Five novel experimental placebo-controlled studies have been conducted in Basel, London, and Zurich in a total of 95 normal subjects (Table 2). All studies used a crossover design and were placebo-controlled. The Basel and Zurich studies were randomized and double-blind, whereas the London studies were non-randomized and single-blind. Low–moderate doses of LSD base of 40–80 μg intravenously (London) or 100 μg orally (Basel and Zurich) were used in studies including brain imaging and a relatively high dose of 200 μg LSD base was used in one study in Basel without brain imaging. A full LSD reaction is expected at doses of 100–200 μg (Pahnke et al, 1969; Passie et al, 2008). Similar and higher doses of LSD were used in patients in the 1950s–1970s (Krebs and Johansen, 2012; Pahnke et al, 1970).

Table 2 Modern Clinical Placebo-Controlled LSD Studies Full size table

Subjective Effects

Modern placebo-controlled studies using validated psychometric scales have only recently been conducted (Carhart-Harris et al, 2016b; Kraehenmann et al, 2017; Preller et al, 2017; Schmid et al, 2015). In a controlled setting, the subjective effects of LSD were predominantly positive (Dolder et al, 2016; Schmid et al, 2015). Mean group ratings of ‘good drug effect’ and ‘drug liking’ on visual analog scales (VASs) reached 90% of maximal possible VAS scores after administration of 200 μg LSD (Schmid et al, 2015). In contrast, LSD produced only small (<25%) mean group increases in ‘negative drug effect’ and ‘fear’ (Dolder et al, 2017; Schmid et al, 2015). However, transiently greater ratings of negative drug effects (>50%) are seen in approximately half of the subjects at a 200 μg dose of LSD (Dolder et al, 2017). Thus, within a session all subjects experience positive drug effects but some also negative drug effects. Profound anxiety or panic was not experienced, and pharmacological sedation was not needed (Dolder et al, 2016; Schmid et al, 2015). LSD increased ratings on all dimensions and subscales of the 5-dimension altered states of consciousness (5D-ASC) scale that has been used in all modern studies (Carhart-Harris et al, 2016b; Kraehenmann et al, 2017; Liechti et al, 2017; Preller et al, 2017; Schmid et al, 2015) (Figure 1). LSD mainly induced a blissful state, audiovisual synesthesia, changes in the meaning of perceptions, and positively experienced derealization and depersonalization (Carhart-Harris et al, 2016b; Liechti et al, 2017; Schmid et al, 2015). An oral dose of 200 μg LSD produced significantly greater bliss, changes in the meaning of perceptions, and insightfulness compared with 100 μg (Liechti et al, 2017). Intravenous LSD at a dose of 75 μg (Carhart-Harris et al, 2016b) produced similar ratings on the 5D-ASC as an oral dose of 100 μg (Liechti et al, 2017) but lower ratings compared with an oral dose of 200 μg (Schmid et al, 2015) (Figure 1). Pretreatment with the 5-HT 2A receptor antagonist ketanserin fully prevented the effects of 100 μg LSD on the 5D-ASC (Kraehenmann et al, 2017; Preller et al, 2017), indicating that the mind-altering effects of LSD in humans are primarily mediated by 5-HT 2A receptors. LSD elicited spontaneous synesthesia-like experiences (Carhart-Harris et al, 2016b; Liechti et al, 2017; Preller et al, 2017; Schmid et al, 2015), but it did not induce more vivid color experiences in response to grapheme or sound stimuli (Terhune et al, 2016). These findings indicate that LSD alters spontaneous processes rather than induced responses (Terhune et al, 2016). LSD at 40–80 μg, i.v., increased suggestibility (vividness of imagination) but not cued imagery (Carhart-Harris et al, 2015). LSD at 200 μg, p.o. acutely induced mystical experiences in healthy subjects and patients during LSD-assisted psychotherapy (Liechti et al, 2017). Studies of psilocybin showed that greater acute mystical experiences were significantly associated with positive long-term effects on mood and personality in healthy subjects (Griffiths et al, 2011) and better therapeutic outcomes in patients with anxiety, depression, and substance use disorder (Garcia-Romeu et al, 2015; Griffiths et al, 2011, 2016; Ross et al, 2016). Thus, acute substance-induced mystical-type effects during therapeutic sessions appear to predict the long-term effects of hallucinogens. However, LSD-induced mystical-type effects were highly correlated with other alterations of consciousness and particularly the blissful state on the 5D-ASC (Liechti et al, 2017), indicating that greater positive acute responses to hallucinogens and not specifically mystical-type effects may generally be associated with any better long-term effects on mood. Furthermore, LSD increased feelings of well-being, happiness, closeness to others, openness, and trust (Dolder et al, 2016; Schmid et al, 2015). Such empathogenic effects on mood are typically produced by 3,4-methylenedioxymethamphetamine (MDMA; ecstasy) (Hysek et al, 2014a) and may facilitate psychotherapy. A 200 μg dose of LSD produced greater feelings of closeness to others, happiness, openness, and trust than a 100 μg dose (Dolder et al, 2016). Consistently, an LSD dose of 200 μg is currently used in LSD-assisted psychotherapy in Switzerland (Gasser et al, 2014, 2015).

