Possible Interactions Between 5-HT2A Receptors and the Endocannabinoid System in Humans: Preliminary Evidence of Interactive Effects of Ayahuasca and Endocannabinoids in a Healthy Human Subject

Ayahuasca is a traditional Amazonian psychoactive decoction that combines Banisteriopsis caapi, which contains reversible monoamine oxidase A (MAO-A) inhibitors (β-carbolines), with Psychotria viridis, which supplies N,N-dimethyltryptamine (DMT), a 5-HT1A/2A/2C agonist. Because the β-carbolines inhibit peripheral MAO-A, orally administered DMT can reach the brain; earlier clinical work has reported rapid antidepressant effects after a single ayahuasca dose in patients with treatment-resistant depression. Basic pharmacology indicates that activation of cortical 5-HT2A receptors can trigger formation and release of endocannabinoids (ECs), particularly 2-arachidonoylglycerol (2-AG), and both the 5-HT2A receptor and the endocannabinoid system (ECS) are coexpressed in brain regions implicated in emotional processing, suggesting a plausible interaction that could contribute to ayahuasca’s effects on mood and anxiety. Romanos and colleagues set out to probe whether acute 5-HT2A agonism via ayahuasca alters peripheral EC levels in humans and to relate any changes to subjective and physiological responses. The study used an open-label, single-subject design in a healthy volunteer to obtain preliminary evidence of interactive effects between ayahuasca and the ECS and to assess tolerability and subjective mood/anxiety changes over the acute time course of the drug’s action.

The investigation was an open-label single-subject experiment in a healthy 34-year-old man. After providing written consent, the volunteer fasted and abstained from alcohol, tobacco and caffeinated drinks for 12 hours prior to the session. Urine screening for illicit drugs (cannabis and cocaine) was negative. A peripheral intravenous cannula was placed for serial blood sampling. No psychological intervention or preparatory psychotherapy was given before, during or after the session; the participant remained seated in a reclining chair in a quiet dimly lit room and was observed for the duration of the experiment and until effects had subsided. Ayahuasca was administered orally at a dose of 1 mL/kg at approximately 08:00 and the experimental session lasted about five hours, with discharge around six hours post-dose. The ayahuasca sample was chemically characterised by gas chromatography with nitrogen–phosphorus detection and contained (mg/mL) 0.702 DMT, 1.748 harmine, 0.780 tetrahydroharmine and 0.039 harmaline. Endocannabinoid analysis in plasma (anandamide, AEA; and 2-AG) was performed using ultrahigh-performance liquid chromatography–tandem mass spectrometry (UHPLC-MS/MS). Detailed assay procedures are reported in the supplemental material (as noted in the extracted text). Subjective and physiological measures were collected at baseline and at prespecified post-dose times. Subjective measures included the Visual Analog Mood Scale (VAMS) and a Bodily Symptoms Scale, with VAMS, Bodily Symptoms, blood pressure and heart rate assessed at baseline and 40, 90, 120, 150 and 240 minutes after intake; the Beck Anxiety Inventory (BAI) was measured at baseline and at 240 minutes. Blood samples for AEA and 2-AG were taken at baseline, 90 minutes and 240 minutes after ingestion. Tolerability was monitored via self-report and vital signs, and the investigators conducted a follow-up telephone call 24 hours after the session.

Acute ayahuasca administration was associated with a clear reduction on the VAMS anxiety factor; other VAMS domains (sedation, cognitive impairment and discomfort) changed little. The BAI showed no appreciable change at the single post-dose measurement reported. Somatic effects were limited to nausea and gastrointestinal discomfort, which began within the first hour after ingestion and subsided by about three hours, returning to baseline by the end of the session. The volunteer reported feeling calm, relaxed and described the experience as pleasant. Measured plasma ECs showed differing time courses. Anandamide (AEA) declined from 0.730 ± 0.016 ng/mL at baseline to 0.647 ± 0.051 ng/mL at 90 minutes and further to 0.281 ± 0.008 ng/mL at 240 minutes. By contrast, 2-AG decreased slightly from 0.174 ± 0.023 ng/mL at baseline to 0.118 ± 0.015 ng/mL at 90 minutes, then rose above baseline to 0.249 ± 0.039 ng/mL at 240 minutes. Cardiovascular parameters remained within modest limits: blood pressure did not exceed 140/90 mm Hg and heart rate never rose above 100 beats per minute. No serious adverse events occurred; nausea and gastrointestinal discomfort were the only reported adverse effects.

Romanos and colleagues interpret these observations as the first human evidence that acute 5-HT2A receptor agonism via ayahuasca can modulate peripheral measures of the ECS. They highlight that 2-AG showed the expected pattern reported in preclinical studies—an initial small decrease followed by an increase—whereas AEA exhibited a continuous decrease over time, a direction opposite to some prior expectations. The reduction in subjective anxiety aligns with findings from earlier human studies, including a randomised controlled trial in healthy experienced users and open-label work in depressed patients, and the investigators note that DMT’s agonism at 5-HT2A receptors in emotion-related brain regions and possible modulation of the hypothalamic–pituitary–adrenal axis may contribute to anxiolytic or antidepressant effects. The authors acknowledge important limitations that temper causal inference: the open-label design, the inclusion of only a single subject and known interindividual variability in EC levels related to factors such as age, sex and ethnicity. They therefore caution that the unexpected AEA decline might reflect individual variability or an outlier value rather than a generalisable pharmacological effect. Given these uncertainties, the investigators state that larger, controlled studies are required to clarify the interaction between ayahuasca, 5-HT2A receptor activation and the ECS, and they report that two randomised controlled trials—one in healthy volunteers and another in socially anxious individuals—are underway to further explore these questions.