over the energy range of 0.015 to 15 MeV. The quantities evaluated included the mass attenuation coefficient, the linear attenuation coefficient, the half-value layer, the effective atomic number, the effective electron density, the equivalent atomic number, the total kerma coefficient, and both the exposure buildup factor and the energy absorption buildup factor. All fifteen substances were treated under identical computational conditions, allowing the effects of elemental composition and density to be separated cleanly.

The fifteen compounds were deliberately chosen to span different pharmacological classes and different elemental makeups. Amphetamine and nicotine are liquids composed almost entirely of carbon, hydrogen, and nitrogen. Caffeine, meprobamate, and the barbiturates pentobarbital and phenobarbital contain substantial oxygen fractions. Diazepam and chlordiazepoxide stand apart because they include chlorine, an element with a considerably higher atomic number than any other constituent in the dataset. Densities ranged from 0.936 g cm⁻³ for amphetamine to 1.56 g cm⁻³ for heroin, and the author notes that where directly traceable experimental densities were unavailable, database-derived adopted values were used rather than new measurements.

The most striking compositional effect appears at low photon energy. At 15 keV, diazepam shows the highest mass attenuation coefficient of the entire set, 3.0084 cm² g⁻¹, followed closely by chlordiazepoxide at 2.9168 cm² g⁻¹. These values are far above those of the chlorine-free compounds because even a modest chlorine fraction, roughly 12 percent by weight in diazepam and 11.8 percent in chlordiazepoxide, strongly enhances photoelectric absorption, which scales steeply with atomic number. Among the chlorine-free substances, those with higher oxygen or nitrogen content, such as meprobamate at 1.1282 cm² g⁻¹ and caffeine at 1.0782 cm² g⁻¹, outperform the carbon-rich compounds. Phencyclidine, or PCP, gives the lowest value at 0.7875 cm² g⁻¹, with amphetamine, methadone, and nicotine close behind.

This separation collapses rapidly as photon energy rises. In the Compton-dominated region around 1.5 MeV, the mass attenuation coefficients of all fifteen compounds fall within a narrow band from 0.05391 to 0.05706 cm² g⁻¹, and at 15 MeV they span only 0.01766 to 0.01854 cm² g⁻¹. The difference between the highest and lowest values shrinks from nearly a factor of four at 15 keV to just a few percent in the megaelectronvolt range, because Compton scattering depends mainly on electron density rather than atomic number. Once the mass coefficients converge, bulk density becomes the dominant discriminator of attenuation per unit thickness. Heroin, with the highest density in the series, then delivers the highest linear attenuation coefficient and the smallest half-value layer, 24.82 cm at 15 MeV, while low-density amphetamine shows the weakest performance with a half-value layer of 41.41 cm.

The effective atomic number tells the same story in a different language. At 15 keV, diazepam and chlordiazepoxide reach values of 11.180 and 11.001 respectively, while PCP and amphetamine sit at the bottom with 4.860 and 4.989. Around 1 MeV, all curves converge to a broad minimum as Compton scattering takes over, with values ranging from about 3.117 for PCP to 4.486 for diazepam. Above the pair-production threshold of 1.022 MeV, the effective atomic numbers climb again, reaching 3.476 to 5.089 at 15 MeV, because pair production carries a stronger dependence on atomic number than Compton scattering. The effective electron density follows a related but not identical pattern, since it counts electrons per unit mass and therefore weights both elemental fractions and atomic masses.

The total kerma coefficient, which quantifies the kinetic energy transferred from photons to secondary charged particles per unit mass, behaves differently from the attenuation coefficients altogether. Rather than declining monotonically with energy, KermaTot falls from its 15 keV maximum to a minimum near 50 to 80 keV and then rises steadily toward 15 MeV, where values reach approximately 3.28 to 3.48 × 10⁻¹¹ Gy·cm² per photon. At 15 keV, diazepam and chlordiazepoxide again lead with 6.4387 × 10⁻¹² and 6.2254 × 10⁻¹² Gy·cm² per photon, while PCP records the lowest value at 1.2716 × 10⁻¹² Gy·cm² per photon. This U-shaped profile reflects the increasing energy carried away by secondary charged particles at high photon energies even as the overall interaction probability drops.