Figure 1 Effects of LSD on the 5D-ASC scale. The data are derived from three studies using doses of 75 μg i.v. (Carhart-Harris et al, 2016b), 100 μg p.o. (Liechti et al, 2017), and 200 μg p.o. (Schmid et al, 2015) LSD in healthy subjects. LSD predominantly increased ratings in subscales of the dimensions oceanic boundlessness and visionary restructuralization. LSD-induced increases in subscales of the anxious ego-dissolution dimension and in particular in the anxiety scale were relatively small. LSD-induced changes on the 5D-ASC scale were significant compared with placebo for all LSD doses and all of the scales, with the exception of the effects of the 200 μg dose on anxiety. There were no statistical differences in the effects of the intravenous 75 μg (Carhart-Harris et al, 2016b) and oral 100 μg (Liechti et al, 2017) dose of LSD on 5D-ASC scale ratings (data provided by the authors). At 200 μg, LSD produced significant and relevantly higher ratings of blissful state, insightfulness, and changed meaning of percepts compared with 100 μg. The data are expressed as the mean in 24, 16, and 20 subjects for the 100 μg, 200 μg, and 75 μg doses of LSD, respectively. PowerPoint slide Full size image

No differences in subjective VAS-rated responses to LSD were found between subjects with no prior hallucinogen use and subjects with moderate experience (1–3 prior uses) (Schmid et al, 2015). The effects of LSD on the 5D-ASC were also similar between subjects with no prior hallucinogen use (n=21) (Dolder et al, 2016) and subjects who had used LSD 14±18 times (mean±SD) (Carhart-Harris et al, 2016b). No correlations were found between past LSD use and the acute effects of LSD on functional magnetic resonance imaging (fMRI) study outcomes across subjects with prior LSD use (Speth et al, 2016; Tagliazucchi et al, 2016).

Music has typically been used in substance-assisted psychotherapy (Gasser et al, 2014, 2015; Johnson et al, 2008). Several modern studies assessed the interactive effects of LSD and listening to music. LSD enhanced the emotional response to music and produced greater feelings of wonder and transcendence compared with listening to music after placebo (Kaelen et al, 2015). LSD increased eyes-closed imagery or seeing scenes from the past, but listening to music did not interact with these subjective effects of LSD on imagery (Kaelen et al, 2016). Other researchers found that LSD significantly increased ratings of music excerpts that were previously rated as personally meaningless or neutral (Preller et al, 2017). Thus, LSD attributed meaning to previously meaningless stimuli (Preller et al, 2017).

