Human behavioral pharmacology of psychedelics

Psychedelics are a group of psychoactive compounds that primarily act as agonists or partial agonists at the serotonin 2A receptor (5-HT2AR). Interest in their clinical potential has resurged after a long hiatus following mid-20th century research; contemporary work combines modern neuroscience and rigorous clinical methods and is focused on therapeutic applications in conditions that have been difficult to treat. Strickland and colleagues situate their chapter within this renewed effort and emphasise both the historical context and the methodological advances that distinguish modern human research from the earlier era. This paper sets out to provide an overview of the human behavioural pharmacology of three prototypic classic psychedelics—psilocybin, LSD, and DMT. The authors outline nomenclature and classification, highlight special methodological considerations for human studies (for example, set and setting, measurement of mystical experiences, blinding, and abuse liability), and then review the subjective, physiological, and emerging clinical findings for each compound. The chapter concludes by identifying outstanding questions and priorities for future research, with an emphasis on behavioural pharmacology approaches that explore interactions between pharmacology and environment.

The extracted text indicates that this work is a narrative chapter-style overview rather than a systematic review or meta-analysis with explicit search methods. The authors describe conceptual frameworks, methodological challenges, and contemporary human experimental and clinical studies that illustrate those points, but the extraction does not report a prespecified literature search strategy, inclusion/exclusion criteria, or databases searched. Coverage is organised thematically. The authors first address classification and nomenclature of classic psychedelics, then review four "special considerations" relevant to human studies (set and setting; mystical experience measurement; blinding and placebos; and abuse liability). Empirical literature is then summarised for each of the three focal compounds (psilocybin, LSD, and DMT), with sub-sections on subjective and physiological effects and separate sections on clinical research. When available, the chapter draws on double-blind, placebo-controlled trials, dose–response studies, pooled analyses of laboratory experiments, open-label clinical trials, and randomized clinical trials in patient populations. The text reports experimental designs and sample sizes for several cited studies, but the extraction does not present a uniform methods appendix or formal risk-of-bias assessment for included trials.

