Metabolism of lysergic acid diethylamide (LSD): an update

Characterization of iso-LSD metabolism using human liver microsomes in comparison to LSD and its applicability as urinary biomarker for LSD consumption

Urinalysis of lysergic acid diethylamide (LSD) poses a challenge due to its rapid metabolism, resulting in little to no LSD detectable in urine. Instead, its primary metabolite, 2-oxo-3-hydroxy-LSD, is predominantly detected. In this study, we observed several urine profiles with iso-LSD detected together with 2-oxo-3-hydroxy-LSD. Iso-LSD is derived from illicit preparation of LSD as a major contaminant, and it was detected at higher abundance than LSD and 2-oxo-3-hydroxy-LSD in certain urine samples. Therefore, the metabolism of iso-LSD and its potential as a viable urinary biomarker for confirming LSD consumption is of interest. For metabolism studies, LSD and iso-LSD were incubated in human liver microsomes (HLMs) at 0 min, 60 min and 120 min to characterize their metabolites using LC-QTOF-MS. For urinary analysis, 500 µL of urine samples underwent enzymatic hydrolysis and clean-up using supported-liquid extraction (SLE) prior to analysis by LC-QTOF-MS. From HLM incubation study of LSD, the metabolites detected were dihydroxy-LSD, 2-oxo-LSD, N-desmethyl-LSD (nor-LSD) and 2-oxo-3-hydroxy-LSD with LSD levels decreasing significantly throughout all time points, consistent with the existing literatures. For HLM study of iso-LSD, metabolites eluting at retention times after the corresponding metabolites of LSD were detected, with iso-LSD levels showing only a slight decrease throughout all time points, due to a slower metabolism of iso-LSD compared to LSD. These findings corroborate with the urinalysis of 24 authentic urine samples, where iso-LSD with 2-oxo-3-hydroxy-LSD was detected in the absence of LSD. Based on our findings, iso-LSD is commonly detected in urine (18 out of 24 samples) sometimes with traces of possible 2-oxo-3-hydroxy-iso-LSD. The slower metabolism and high detection rate in urine make iso-LSD a viable urinary biomarker for confirming LSD consumption, especially in the absence of LSD and/or 2-oxo-3-hydroxy-LSD.

Psychedelic Therapy: A Primer for Primary Care Clinicians-Lysergic Acid Diethylamide (LSD)

BACKGROUND

Lysergic acid diethylamide (LSD) is a hallucinogenic agent. In the mid-20th century, it was used to augment psychoanalysis and to treat alcohol use disorder. However, LSD was banned in 1970 in part because of concerns that it could bring about or exacerbate mental illness. Its therapeutic potential remains incompletely understood.

AREAS OF UNCERTAINTY

While uncontrolled recreational use of LSD can, in rare instances, lead to long-term psychosis, adverse events in clinical trials of LSD, such as anxiety, headache, and nausea, have almost always been mild and transient. Serious adverse events, such as intense panic, suicidal ideation, and psychosis, were reported in either none or very few of the participants. However, patient selection criteria, optimal dosing strategy, and appropriate clinical follow-up guidelines remain to be established.

THERAPEUTIC ADVANCES

Preliminary data suggest that LSD may be effective for the management of alcohol use disorder, anxiety, and depression. In trials of LSD for treating anxiety and depression associated with life-threatening illnesses, 77% of participants demonstrate durable relief at 1 year post-treatment. Top-line data from a large-scale phase IIb trial (n = 198) indicate that 50% of participants experience remission from generalized anxiety disorder after a single 100 μg dose of LSD. According to a meta-analysis of RCTs on LSD from the mid-20th century, single-dose regimens of LSD significantly improve alcohol use disorder (P < 0.0003) with an odds ratio (OR) of 1.96.

LIMITATIONS

Only one large-scale clinical trial (>50 participants) has been conducted on LSD in the contemporary era of psychedelic research. Further studies with large sample sizes are needed to explore potential clinical applications.

CONCLUSIONS

Preliminary data suggest that LSD may be one of the most potent treatments for anxiety in patients both with and without a life-threatening illness. LSD may also be beneficial for treating depression and substance use disorders.