Autonomic and Adverse Effects

LSD moderately increased blood pressure, heart rate, body temperature, and pupil size (Dolder et al, 2016; Kaelen et al, 2015; Schmid et al, 2015). The sympathomimetic effects of 100 and 200 μg doses of LSD were similar (Dolder et al, 2016, 2017) and less pronounced than those of MDMA and stimulants (Hysek et al, 2014b). Acute adverse effects up to 10–24 h after LSD administration included difficulty concentrating, headache, dizziness, lack of appetite, dry mouth, nausea, imbalance, and feeling exhausted. Headaches and exhaustion may last up to 72 h (Dolder et al, 2016; Schmid et al, 2015). No severe adverse reactions were reported in modern LSD studies (Carhart-Harris et al, 2016b; Dolder et al, 2016; Kaelen et al, 2015; Preller et al, 2017; Schmid et al, 2015). This is consistent with the view that LSD is relatively safe when used in medical settings and according to safety guidelines (Johnson et al, 2008). LSD is physically non-toxic, but there are psychological risks especially when it is used in unsupervised settings. In addition, it is important to note that many novel hallucinogens are being used and may even be sold as LSD but have a different pharmacology and possibly risk profile than LSD (Rickli et al, 2015, 2016). LSD has typically been reported to produce flashbacks. Flashbacks after LSD can be defined as episodic and short (seconds or minutes) replications of elements of previous substance-related experiences (Holland and Passie, 2011). In a web-based survey among hallucinogen users, greater past LSD use was a predictor of the probability of experiencing unusual substance-free visual experiences (Baggott et al, 2011). Clinically significant flashbacks are also defined as hallucinogen persisting perception disorder (HPPD). This disorder is considered rare and occurs almost exclusively in the context of illicit recreational use or/and in patients with anxiety disorders and it typically will have a limited course of months to a year (Halpern and Pope, 1999; Holland and Passie, 2011; Johnson et al, 2008). In controlled non-therapeutic research settings, psilocybin did not produce HPPD or flashbacks (Studerus et al, 2011). However, the prevalence and relevance of HPPD is unclear and needs to be studied (Halpern et al, 2016).

Endocrine Effects

LSD acutely increased plasma concentrations of cortisol (Strajhar et al, 2016), prolactin, oxytocin, and epinephrine (Schmid et al, 2015). LSD does not increase plasma concentrations of norepinephrine (Schmid et al, 2015), testosterone, or progesterone (Strajhar et al, 2016). The endocrine effects of LSD are consistent with those of other serotonergic substances including psilocybin, DMT, and MDMA (Hasler et al, 2004; Hysek et al, 2014b; Seibert et al, 2014; Strassman and Qualls, 1994a).

Model Psychosis

LSD (75 μg, i.v.) increased subjective ratings of cognitive disorganization and delusional thinking (Carhart-Harris et al, 2016b). Disordered cognition has been suggested to be a more fundamental characteristic of LSD’s effects than positive or negative mood (Carhart-Harris et al, 2016b). Nevertheless, the LSD experience was not dominated by unpleasant psychosis-like phenomena but rather characterized by an overall positive mood state in the majority of subjects (Carhart-Harris et al, 2016b). Investigators rated subjects as more distant from reality and happy after administration of 200 μg LSD, whereas ratings of anxiety and paranoid thinking did not increase (Schmid et al, 2015). Patients with schizophrenia present deficits in sensorimotor gating, reflected by prepulse inhibition (PPI) of the startle response. LSD acutely disrupts PPI in both animals (Halberstadt and Geyer, 2010) and healthy human subjects (Schmid et al, 2015), producing deficits in information processing that are similar to those observed in schizophrenia. Similarly, inhibitory processes are impaired in schizophrenia and in healthy subjects after administration of LSD (Schmidt et al, 2017).