The buildup factors reveal the most dramatic behavior in the entire study. Unlike the Beer–Lambert law, which counts only uncollided primary photons, buildup factors account for scattered photons that reach a point of interest after multiple interactions within the material. Using the geometric progression fitting method with equivalent atomic numbers interpolated from reference elements, the study calculated both exposure and energy absorption buildup factors at penetration depths from 1 to 40 mean free paths. At shallow depths the differences are modest, but they explode with penetration. At 80 keV and 40 mean free paths, PCP produces an energy absorption buildup factor of 1.182 × 10⁵ and an exposure buildup factor of 2.654 × 10⁵, meaning scattered photons multiply the dose by more than five orders of magnitude. Amphetamine follows at 1.069 × 10⁵, while chlordiazepoxide and diazepam remain more than an order of magnitude lower, at 3.752 × 10³ and 3.341 × 10³ respectively.

The physical explanation lies in the competition between absorption and scattering. PCP and amphetamine, being carbon-hydrogen-nitrogen-rich with low equivalent atomic numbers, absorb few low-energy photons photoelectrically, so photons survive to scatter repeatedly and accumulate. The chlorine-containing benzodiazepines, by contrast, remove a larger fraction of photons through photoelectric absorption before they can multiply. By 15 MeV and 40 mean free paths, the buildup factors converge to narrow ranges of roughly 7.8 to 8.2 for both quantities, and the ordering even reverses slightly, with diazepam highest and PCP lowest, showing that PCP’s dominance is confined to the low and intermediate energy region where Compton multiple scattering outpaces photoelectric removal.

The author emphasizes that these compounds are not proposed as practical radiation shielding materials, and that the findings are limited to theoretical calculations for idealized, homogeneous bulk forms defined by the adopted compositions and densities. Experimental verification, along with assessment of physical, chemical, and mechanical properties, would be needed before any shielding application could be considered. Nevertheless, the dataset offers a quantitative reference for computational photon transport, dosimetric evaluations, and radiation protection analyses involving pharmaceutical and organic media, and it demonstrates that no single parameter can capture the full photon interaction response of a material. Low-energy attenuation is governed by composition, thickness-dependent attenuation by density, and buildup by the interplay of absorption and multiple scattering across both energy and depth.

Subject of Research: Photon interaction parameters and radiation shielding characteristics of psychoactive substances

Article Title: Comparative investigation of photon interaction parameters in psychoactive substances: the role of molecular composition

Article References: Comparative investigation of photon interaction parameters in psychoactive substances: the role of molecular composition. Original publication (Bibliographic details unavailable.)

Image Credits: AI Generated

DOI: Not provided

Keywords: radiation physics, photon attenuation, psychoactive substances, mass attenuation coefficient, buildup factors, effective atomic number, EpiXS, EPICS2017, diazepam, PCP, radiation shielding, dosimetry

Theory of Mind, often abbreviated as ToM, is the cognitive machinery that allows humans to attribute mental states to others and to predict behavior on that basis. First described in the landmark 1978 question of whether chimpanzees possess a theory of mind, the capacity depends on a distributed network of cortical regions, particularly the prefrontal, insular and cingulate areas. Those same regions are among the most consistently altered in people with substance use disorder, whether through reduced cortical volume, disrupted connectivity or changes in neurochemistry. This anatomical overlap is one reason researchers have long suspected that psychoactive substances could compromise the ability to understand other people.

Previous syntheses had already documented ToM problems among people with alcohol use disorder, with effect sizes ranging from moderate to very large. Meta-analyses of facial emotion recognition, a related social-cognitive skill, also pointed to deficits among alcohol and substance users. But evidence on substances other than alcohol remained scattered. The only earlier systematic review covering the broader landscape, published in 2016, found relatively weak evidence for ToM impairments linked to methamphetamine and no impairments among recreational cocaine users. The new study, led by José Vitor de Lima Neto and Flavia de Lima Osório of the University of São Paulo, set out to resolve that uncertainty with a formally registered, PRISMA-guided search of eight databases including Embase, Scopus, PsycINFO, Web of Science, PubMed, PsycNet, LILACS and SciELO.

The review included original quantitative studies comparing Theory of Mind performance between adult psychoactive substance users and controls. After screening, 37 studies met the criteria, and 30 of them—covering 35 distinct samples—provided data suitable for meta-analysis. Most of the research originated in Europe, with 70 percent of studies conducted there, and the field’s first study dates only to 2006, making this a young but rapidly growing literature. Alcohol was the predominant substance studied, accounting for roughly half of the samples, followed by stimulants, opioids, cannabis and a single study involving the hallucinogen MDMA. Participants were mostly men, with a mean age of about 37 years, and were largely recruited from inpatient detoxification clinics.