Small molecules targeting selected histone methyltransferases (HMTs) for cancer treatment: Current progress and novel strategies

Histone methyltransferases (HMTs) play a critical role in gene post-translational regulation and diverse physiological processes, and are implicated in a plethora of human diseases, especially cancer. Increasing evidences demonstrate that HMTs may serve as a potential therapeutic target for cancer treatment. Thus, the development of HMTs inhibitor have been pursued with steadily increasing interest over the past decade. However, the disadvantages such as insufficient clinical efficacy, moderate selectivity, and propensity for acquired resistance have hindered the development of conventional HMT inhibitors. New technologies and methods are imperative to enhance the anticancer activity of HMT inhibitors. In this review, we first review the structure and biological functions of the several essential HMTs, such as EZH2, G9a, PRMT5, and DOT1L. The internal relationship between these HMTs and cancer is also expounded. Next, we mainly focus on the latest progress in the development of HMT modulators encompassing dual-target inhibitors, targeted protein degraders and covalent inhibitors from perspectives such as rational design, pharmacodynamics, pharmacokinetics, and clinical status. Lastly, we also discuss the challenges and future directions for HMT-based drug discovery for cancer therapy.

Present and future of metabolic and metabolomics studies focused on classical psychedelics in humans

Psychedelics are classical hallucinogen drugs that induce a marked altered state of consciousness. In recent years, there has been renewed attention to the possible use of classical psychedelics for the treatment of certain mental health disorders. However, further investigation to better understand their biological effects in humans, their mechanism of action, and their metabolism in humans is needed when considering the development of future novel therapeutic approaches. Both metabolic and metabolomics studies may help for these purposes. On one hand, metabolic studies aim to determine the main metabolites of the drug. On the other hand, the application of metabolomics in human psychedelics studies can help to further understand the biological processes underlying the psychedelic state and the mechanisms of action underlying their therapeutic potential. This review presents the state of the art of metabolic and metabolomic studies after lysergic acid diethylamide (LSD), mescaline, N,N-dimethyltryptamine (DMT) and β-carboline alkaloids (ayahuasca brew), 5-methoxy-DMT and psilocybin administrations in humans. We first describe the characteristics of the published research. Afterward, we reviewed the main results obtained by both metabolic and metabolomics (if available) studies in classical psychedelics and we found out that metabolic and metabolomics studies in psychedelics progress at two different speeds. Thus, whereas the main metabolites for classical psychedelics have been robustly established, the main metabolic alterations induced by psychedelics need to be explored. The integration of metabolomics and pharmacokinetics for investigating the molecular interaction between psychedelics and multiple targets may open new avenues in understanding the therapeutic role of psychedelics.

Systematic characterization of Lysergic Acid Diethylamide metabolites in Caenorhabditis elegans by ultra-high performance liquid chromatography coupled with high-resolution tandem mass spectrometry

Psychedelic compounds have gained renewed interest for their potential therapeutic applications, but their metabolism and effects on complex biological systems remain poorly understood. Here, we present a systematic characterization of Lysergic Acid Diethylamide (LSD) metabolites in the model organism Caenorhabditis elegans using state-of-the-art analytical techniques. By employing ultra-high performance liquid chromatography coupled with high-resolution tandem mass spectrometry, we putatively identified a range of LSD metabolites, shedding light on their metabolic pathways and offering insights into their pharmacokinetics. Our study demonstrates the suitability of Caenorhabditis elegans as a valuable model system for investigating the metabolism of psychedelic compounds and provides a foundation for further research on the therapeutic potential of LSD.

The Bright Side of Psychedelics: Latest Advances and Challenges in Neuropharmacology

The need to identify effective therapies for the treatment of psychiatric disorders is a particularly important issue in modern societies. In addition, difficulties in finding new drugs have led pharmacologists to review and re-evaluate some past molecules, including psychedelics. For several years there has been growing interest among psychotherapists in psilocybin or lysergic acid diethylamide for the treatment of obsessive-compulsive disorder, of depression, or of post-traumatic stress disorder, although results are not always clear and definitive. In fact, the mechanisms of action of psychedelics are not yet fully understood and some molecular aspects have yet to be well defined. Thus, this review aims to summarize the ethnobotanical uses of the best-known psychedelic plants and the pharmacological mechanisms of the main active ingredients they contain. Furthermore, an up-to-date overview of structural and computational studies performed to evaluate the affinity and binding modes to biologically relevant receptors of ibogaine, mescaline, N,N-dimethyltryptamine, psilocin, and lysergic acid diethylamide is presented. Finally, the most recent clinical studies evaluating the efficacy of psychedelic molecules in some psychiatric disorders are discussed and compared with drugs already used in therapy.