Emotional Processing

LSD impaired the recognition of sad and fearful faces (Dolder et al, 2016) and enhanced emotional empathy (Dolder et al, 2016), similar to psilocybin (Kometer et al, 2012; Preller et al, 2015) and MDMA (Hysek et al, 2014a; Kuypers et al, 2017). These effects of LSD on emotion processing may be considered useful in LSD-assisted psychotherapy. However, LSD also impaired the identification of complex emotions (Dolder et al, 2016).

Functional Brain Imaging

LSD acutely decreased the functional integrity of brain networks (Figure 2a) and the separation between networks (Carhart-Harris et al, 2016c; Tagliazucchi et al, 2016) (Figure 2b). At the whole-brain level, LSD increased functional connectivity between various brain regions (Figure 3). LSD also increased measures of functional ‘brain entropy’ (ie, the predictability of resting-state fMRI time series) across many functional systems (Lebedev et al, 2016). The acute LSD-induced global increase in ‘brain entropy’ was associated with trait openness that was assessed 14 days later (Lebedev et al, 2016). LSD increased thalamocortical resting-state functional connectivity (RSFC) (Mueller et al, 2017b; Tagliazucchi et al, 2016), overall connectivity in high-level cortical regions and the thalamus, and connectivity between normally more dissociated resting-state networks (Tagliazucchi et al, 2016). These findings indicate more globally synchronized activity within the brain and a reduction of network separation while under the pharmacological effects of LSD. Similar decreases in within-network integrity (Carhart-Harris et al, 2014; Muthukumaraswamy et al, 2013) and increases in between-network connectivity (Carhart-Harris et al, 2013; Roseman et al, 2014) have been observed under psilocybin. The LSD-induced increases in global connectivity, particularly in the temporo-parietal junction and insular cortex, correlated with feelings of moderate ‘ego dissolution’ that were produced by LSD (Tagliazucchi et al, 2016). ‘Ego dissolution’ refers to a disintegration of the sense of possessing a ‘self’ or identity that is distinct from others and from the environment (Preller and Vollenweider, 2016; Tagliazucchi et al, 2016). In addition, LSD-induced RSFC between the thalamus and right fusiform gyrus and insula correlated with subjective visual and auditory alterations, respectively (Mueller et al, 2017b). Remaining to be determined is the way in which LSD-induced increases in thalamocortical connectivity may be linked to the thalamic gating of perceptions (Mueller et al, 2017b). In contrast to the higher connectivity between neural networks while under the effects of LSD, LSD globally decreased within-network RSFC (integrity) and within-network signal variance (Carhart-Harris et al, 2016c) (Figure 2a). Specifically, LSD decreased default mode network (DMN) integrity (Carhart-Harris et al, 2016c) as previously shown for psilocybin (Carhart-Harris et al, 2014), and this LSD-induced disintegration of the DMN correlated with ratings of ego dissolution (Carhart-Harris et al, 2016c; Tagliazucchi et al, 2016). Furthermore, reductions of RSFC in the DMN (ie, DMN disintegration) were associated with fewer mental spaces for the past (ie, decreased mental time travel to the past) while under the effects of LSD (Speth et al, 2016). Increases in DMN RSFC have been described in depression, and decreases in DMN RSFC that are induced by LSD may be linked to its potential antidepressant effects (Carhart-Harris et al, 2016a).

Figure 2 (a) Mean percentage differences (+SEM) in CBF (red), integrity (blue), and signal variance (green) in 12 different resting-state networks (RSNs) under LSD relative to placebo (red asterisks indicate statistical significance, *P<0.05; **P<0.01, Bonferroni corrected). (b) Differences in between-RSN RSFC or RSN ‘segregation’ under LSD vs placebo. Each square in the matrix represents the strength of functional connectivity (positive=red, negative=blue) between a pair of different RSNs (parameter estimate values). The matrix on the far right displays the between-condition differences in covariance (t values): red=reduced segregation and blue=increased segregation under LSD. White asterisks represent significant differences (P<0.05, FDR corrected; n=15). Reproduced from Carhart-Harris et al (2016c). PowerPoint slide Full size image