The pooled result was striking. Across the meta-analyzed samples, substance users showed significantly poorer Theory of Mind performance than controls, with a standardized mean difference of −0.80, a large effect by conventional statistical standards. The 95 percent confidence interval ran from −1.13 to −0.47, well clear of zero. Heterogeneity was high, at roughly 93 percent, reflecting the diversity of instruments, populations and study designs, but sensitivity analyses removing statistical outliers left the overall estimate essentially unchanged. Publication bias tests, including Begg’s rank correlation test and funnel plot inspection, found no evidence that negative findings were being suppressed.

Subgroup analyses revealed something perhaps even more consequential: the impairment did not appear to depend on which substance was used. Samples of alcohol users produced an effect size of −0.92, opioid users −1.12 and stimulant users −1.29. A formal between-group test found no statistically significant differences among these classes, with overlapping confidence intervals pointing to a common magnitude of deficit. The authors also tested whether the choice of assessment tool mattered, given ongoing debate about the Reading the Mind in the Eyes Test, the most frequently used instrument in this literature. A mixed-effects meta-regression comparing studies that used the Eyes Test with those using other measures found no significant moderating effect, suggesting the deficits are not an artifact of one particular task.

Two theoretical explanations dominate the interpretation of these findings, and they are not mutually exclusive. The first is neurotoxicity: chronic exposure to alcohol, opioids or stimulants damages or dysregulates precisely the prefrontal, insular and cingulate circuits that Theory of Mind depends upon, and decades of neuroimaging work document structural and functional changes in these regions among people with substance use disorder. The second is vulnerability: weak social cognition may predate substance use, increasing susceptibility to social stress, isolation and exclusion, and leading individuals to use substances as a way of coping with relational difficulties or of conforming to perceived social norms. Supporting the vulnerability hypothesis, a 2022 meta-analysis found an inverse relationship between alcohol-related problems and ToM performance even in non-clinical samples, where use never reached diagnostic thresholds.

The new review itself is cross-sectional in its evidence base, with 36 of the 37 included studies using cross-sectional designs, so causality cannot be established. The authors are explicit that their pooled estimate should be read as evidence of association rather than cause. They also note that the two mechanisms likely interact in a self-reinforcing loop: a primary ToM weakness predisposes someone to use substances, chronic use then intensifies the deficit through neurotoxic effects, and the deepened impairment in turn hinders therapeutic adherence and relapse control. Interestingly, one study in the review found that long-term MDMA users actually performed better on Theory of Mind tasks than non-users, a finding consistent with evidence that the pursuit of empathetic social experience motivates MDMA use—though it rests on a single study.

Important gaps remain. Evidence on cannabis, hallucinogens and nicotine is too sparse for meta-analysis; two cannabis studies found no significant differences, though the tasks used assessed affective rather than cognitive ToM, a subdomain that may be selectively vulnerable given cannabis-linked frontal reductions. Methodological quality across the field was only moderate, most samples came from high-income countries, inpatient recruitment may have inflated severity, and polysubstance use and psychiatric comorbidities were rarely controlled. Nevertheless, the authors argue the practical message is clear: sociocognitive skills deserve formal assessment in both prevention programs and addiction treatment, because deficits in reading minds can raise vulnerability to initiating use, undermine therapy and complicate recovery long after detoxification ends.

Subject of Research: Theory of Mind deficits among psychoactive substance users

Article Title: Theory of Mind in Psychoactive Substance Users: A Systematic Literature Review and Meta-analysis

Article References: Neto, J. V. D. L., & Osório, F. D. L. (2026). Theory of Mind in Psychoactive Substance Users: A Systematic Literature Review and Meta-analysis. International Journal of Mental Health and Addiction. https://doi.org/10.1007/s11469-026-01717-0

Image Credits: AI Generated

DOI: 10.1007/s11469-026-01717-0

Keywords: Theory of Mind, psychoactive substances, substance use disorder, social cognition, meta-analysis, alcohol, opioids, stimulants, addiction, prefrontal cortex, mental state attribution, systematic review