Dose-response study of N,N-dimethyltryptamine in humans: subjective effects and preliminary results of a new rating scale

Earlier human research on classical hallucinogens (for example LSD, psilocybin, mescaline) documented a consistent constellation of perceptual, emotional, cognitive and interoceptive effects, but studies used varied subject samples and assessment methods, limiting comparability. Neuropharmacologic work has focused on serotonergic mechanisms, especially 5-HT receptor subtypes, yet translating animal and biochemical data to human subjective effects remained problematic. N,N-dimethyltryptamine (DMT) was identified as a practical probe for revisiting human hallucinogen phenomenology because of its prototypical tryptamine structure, short duration of action, prior clinical safety data, and relative obscurity in recreational use. Strassman and colleagues set out to characterise systematically the subjective effects of graded intravenous DMT doses in experienced hallucinogen users and to present preliminary psychometric results from a new instrument, the Hallucinogen Rating Scale (HRS). The investigators aimed both to describe dose-dependent phenomenology clustered by clinical mental-status domains and to examine whether the HRS could quantify and discriminate DMT dose effects, thereby providing a bridge between human subjective reports and biological measures of drug action.

The study recruited experienced hallucinogen users by word-of-mouth and excluded individuals with current Axis I disorders (except one with Adjustment Disorder), ongoing medical illness, or long-term medication; one participant was withdrawn after developing recurrent major depression, leaving 11 analysed subjects (one woman, ten men). Subjects reported between six and “hundreds” of prior hallucinogen exposures; prior MDMA use was categorised as high (six subjects with five or more uses) or low-no exposure (five subjects with two or fewer uses) to permit exploratory assessment of prior serotonergic probe exposure. Initial nonblind inpatient administrations consisted of two intravenous (IV) DMT doses (0.04 and 0.4 mg/kg) infused over 30 seconds with a 15-second saline flush, during which the team did not question subjects so participants could observe onset, plateau and resolution. In the double-blind, placebo-controlled, randomised phase subjects received saline and four active IV doses (0.05, 0.1, 0.2, 0.4 mg/kg), with sessions separated by ≥1 week for men and ≥1 month for the single woman (studied in the early follicular phase). Physiological monitoring included a rectal temperature probe and blood sampling for 60 minutes post-injection; the HRS was administered approximately 30 minutes postinfusion after lines and probes were removed. The Hallucinogen Rating Scale was drafted from interviews with an independent sample of experienced DMT users and finalised as a 126-item instrument scored 0–4 (0 = not at all; 4 = extremely). Items were grouped both by a priori clinical mental-status clusters (Somaesthesia, Affect, Perception, Cognition, Volition, Intensity) and by empirical principal components factor analysis (VARIMAX rotation), with the number of factors set to six to match clinical clusters. Statistical analyses used PC-SAS Version 6, two-tailed P<.05. Screening for dose effects used one-way ANOVA on individual items; items with dose-related P<.05 were retained. Major inferential tests were one-way repeated measures ANOVAs on cluster and factor scores, and a two-way repeated measures ANOVA (dose repeated; MDMA history as class variable) tested for effects of prior MDMA exposure.

Eleven subjects completed the double-blind portion. The principal behavioural finding was a clear dose-response: 0.2 and 0.4 mg/kg IV DMT produced near-instantaneous visual hallucinatory phenomena, bodily dissociation and pronounced mood shifts that effectively replaced subjects’ ongoing mental experience; auditory effects occurred in about half the subjects. Effects were brief and their time course paralleled previously reported DMT blood levels. Lower doses (0.05 and 0.1 mg/kg) were not hallucinogenic and were dominated by somaesthetic and affective changes; the 0.1 mg/kg dose elicited the most aversive or tensely dysphoric physical sensations without the desired perceptual “breakthrough,” whereas 0.05 mg/kg produced mild relaxing and calming effects in some subjects. Qualitatively, the 0.4 mg/kg dose elicited an almost universally reported rapid “rush” during infusion that disrupted normal mental function within seconds, often producing loss of bodily awareness and transient dissociation. Visual imagery predominated and was described as vividly formed or kaleidoscopic, with greatly intensified colour saturation and merged foreground–background organisation; auditory changes were usually non-formed high-pitched or mechanical sounds or increased sensitivity to ambient noises. Somaesthetic responses included breath‑catching, chest constriction, alternating hot/cold sensations, and sometimes eroticised genital warmth; many subjects reported alternating or simultaneous fear and euphoria. Cognition and reality testing were powerfully affected at higher doses, with some subjects reporting the experience felt more “real” than dreams or waking perception. Volitional control was commonly lost at the highest dose, leaving subjects feeling regressed and helpless. Intensity often followed a wavelike pattern across the episode. On the HRS, 75 of 126 items showed a significant dose effect by one-way ANOVA and were used for subsequent grouping analyses. Five of six clinical clusters discriminated between placebo and 0.05 mg/kg; Intensity and Cognition discriminated among all doses. The statistical “break point” between hallucinogenic and non‑hallucinogenic doses (0.2 vs 0.1 mg/kg) was separable for Intensity, Affect, Perception and Cognition. Comparatively, principal components factors offered somewhat greater discrimination of the hallucinogenic threshold (six of six factors separated 0.2 from 0.1 mg/kg, versus four of six clinical clusters). A two-way repeated measures ANOVA found no effect of prior MDMA exposure on HRS responses across doses.

Strassman and colleagues interpret their findings as demonstrating that intravenous DMT produces a rapid, dose-dependent alteration of perception, somatic experience, affect and volition in experienced hallucinogen users, with 0.2–0.4 mg/kg acting as hallucinogenic doses and 0.05–0.1 mg/kg producing primarily somaesthetic or affective effects. The reported phenomenology aligns with prior intramuscular and “field” descriptions of DMT and emphasises the drug’s characteristic instant onset and very short duration, attributes the investigators link mechanistically to fast cortical entry and rapid monoamine oxidase metabolism. The authors note similarities and differences relative to LSD (mainly onset and duration) and discuss serotonergic receptor subtypes as plausible mediators while acknowledging that receptor-level causality remains incompletely resolved. Regarding measurement, the preliminary HRS results suggest the scale can quantify dose-dependent subjective effects and that clustering items clinically or via principal components provides useful discrimination across doses; in these pilot data the HRS-based groupings discriminated dose effects more clearly than the biological measures reported in the preceding article. The authors acknowledge several limitations: the small sample size, selection of highly experienced and motivated hallucinogen users (which limits generalisability), the HRS being developed from DMT user descriptions so far tested only with DMT, and the preliminary nature of psychometric analyses (for example, affect was measured as magnitude rather than parsed into qualitative subtypes such as euphoria vs dysphoria). They also emphasise the importance of ‘‘set’’ and ‘‘setting’’ in interpreting hallucinogen effects. Implications discussed by the investigators include using the dose‑response normative data and the HRS to test hypotheses from animal or prior human studies, to compare effects across classes of psychoactive drugs (including ketamine, PCP, cannabinoids, amphetamines and phenethylamine hallucinogens), and to explore receptor-specific manipulations or sensitivity in psychiatric populations. They stress the need for further validation of the HRS in independent samples, comparison with other hallucinogens, and studies including control subjects with no drug-abuse history to address questions such as the impact of prior MDMA exposure on serotonergic function.