Figure 3 Connectome ring showing functional connectivity between 132 regions covering the whole brain. Contrast LSD vs placebo, P<0.05, FDR. Yellow-red indicates increased functional connectivity, blue indicates decreased connectivity. Figure provided by F Mueller from the Basel fMRI study (Mueller et al, 2017b). PowerPoint slide Full size image

Arterial spin labeling analyses revealed greater cerebral blood flow in the visual cortex that was induced by LSD, and this increase was associated with ratings of complex imagery on the 5D-ASC (Carhart-Harris et al, 2016c). LSD also strongly increased RSFC between the primary visual cortex (V1) and cortical and subcortical brain regions, and this effect correlated with 5D-ASC ratings of elementary or complex hallucinations (Carhart-Harris et al, 2016c). Greatly expanded V1 functional connectivity that is induced by LSD may indicate that a greater proportion of the brain processes visual information than under normal conditions (Carhart-Harris et al, 2016c). Further analyses found that LSD administration altered eyes-closed spontaneous activity within retinotopically organized patches of the V1 and neighboring visual regions (V3), similar to visual stimulation (Roseman et al, 2016). Thus, the primary visual system is altered by LSD and behaves as if it perceives spatially localized visual information when in fact there is none (Roseman et al, 2016), which is consistent with the notion of ‘seeing with the eyes shut’ (Carhart-Harris et al, 2016c; Roseman et al, 2016).

LSD-induced decreases in RSFC between the parahippocampus and the rest of the brain (particularly the retrosplenial and posterior cingulate cortex) correlated with VAS ratings of ego dissolution and altered meaning on the 5D-ASC (Carhart-Harris et al, 2016c). Similarly, psilocybin altered activity in parahippocampal-retrosplenial cortex circuit measured with EEG and this effect correlated with spirituality and insigthfulness ratings on the 5D-ASC (Kometer et al, 2015). LSD increased blood oxygen-level-dependent activity of the supplementary motor area and prefrontal cortex in response to music without personal meaning or relevance compared with personally meaningful and neutral music, indicating enhanced activity in brain areas that are involved in self-referential cognition and processing (Preller et al, 2017). LSD reduced left amygdala reactivity to the presentation of fearful faces (Mueller et al, 2017a). Psilocybin similarly decreased amygdala reactivity to negative facial expressions (Kraehenmann et al, 2015). Lower fear perception (Dolder et al, 2016) and amygdala reactivity may be useful during psychotherapy. Magnetoencephalography showed that LSD decreased oscillatory power throughout the brain during eyes-closed rest (Carhart-Harris et al, 2016c) as similarly shown for psilocybin (Kometer et al, 2015; Muthukumaraswamy et al, 2013) and ayahuasca (Riba et al, 2002). After LSD administration, lower alpha power correlated with subjective ratings of simple hallucinations (Carhart-Harris et al, 2016c). Lower alpha power in occipital sensors correlated with increases in primary visual cortex RSFC (Carhart-Harris et al, 2016c). Modern positron emission tomography (PET) and single-photon emission computed tomography studies of LSD have not yet been conducted. Other 5-HT hallucinogens, such as psilocybin, ayahuasca, and mescaline, increased metabolic indices in frontal brain areas Hallucinogen-induced hyperfrontality is hypothesized to reflect increased frontal activity due to flooding with information (Geyer and Vollenweider, 2008; Vollenweider et al, 1997). In contrast, in an fMRI study, psilocybin decreased blood flow and BOLD signal in the thalamus, anterior cingulate, medial prefrontal, and cingulate cortices (Carhart-Harris et al, 2012). It is not yet clear what the different imaging modalities represent and how these inconsistencies can be explained. It has been proposed that the PET study findings of hyperfrontality reflect the increased neuronal firing activity while fMRI BOLD measures correlate with cortical oscillatory activity (Halberstadt, 2015). Altogether, the first modern imaging studies of LSD have provided preliminary information on the neural correlates of altered states of mind that are induced by LSD. However, there are many limitations. Much data have been derived from only a few small studies. Chance findings should be expected especially with regard to the RSFC data. LSD may also have direct actions on vascular resistance and blood flow that may confound neuroimaging data. These preliminary findings need to be confirmed in larger studies and by different research groups.