Potential Treatment for Obsessive-Compulsive Disorder

Obsessive-compulsive disorder (OCD) affects approximately 2% of the population worldwide and is also prevalent in other neuropsychiatric disorders. Serotonin reuptake inhibitors have limited widespread success. Alternate treatment options for OCD and other indications are highlighted using 3-bromo-lysergic acid diethylamide.

The chemical induction of synaesthesia

OBJECTIVE

Preliminary research suggests that experiences resembling synaesthesia are frequently reported under the influence of a diverse range of chemical substances although the incidence, chemical specificity, and characteristics of these effects are poorly understood.

METHODS

Here we surveyed recreational drug users and self-reported developmental synaesthetes regarding their use of 28 psychoactive drugs from 12 different drug classes and whether they had experienced synaesthesia under the influence of these substances.

RESULTS

The drug class of tryptamines exhibited the highest incidence rates of drug-induced synaesthesia in controls and induction rates of novel forms of synaesthesia in developmental synaesthetes. Induction incidence rates in controls were strongly correlated with the corresponding induction and enhancement rates in developmental synaesthetes. In addition, the use of lysergic acid diethylamide (LSD) was the strongest predictor of drug-induced synaesthesia in both controls and developmental synaesthetes. Clear evidence was observed for a clustering of synaesthesia-induction rates as a function of drug class in both groups, denoting non-random incidence rates within drug classes. Sound-colour synaesthesia was the most commonly observed type of induced synaesthesia. Further analyses suggest the presence of synaesthesia-prone individuals, who were more likely to experience drug-induced synaesthesia with multiple drugs.

CONCLUSIONS

These data corroborate the hypothesized link between drug-induced synaesthesia and serotoninergic activity, but also suggest the possibility of alternative neurochemical pathways involved in the induction of synaesthesia. They further imply that the induction and modulation of synaesthesia in controls and developmental synaesthetes share overlapping mechanisms and that certain individuals may be more susceptible to experiencing induced synaesthesia with different drugs.

High-sensitivity method for the determination of LSD and 2-oxo-3-hydroxy-LSD in oral fluid by liquid chromatography‒tandem mass spectrometry

PURPOSE

We have developed and validated a high-sensitivity method to quantify lysergic acid diethylamide (LSD) and 2-oxo-3-hydroxy-LSD (OH-LSD) in oral fluid samples using liquid-liquid extraction and liquid chromatography-tandem mass spectrometry (LC‒MS/MS). The method was applied to the quantification of both substances in 42 authentic oral fluid samples.

METHODS

A liquid-liquid extraction was performed using 500 µL each of samples (oral fluid samples collected using Quantisal™ device) and dichloromethane/isopropanol mixture (1:1, v/v). Enzymatic hydrolysis was evaluated to cleave glucuronide metabolites.

RESULTS

The limit of quantification was 0.01 ng/mL for both LSD and OH-LSD. The linearity was assessed between 0.01 and 5 ng/mL. Imprecision and bias were not higher than 10.2% for both analytes. Extraction recovery was higher than 69%. The analytes were stable in the autosampler at 10 °C for 24 h, and up to 30 days at 4 and -20 °C. The method was applied to the analysis of 42 oral fluid samples. LSD was detected in all samples (concentrations between 0.02 and 175 ng/mL), and OH-LSD was detected in 20 samples (concentrations between 0.01 and 1.53 ng/mL).

CONCLUSIONS

A high-sensitive method was fully validated and applied to authentic samples. To our knowledge, this is the first work to report concentrations of LSD and OH-LSD in authentic oral fluid samples.

Analytical profile, in vitro metabolism and behavioral properties of the lysergamide 1P-AL-LAD