Classification and general features: The authors clarify terminology and group classic psychedelics into two structural families, indoleamines (including psilocybin, DMT, and LSD) and phenylalkylamines (including mescaline and some synthetic compounds). All are grouped primarily on their 5-HT2AR agonist activity. Some other compounds (for example ketamine or MDMA) are sometimes included in broader discussions of psychedelics but differ in mechanism and subjective profile. Special considerations: Set (psychological state) and setting (environment) are presented as central determinants of acute subjective experience and longer-term outcomes. The authors link these ideas to behavioural pharmacology concepts such as expectancy and cite balanced-placebo logic as relevant. Measurement of mystical-type experiences is commonly used in contemporary research; the Mystical Experience Questionnaire (MEQ-30) is described as a validated instrument capturing unity/noetic quality, positive mood, transcendence of time/space, and ineffability. Blinding remains a persistent challenge: trials have used inert placebos, low-dose placebos, or active comparators, but evidence from older studies suggests blinds can break (for example, therapists identified LSD versus placebo at high rates), and the extraction highlights ongoing debate about suitable placebo controls or alternative designs. On abuse liability, the chapter summarises evidence for generally low dependence potential across classic psychedelics, noting lack of typical withdrawal and low rates of laboratory-reported increases in recreational use after controlled administration (a pooled analysis found 90% of participants reported no change in psilocybin use after laboratory sessions; 5.6% used less, 3.3% used more). Psilocybin: Subjective and physiological effects are consistently dose-dependent and mediated largely by 5-HT2AR activity; ketanserin pretreatment attenuated or prevented many subjective effects in several studies. Preclinical safety metrics cited include a rodent LD50 estimated at roughly 2,000–3,000 times the usual human dose on a mg/kg basis; human studies report low toxicity and mainly transient, mild-to-moderate adverse effects (nausea, dizziness, transient headache) and dose-related cardiovascular increases within clinically tolerable ranges. Acute peak effects after oral use typically occur within 2–3 hours and dissipate by about 6–7 hours according to the extracted text. Several controlled laboratory experiments and pooled analyses are reported: a pooled analysis of eight double-blind studies in healthy participants (N = 110; 1999–2008) showed dose-dependent, predominantly positive subjective effects. A pooled dataset of ten Johns Hopkins studies (N = 288) analysing weight-adjusted dosing (20–30 mg/70 kg) found no significant associations between subjective effects and body weight or sex across a broad weight range, suggesting body weight may not strongly influence subjective intensity at these dosing schemas. Comparative work showed psilocybin produced greater mystical-type and insight-related ratings than high-dose dextromethorphan (DXM) although some cognitive and dissociative effects overlapped. Psilocybin clinical studies: Studies in cancer-related depression and anxiety reported immediate and sustained improvements, with some trials reporting mediation of clinical benefit by mystical-experience ratings. In treatment-resistant depression, open-label and randomized designs reported symptom reductions following one or two doses; a double-dummy trial comparing two psilocybin sessions versus escitalopram found no significant difference on the prespecified primary outcome but reported superior outcomes for psilocybin on many secondary measures of depression, well-being, and quality of life. A wait-list controlled trial reported robust reductions in depression with 17 of 24 completers achieving a clinically significant response (50% reduction in GRID-HAMD) four weeks after the first psilocybin session. For substance use, an open-label alcohol use disorder study reported reductions in drinking days and heavy drinking days after psilocybin sessions (mean percentage heavy drinking days fell from about 26% in the four weeks before treatment to about 8% in the four weeks after the first session, and ~13% at final follow-up 21–32 weeks later). Psilocybin-assisted smoking cessation in a small open-label trial showed biologically verified abstinence in 80% of participants at six months and 60% at approximately 2.5 years, with qualitative reports of enduring changes in self-concept and motivation. LSD: Behavioural pharmacology studies, largely placebo-controlled crossover designs, found dose-dependent subjective and physiological effects similar to psilocybin and mediated by 5-HT2AR (ketanserin attenuated subjective and cardiovascular responses). Reported durations vary by dose; the extraction indicates peak effects at about 2–3 hours with total durations up to roughly 12 hours for higher doses, and a recent dose–response study (25–200 units as reported in the extraction) observed dose-dependent subjective increases and cardiovascular effects, with a possible ceiling for positive effects between higher dose levels and greater ego dissolution and anxiety at the highest dose. Microdosing laboratory studies with low doses of LSD found minimal effects on cognition, physiology, and most mood measures in acute administration, though participants reported small dose-related increases in subjective drug effects; observational studies suggesting well-being gains were noted to be confounded by baseline expectancies. LSD clinical studies: Historical randomized trials from the 1950s–1970s of LSD for alcohol use and other conditions showed trends favouring LSD; a modern meta-analysis of six older studies (n = 536) reported odds-ratio effects around 2 for alcohol misuse at 1 month. Contemporary clinical work is limited but includes studies in cancer-related anxiety showing reductions in anxiety and some longer-term benefits in small samples. DMT and ayahuasca: DMT produces rapid-onset, short-duration psychedelic effects when inhaled or administered intravenously, with inhaled/smoked effects peaking within a minute and dissipating within 15–30 minutes; oral DMT requires an MAOI (as in ayahuasca) and has a longer time course. Double-blind and single-blind studies reported dose-dependent subjective and cardiovascular effects and evidence that ketanserin attenuates some ayahuasca effects, supporting a role for 5-HT2AR. Clinical evidence is more limited but includes open-label and small randomized trials: two open-label trials in refractory depression reported substantial reductions in depressive symptoms within days of administration, and one randomized, double-blind placebo-controlled trial in treatment-resistant depression found significantly greater reductions in depression in the ayahuasca arm at 1, 2, and 7 days post-intervention, with the largest difference at day 7. Cross-cutting pharmacology: Across the three compounds, the chapter emphasises convergent evidence for primary mediation via 5-HT2AR, supported by ketanserin antagonism studies and PET imaging. Physiological safety profiles are presented as acceptable in controlled settings, with dose-related but generally manageable cardiovascular and transient adverse effects reported.

Strickland and colleagues interpret the reviewed literature as indicating that classic psychedelics produce a unique and complex constellation of subjective states that can have persistent effects on mood, cognition, and behaviour when administered in controlled contexts. They highlight consistent mechanistic evidence implicating the 5-HT2AR while also noting pharmacodynamic differences among compounds that may influence duration and qualitative features of experience. The authors frame mystical-type experiences as a commonly measured and often predictive subjective phenomenon in therapeutic trials, but they explicitly acknowledge ongoing debate about whether such experiences are necessary for clinical benefit or may be epiphenomena related to other biological mechanisms. Methodological challenges receive considerable attention. The authors underline the centrality of set and setting and the difficulty of maintaining effective blinds in clinical trials, noting that broken expectancy could bias outcomes. They also point to unresolved questions about the abuse liability of classic psychedelics despite evidence for low dependence potential. Limitations of the existing evidence base that the chapter identifies include reliance on small samples in many clinical trials, frequent open-label designs, variation in dosing and psychosocial support across studies, and the lack of systematic mechanistic experiments that disentangle subjective experience from neurobiological changes. In terms of implications, the authors argue that behavioural pharmacology is well positioned to address key questions at the intersection of environment and pharmacology—specifically, to clarify how set and setting, expectancy, psychotherapeutic support, and acute subjective phenomena interact with drug action to produce longer-term clinical effects. They recommend further experimental work to test assumptions (for example, manipulating environmental factors or using designs that separate subjective experience from drug exposure) and call for expanded, rigorous clinical trials and mechanistic studies, including investigation of alternative routes of administration for compounds such as DMT. The extracted text concludes with a call for continued, carefully controlled research to elucidate mechanisms of behaviour change and to refine therapeutic applications of psychedelics.