Clinical Pharmacology

The pharmacokinetics of LSD have been well investigated only for oral doses of 100 and 200 μg (Dolder et al, 2015b, 2017; Steuer et al, 2016). LSD concentration-time and subjective effect-time curves are shown in Figure 4. No data are available on the concentration-time course of the intravenous dose of 75 μg LSD that was used in the London studies. The pharmacokinetics of LSD are dose-proportional, and elimination kinetics are linear up to 12 h (Dolder et al, 2015b, 2017; Steuer et al, 2016). Maximal plasma concentrations are reached 1.5 h after oral administration (Dolder et al, 2015b, 2017) (Figure 4). The elimination half-life is ~3 h (Dolder et al, 2015b, 2017). LSD can be detected in blood plasma up to 12–24 h after administration, depending on the dose (Dolder et al, 2017). 2-Oxo-3-hydroxy-LSD (Oxo-HO-LSD) is the major metabolite of LSD and is detectable in urine for a longer time than LSD (Dolder et al, 2015a; Steuer et al, 2016). Oxo-HO-LSD and minor metabolites of LSD can only be detected at very low concentrations in blood plasma and serum (<0.3 ng/ml) (Dolder et al, 2015a; Steuer et al, 2016) but are present at higher concentrations in urine (Dolder et al, 2015a). The intravenous dose of 75 μg LSD that was used in the London studies likely corresponds to the oral dose of 100 μg that was used in the Basel and Zurich studies, based on the comparable effects on the 5D-ASC (Carhart-Harris et al, 2016b; Liechti et al, 2017) (Figure 1). The subjective, cognitive, and sympathomimetic effects of oral LSD closely reflected the time course of LSD concentrations in plasma (Dolder et al, 2015b, 2017) (Figure 4). Subjective effects of LSD peaked 2.5 h after administration and lasted for 8 h and 12 h after administration of 100 μg and 200 μg, respectively (Dolder et al, 2017) (Figure 4). After intravenous administration of 75 μg LSD, subjective effects peaked at 45–120 min and lasted 7–8 h (Carhart-Harris et al, 2016b; Kaelen et al, 2015). After a single dose of LSD, the pharmacodynamic effects lasted as long as LSD was present in the body, with no evidence of acute tolerance to the effects of LSD (Dolder et al, 2017). Tolerance has been reported with repeated daily LSD administration over 3–7 days (Belleville et al, 1956).

Mid- and Long-Term Effects

In comparison to other illicit substances, epidemiological studies indicate that the use of classic hallucinogens is associated with lower psychological distress, lower suicidality, and lower mental health problems (Hendricks et al, 2015). Long-lasting positive effects were documented in modern studies after controlled administration of psilocybin (Griffiths et al, 2011; MacLean et al, 2011) and ayahuasca (Bouso et al, 2012) but have not yet been reported in modern experimental laboratory studies of LSD. Controlled administration of LSD in healthy subjects increased optimism and trait openness 2 weeks after administration and produced trends toward decreases in distress and delusional thinking (Carhart-Harris et al, 2016b). In addition, the greatest increases in openness were observed in subjects who presented both the highest acute LSD-induced enhancements of ego dissolution during music listening and greater brain entropy in frontal areas (Lebedev et al, 2016). However, the reported increases in optimism and personality trait openness 14 days after LSD administration were observed in subjects with on average already 14 previous uses of LSD (Carhart-Harris et al, 2016b; Lebedev et al, 2016) raising the question of how open and optimistic participants can actually become or whether these effects are rather transient.