Lysergic acid diethylamide (LSD) is known to induce powerful psychoactive effects in humans, which cemented its status as an important tool for clinical research. A range of analogues and derivatives has been investigated over the years, including those classified as new psychoactive substances. This study presents the characterization of the novel lysergamide N,N-diethyl-1-propanoyl-6-(prop-2-en-1-yl)-9,10-didehydroergoline-8β-carboxamide (1P-AL-LAD) using various mass spectrometric, gas- and liquid chromatographic and spectroscopic methods. In vitro metabolism studies using pooled human liver microsomes (pHLM) confirmed that 1P-AL-LAD converted to AL-LAD as the most abundant metabolite consistent with the hypothesis that 1P-AL-LAD may act as a prodrug. Fourteen metabolites were detected in total; metabolic reactions included hydroxylation of the core lysergamide ring structure or the N⁶ -allyl group, formation of dihydrodiol metabolites, N-dealkylation, N¹ -deacylation, dehydrogenation, and combinations thereof. The in vivo behavioral activity of 1P-AL-LAD was evaluated using the mouse head twitch response (HTR), a 5-HT_2A -mediated head movement that serves as a behavioral proxy in rodents for human hallucinogenic effects. 1P-AL-LAD induced a dose-dependent increase in HTR counts with an inverted U-shaped dose-response function, similar to lysergic acid diethylamide (LSD), psilocybin, and other psychedelics. Following intraperitoneal injection, the median effective dose (ED₅₀ ) for 1P-AL-LAD was 491 nmol/kg, making it almost three times less potent than AL-LAD (174.9 nmol/kg). Previous studies have shown that N¹ -substitution disrupts the ability of lysergamides to activate the 5-HT_2A receptor; based on the in vitro metabolism data, 1P-AL-LAD may induce the HTR because it acts as a prodrug and is metabolized to AL-LAD after administration to mice.

Mycotherapy: Potential of Fungal Bioactives for the Treatment of Mental Health Disorders and Morbidities of Chronic Pain

Mushrooms have been used as traditional medicine for millennia, fungi are the main natural source of psychedelic compounds. There is now increasing interest in using fungal active compounds such as psychedelics for alleviating symptoms of mental health disorders including major depressive disorder, anxiety, and addiction. The anxiolytic, antidepressant and anti-addictive effect of these compounds has raised awareness stimulating neuropharmacological investigations. Micro-dosing or acute dosing with psychedelics including Lysergic acid diethylamide (LSD) and psilocybin may offer patients treatment options which are unmet by current therapeutic options. Studies suggest that either dosing regimen produces a rapid and long-lasting effect on the patient post administration with a good safety profile. Psychedelics can also modulate immune systems including pro-inflammatory cytokines suggesting a potential in the treatment of auto-immune and other chronic pain conditions. This literature review aims to explore recent evidence relating to the application of fungal bioactives in treating chronic mental health and chronic pain morbidities.

Lysergic Acid Diethylamide, Psilocybin and Dimethyltryptamine in Depression Treatment: A Systematic Review

Despite many different kinds of substances available for depression treatment, depression itself still appears to be a clinical challenge. Recently, formerly illicit substances came to scientists' attention, including lysergic acid diethylamide (LSD), psilocybin and dimethyltryptamine (DMT). Some studies suggest that these substances might be effective in depression treatment. The aim of this study was to evaluate the efficiency of LSD, psilocybin and DMT in depression treatment in the light of current medical literature. The authors followed the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guidelines for this systematic review. The authors searched the PubMed and Cochrane Library databases to identify relevant publications. Finally, 10 papers were included. Most of the selected studies showed significant correlation between psilocybin and DMT use and reduction in depression symptom intensity. By analyzing qualified studies, it can be concluded that psilocybin and DMT could be useful in depression treatment, but further observations are still required.

Classic Psychedelic Coadministration with Lithium, but Not Lamotrigine, is Associated with Seizures: An Analysis of Online Psychedelic Experience Reports

INTRODUCTION

Psychedelics show promise in treating unipolar depression, though patients with bipolar disorder have been excluded from recent psychedelic trials. There is limited information on the use of classic psychedelics (e. g., LSD or psilocybin) in individuals using mood stabilizers to treat bipolar disorder. This is important to know, as individuals with bipolar depression may attempt to treat themselves with psychedelics while on a mood stabilizer, particularly given enthusiastic media reports of the efficacy of psilocybin for depression.

METHODS

This study analyzed reports of classic psychedelics administered with mood stabilizers from 3 websites (Erowid.org, Shroomery.org, and Reddit.com).

RESULTS

Strikingly, 47% of 62 lithium plus psychedelic reports involved seizures, and an additional 18% resulted in bad trips while none of 34 lamotrigine reports did. Further, 39% of lithium reports involved medical attention. Most of the lamotrigine reports (65%) but few (8%) of the lithium reports were judged to not affect the psychedelic experience.

DISCUSSION

Although further research is needed, we provisionally conclude that psychedelic use may pose a significant seizure risk for patients on lithium.

