Trends in the Top-Cited Articles on Classic Psychedelics

The researchers constructed two complementary databases. PubMed was used to estimate the annual number of publications on classic psychedelics per year, and Thomson Reuters Web of Science (Clarivate Analytics) was used to extract citation metrics for bibliometric analyses. A classic psychedelic was defined for the search as a 5-HT2A receptor agonist and the search terms included psilocybin, psilocin, LSD, DMT (including 5‑MeO‑DMT), mescaline and natural products such as peyote and ayahuasca. The amphetamine-derived DOI was excluded despite its 5‑HT2A activity because of its classification within the amphetamine derivatives. The Web of Science search returned 8,074 records; the PubMed search identified 8,889 publications. Search dates for citation metrics and total publication counts were current as of October 22, 2020 and January 25, 2020, respectively. From the Web of Science results the authors generated citation reports and rank-ordered articles by two metrics separately: total number of citations (TNC) and annual citation rate (ACR, an estimate of citations per year). Titles and abstracts were screened in descending order of TNC and ACR until 50 articles were identified for each metric (50 for TNC, 50 for ACR). Full-text review of these 50+50 articles was then completed; none were excluded at that stage. For each included article the investigators extracted lead author, title, publication type, journal, year, country of lead author, total citations and annual citation rate. Inclusion criteria required peer-reviewed, English-language articles with accessible full texts; exclusions included non-peer-reviewed items, books, non-English manuscripts, duplicates and inaccessible full texts. Two reviewers applied a grounded theory approach to code study design and outcomes for each publication. Each reviewer independently coded manuscripts into meaningful units which were then grouped into categorical themes; consensus was reached after category development. To examine temporal trends among the highly cited works, the unique articles identified across both cohorts (n = 77) were pooled and dichotomised at the median year of publication (2010) into a Recent Cohort (published 2010 or later) and an Older Cohort (published before 2010). Descriptive statistics described variable distributions across cohorts and comparisons used the Wilcoxon Rank Sum test with continuity correction for continuous measures and Fisher-type tests (Fisher‑Freeman‑Halton) for categorical variables. A significance threshold of α < 0.05 was applied and analyses were conducted in R v3.5.2. The authors state that research ethics board approval was not required because the study analysed existing publications.

Using the PubMed search strategy, 8,889 publications on classic psychedelics were identified. Annual publication rates peaked around 1965–1975, declined during a roughly 25‑year period that followed the UN Schedule 1 classification in 1967, and then rose from about 2010 onward, reaching an all‑time high in 2020. The Web of Science search produced a primary dataset of 8,074 items; from this the investigators screened 69 titles/abstracts to identify the top 50 by total citations and 61 titles/abstracts to identify the top 50 by annual citation rate. Following full‑text review, each cohort comprised 50 publications and no articles were excluded. There was overlap between the two ranked lists: 23 studies (23.0%) appeared in both the TNC and ACR cohorts, giving 77 unique articles for pooled analysis. The pooled set had a median year of publication of 2010 and was split into a Recent Cohort (n = 40; published 2010 or later) and an Older Cohort (n = 37; published before 2010). The extracted text does not clearly report the interquartile range for the Recent Cohort median year. The Recent Cohort had a significantly lower total number of citations (median 24.9, IQR 18.4 to 190.5) than the Older Cohort (median 209, IQR 166.0 to 377.0), p < .001. In contrast, the Recent Cohort had a significantly higher annual citation rate (median 76.5, IQR 43.8 to 103.3) than the Older Cohort (median 8.8, IQR 4.2 to 17.2), p < .001. Geographic origin differed between cohorts (p = .003): 78.4% (n = 29) of Older Cohort studies were led from the USA versus 37.5% (n = 15) of the Recent Cohort, while the Recent Cohort included more studies originating in the UK (32.5%, n = 13) and Brazil (12.5%, n = 5) compared with the Older Cohort (UK 2.7%, n = 1; Brazil 0%). Study design distributions also diverged (p < .001): the Recent Cohort contained more clinical studies (n = 27, 67.5%) and fewer preclinical studies (n = 3, 7.5%) compared with the Older Cohort (clinical n = 10, 27.0%; preclinical n = 22, 59.5%). The class of psychedelic investigated differed between the cohorts (p = .028). Psilocybin appeared most frequently in the Recent Cohort (n = 26, 40.6%) compared with the Older Cohort (n = 8, 17.4%), whereas LSD predominated in the Older Cohort (n = 26, 56.5%) versus the Recent Cohort (n = 19, 29.7%). Regarding outcomes, the Recent Cohort included more studies of therapeutic effects—affective disorders (n = 16, 25.8%) and substance use disorders (n = 6, 9.7%)—while the Older Cohort mainly comprised investigations of pharmacology and pharmacokinetics (n = 26, 70.3% in the Older Cohort vs. n = 11, 17.7% in the Recent Cohort).

Lawrence and colleagues interpret these findings as evidence of a shift in highly cited psychedelic research from foundational preclinical pharmacology toward clinical investigations of therapeutic effects. More recent influential articles tend to have higher annual citation rates, are more often clinical in design, focus more frequently on psilocybin, and originate from a broader set of countries than older influential work, which was concentrated in the USA and dominated by LSD and basic pharmacological studies. The observed temporal pattern of publications—an early peak in the 1960s–1970s, stagnation during restrictive regulatory periods, and renewed growth from 2010 onwards—is consistent with prior bibliometric work and with changes in regulatory and institutional access that have facilitated contemporary research activity. The authors note that the progression from preclinical pharmacology to clinical trials mirrors a conventional drug‑development pathway in which mechanisms, receptor targets and pharmacokinetics are characterised prior to clinical safety and efficacy studies. They also highlight differences in funding context compared with typical pharmaceutical development: many classic psychedelics are off‑patent or naturally occurring (for example, LSD and psilocybin), and much research to date has relied on non‑industry funding, including philanthropic support. Several limitations are acknowledged. The bibliometric approach may miss influential recent papers that had not yet accrued enough citations by the search date, and the ACR metric can be affected by the month of publication for newer articles. The study did not perform formal risk‑of‑bias or quality assessments of included articles. Potential biases include omission of relevant papers that used different terminology, unadjusted self‑citation rates, exclusion of non‑English manuscripts which may under‑represent some countries, and the decision not to apply alternative citation indices (for example the g‑index or h‑index) or altmetrics. The authors suggest that future bibliometric work could address these limitations and that continued funding and coordinated efforts will be required to support larger clinical trials to define the therapeutic utility of classic psychedelics. Overall, the investigators conclude that the top‑cited literature reflects a field that established its pharmacological foundations in earlier decades and is now increasingly focused on clinical applications and therapeutic outcomes, with recent publications showing greater citation velocity and broader geographic representation.