Chronic administration of serotonergic antidepressants attenuates the subjective effects of LSD in humans

The investigators used a retrospective case-report approach, recruiting volunteer subjects via announcements on selected Internet newsgroups, a local alternative newspaper, and a psychedelic-interest newsletter; additional participants came by word of mouth or referral. Interested individuals completed a standardised structured interview administered orally or in writing. Confidentiality was assured. The interview solicited four main domains of information: (1) details of antidepressant treatment (drug name, daily dose, duration at time of hallucinogen use, and indication); (2) concomitant regular drug use (prescription or recreational); (3) phenomenology of the hallucinogenic episode taken while on antidepressant treatment, rated as subjectively unchanged, increased or decreased relative to a control condition (either the subject's own prior experience with a similar dose off antidepressant or a friend's contemporaneous experience with the same dose and batch); and (4) any change in LSD response in the weeks following antidepressant discontinuation, when information was available. To be included, a subject needed a usable control comparison: either a prior personal experience with a similar LSD dose while not taking an antidepressant or a friend's same-dose contemporaneous experience. The authors state that a complete copy of the structured interview is available on request. The study is descriptive and qualitative in nature; no formal prospective randomisation or blinding was used and no statistical modelling is reported beyond descriptive counts and percentages.

Thirty-three individuals completed the structured interview (eight women, twenty-four men, one anonymous), aged 15 to 37. Most were prescribed antidepressants for depression; a few had other indications (anxiety, PTSD, attention deficit disorder, or "stress"). Nearly all subjects had taken an antidepressant for at least 3 weeks prior to ingesting a hallucinogen, with one exception who had been on fluoxetine for 1 week. Twenty-six subjects had moderate to extensive prior LSD experience, enabling discriminative comparisons; thirty subjects had friends not on antidepressants who took the same LSD dose from the same batch and thus provided contemporaneous reference experiences. Fifteen subjects had taken LSD from the same batch both before and during antidepressant treatment. The principal finding reported is that chronic administration of serotonergic antidepressants was commonly associated with a subjective reduction in the effects of LSD. Specifically, 28 of 32 subjects (88%) who had taken an SRI for 3 or more weeks described a moderate to marked decrease or virtual elimination of their usual responses to LSD. Fluoxetine was the most common antidepressant in the sample: 15 of 17 subjects taking fluoxetine for over 3 weeks (88%) reported moderate to marked reductions in somatic, hallucinatory and/or psychological effects of moderate to high LSD doses. Sertraline users also showed diminished effects in eight of nine individuals taking 100–200 mg/day; one person on 50 mg/day reported no change. Three subjects on paroxetine reported reduced effects, and a single subject taking trazodone reported a decreased response. Representative case vignettes illustrate typical features: delayed onset of effects, markedly reduced intensity (one subject likened 250 µg while on fluoxetine to his prior experience of 75 µg), and shortened duration (one case reported ~5 hours on fluoxetine versus a normal ~10 hours). In several instances, contemporaneous friends who took the same LSD batch reported normal hallucinatory responses. A few subjects were able to regain full or partial response after antidepressant discontinuation; two reported full return of LSD effects approximately one month after stopping fluoxetine. Tolerance to LSD was deemed an unlikely explanation by the authors because subjects did not use LSD more often than once a week and tolerance typically dissipates within days; concomitant medications (e.g. asthma treatments, oral contraceptives) were not thought to explain the pattern. Exceptionally, one subject who had taken fluoxetine for only 1 week reported an increased response to LSD; the authors note early-onset stimulant-like sensations sometimes reported after initiating SRIs that might resemble early hallucinogen effects. In a minority of cases very high LSD doses (anecdotally ≈400 µg) could overcome the reduced response.

Bonson interprets the pattern of findings as consistent with serotonergic mechanisms by which chronic SRI exposure attenuates LSD's subjective effects. The authors discuss two principal receptor-level hypotheses: chronic SRI treatment may reduce responsiveness at postsynaptic 5-HT2A/5-HT2C receptors (sites implicated in hallucinogenic phenomenology), or it may desensitise presynaptic 5-HT1A autoreceptors, leading to sustained increases in extracellular serotonin that change how LSD interacts with its targets. In this model, raised extracellular serotonin after long-term SRI administration could render LSD's pharmacology more antagonistic or functionally less effective at producing the classic hallucinogenic response. The paper reviews mixed preclinical and binding data: some studies show chronic SRIs reduce 5-HT2 binding in certain brain regions, others show no change or increases; similarly, chronic SRI effects on 5-HT1A binding are inconsistent across studies, though adaptations have been reported in downstream second-messenger systems. Behavioural rodent studies are cited in support of reduced 5-HT2-mediated responses after chronic SRI treatment (for example, diminished head-twitch responses). The authors also note that LSD has activity at multiple serotonergic subtypes beyond 5-HT2 and 5-HT1A (including recently identified 5-HT6 and 5-HT7 sites) and binds to dopamine and adrenergic receptors; consequently, changes in catecholamine systems could also contribute to altered LSD responses. The discussion acknowledges that receptor-binding data are heterogeneous and that further animal experiments are being pursued to test mechanisms. Key limitations are acknowledged: the retrospective, self-selected sample raises potential selection bias; reliance on memory introduces recall bias, although many participants reported confidence in their accounts because they were surprised by the lack of expected effects; and the descriptive design precludes causal inference. The authors also note variability in individual responses and dosing, and that their questionnaires capture subjective reports rather than objective physiological measures. Finally, they contrast the present results with their earlier work showing potentiation of LSD effects with tricyclic antidepressants or lithium and attenuation with MAOIs, emphasising that different antidepressant classes appear to have divergent effects on LSD responsiveness. The paper concludes by recommending further behavioural and biochemical studies in animal models to clarify the mechanisms underlying SRI–LSD interactions.

Chronic, medically prescribed serotonergic antidepressants substantially attenuated the subjective effects of self-administered LSD in this retrospective sample, as measured by a standardised questionnaire. The authors attribute this attenuation to adaptations in serotonergic systems—potentially involving 5-HT1A and 5-HT2A/2C receptors, altered extracellular serotonin concentrations—or contributions from catecholamine systems. These observations contrast with previously reported potentiation of LSD effects during tricyclic antidepressant or lithium treatment and with attenuation seen during MAOI treatment; the authors report that follow-up animal studies are under way to investigate mechanisms.