Molecular and Functional Imaging Studies of Psychedelic Drug Action in Animals and Humans

Hallucinogens are a loosely defined group of compounds including LSD, N,N-dimethyltryptamines, mescaline, psilocybin/psilocin, and 2,5-dimethoxy-4-methamphetamine (DOM), which can evoke intense visual and emotional experiences. We are witnessing a renaissance of research interest in hallucinogens, driven by increasing awareness of their psychotherapeutic potential. As such, we now present a narrative review of the literature on hallucinogen binding in vitro and ex vivo, and the various molecular imaging studies with positron emission tomography (PET) or single photon emission computer tomography (SPECT). In general, molecular imaging can depict the uptake and binding distribution of labelled hallucinogenic compounds or their congeners in the brain, as was shown in an early PET study with N1-([11C]-methyl)-2-bromo-LSD ([11C]-MBL); displacement with the non-radioactive competitor ketanserin confirmed that the majority of [11C]-MBL specific binding was to serotonin 5-HT2A receptors. However, interactions at serotonin 5HT1A and other classes of receptors and pleotropic effects on second messenger pathways may contribute to the particular experiential phenomenologies of LSD and other hallucinogenic compounds. Other salient aspects of hallucinogen action include permeability to the blood-brain barrier, the rates of metabolism and elimination, and the formation of active metabolites. Despite the maturation of radiochemistry and molecular imaging in recent years, there has been only a handful of PET or SPECT studies of radiolabeled hallucinogens, most recently using the 5-HT2A/2C agonist N-(2[11CH3O]-methoxybenzyl)-2,5-dimethoxy- 4-bromophenethylamine ([11C]Cimbi-36). In addition to PET studies of target engagement at neuroreceptors and transporters, there is a small number of studies on the effects of hallucinogenic compounds on cerebral perfusion ([15O]-water) or metabolism ([18F]-fluorodeoxyglucose/FDG). There remains considerable scope for basic imaging research on the sites of interaction of hallucinogens and their cerebrometabolic effects; we expect that hybrid imaging with PET in conjunction with functional magnetic resonance imaging (fMRI) should provide especially useful for the next phase of this research.

Psychedelic Psychiatry and the Consult-Liaison Psychiatrist: A Primer

BACKGROUND

Psychedelic compounds such as lysergic acid diethylamide (LSD), psilocybin, and 3,4-Methylenedioxymethamphetamine (MDMA) share a long and complex history with psychiatry. A half century ago, psychedelics were widely employed by psychiatrists in investigational and clinical settings, with studies demonstrating promising findings for their use in the treatment of mental illness and substance use disorders. However, concerns were also raised about their abuse potential and other adverse effects. Owing to these worries and psychedelics' association with the counterculture movement, psychedelics were largely outlawed in the United States in 1970, bringing research on their therapeutic potential to a halt. However, in recent years, a resurgence of psychedelic research has revealed compelling, though early, evidence for the use of psychedelic-assisted therapy in treating alcohol use disorder, nicotine use disorder, posttraumatic stress disorder, and depression.

OBJECTIVE

Here we provide an overview of psychiatry's complicated relationship with psychedelics, while reviewing contemporary findings on psychedelic-assisted therapy, safety of psychedelic-assisted therapy, and risks of nonmedical use. We also make the case that psychiatry should consider preparing now for the possibility of Food and Drug Administration approval of psychedelic-assisted therapies in the near future. We conclude by discussing how growing societal interest in psychedelics could impact the work of consult-liaison psychiatrists, while also exploring how consult-liaison psychiatrists might contribute to future delivery of psychedelic treatments.

METHODS

We reviewed literature on psychedelic-assisted therapies and adverse events resulting from nonmedical psychedelic use.

RESULTS

We found a small, but rapidly growing literature indicating that psychedelic-assisted therapies may have treatment potential for mental illness and addiction. Our search also revealed a variety of rare adverse events stemming from nonmedical psychedelic use.

CONCLUSIONS

Despite past concerns about psychedelics, current data indicate psychedelic-assisted therapy may potentially reduce suffering owing to mental illness and addiction if administered thoughtfully and cautiously by trained professionals in medical settings.

Neurotoxicology Syndromes Associated with Drugs of Abuse

Substance use disorders-and their associated neurologic complications-are frequently encountered by neurologists as well as emergency room physicians, internists, psychiatrists, and medical intensivists. Prominent neurologic sequelae of drug abuse, such as seizure and stroke, are common and often result in patients receiving medical attention. However, less overt neurologic manifestations, such as dysautonomia and perceptual disturbances, may be initially misattributed to primary medical or psychiatric illness, respectively. This article focuses on the epidemiology, pharmacology, and complications associated with commonly used recreational drugs, including opioids, alcohol, marijuana, cocaine, and hallucinogens.

Classic serotonergic psychedelics for mood and depressive symptoms: a meta-analysis of mood disorder patients and healthy participants

RATIONALE

Major depressive disorder is one of the leading global causes of disability, for which the classic serotonergic psychedelics have recently reemerged as a potential therapeutic treatment option.

OBJECTIVE

We present the first meta-analytic review evaluating the clinical effects of classic serotonergic psychedelics vs placebo for mood state and symptoms of depression in both healthy and clinical populations (separately).

RESULTS

Our search revealed 12 eligible studies (n = 257; 124 healthy participants, and 133 patients with mood disorders), with data from randomized controlled trials involving psilocybin (n = 8), lysergic acid diethylamide ([LSD]; n = 3), and ayahuasca (n = 1). The meta-analyses of acute mood outcomes (3 h to 1 day after treatment) for healthy volunteers and patients revealed improvements with moderate significant effect sizes in favor of psychedelics, as well as for the longer-term (16 to 60 days after treatments) mood state of patients. For patients with mood disorder, significant effect sizes were detected on the acute, medium (2-7 days after treatment), and longer-term outcomes favoring psychedelics on the reduction of depressive symptoms.

CONCLUSION

Despite the concerns over unblinding and expectancy, the strength of the effect sizes, fast onset, and enduring therapeutic effects of these psychotherapeutic agents encourage further double-blind, placebo-controlled clinical trials assessing them for management of negative mood and depressive symptoms.

Trends in LSD use among US adults: 2015-2018

BACKGROUND

The recent use of LSD to treat severe psychological disorders in several clinical applications has proven effective in reducing symptoms and distressing events. Trend analyses are warranted to provide the most current data for clinical and health interventions. The purpose of this study was to examine trends in LSD use among adults in the United States.

METHODS

A secondary analysis of the 2015-2018 National Survey on Drug Use and Health was conducted on 168, 562 adults ages 18 and older.

RESULTS

Past-year LSD use increased 56.4% (P < .0001) from 2015 to 2018. The proportion of LSD users ages 26-34 increased from 19.6% to 31.1% (P < .0001), ages 35-49 increased from 2.73% to 8.82% (P < .0001) and 50 years or older increased from 1.83% to 2.66% (P < .0001). LSD use among bisexual individuals increased from 11.2% to 13.0% (P < .0001). LSD use among individuals with a college degree or more increased from 18.2% to 31.1% (P < .0001). Significant decreases in LSD use were present in individuals who were multi-racial (P < .0001), less than high school education P < .0001), high school education (P < .001), and perceived great risk of drugs (P < .0001).

CONCLUSIONS

LSD use in the US jumped 56.4% from 2015 to 2018. Results from the present study can inform prevention and harm reduction efforts (e.g., co-morbid substance use interventions, health messaging).

Adverse pharmacokinetic interactions between illicit substances and clinical drugs

Adverse pharmacokinetic interactions between illicit substances and clinical drugs are of a significant health concern. Illicit substances are taken by healthy individuals as well as by patients with medical conditions such as mental illnesses, acquired immunodeficiency syndrome, diabetes mellitus and cancer. Many individuals that use illicit substances simultaneously take clinical drugs meant for targeted treatment. This concomitant usage can lead to life-threatening pharmacokinetic interactions between illicit substances and clinical drugs. Optimal levels and activity of drug-metabolizing enzymes and drug-transporters are crucial for metabolism and disposition of illicit substances as well as clinical drugs. However, both illicit substances and clinical drugs can induce changes in the expression and/or activity of drug-metabolizing enzymes and drug-transporters. Consequently, with concomitant usage, illicit substances can adversely influence the therapeutic outcome of coadministered clinical drugs. Likewise, clinical drugs can adversely affect the response of coadministered illicit substances. While the interactions between illicit substances and clinical drugs pose a tremendous health and financial burden, they lack a similar level of attention as drug-drug, food-drug, supplement-drug, herb-drug, disease-drug, or other substance-drug interactions such as alcohol-drug and tobacco-drug interactions. This review highlights the clinical pharmacokinetic interactions between clinical drugs and commonly used illicit substances such as cannabis, cocaine and 3, 4-Methylenedioxymethamphetamine (MDMA). Rigorous efforts are warranted to further understand the underlying mechanisms responsible for these clinical pharmacokinetic interactions. It is also critical to extend the awareness of the life-threatening adverse interactions to both health care professionals and patients.