"Marijuana doesn’t count, does it?” Clinicians are familiar with this common reply when screening for drug use. Cannabis—the most common illicit substance—has managed to exempt itself from the hazardous reputation held by other illicit drugs.1 As mental health practitioners, it is our duty to educate our patients about the potential harms and consequences of cannabis use. This important task is complicated by the disagreement and uncertainty surrounding the nature of the interaction between cannabis and psychotic disorders.
While research suggests that cannabis use can induce an acute psychotic state, there is controversy about whether it may precipitate psychotic disorders, such as schizophrenia. In this article, we provide an update on the literature on this important issue, emphasize areas in need of research, and provide clinically useful recommendations.
More than 16 million Americans use cannabis on a regular basis, typically beginning in adolescence. Notably, it is estimated that approximately 4% of the population have a diagnosis of either cannabis abuse or dependence.1 A history of cannabis misuse is even more common in patients who are schizophrenic than in the general population; 25% of patients with schizophrenia have a comorbid cannabis use disorder. Cannabis use disorders are especially common in younger and first-episode patient samples and in samples with high proportions of males.2
Marijuana contains more than 400 chemical compounds, including over 60 cannabinoids that contribute to its psychopharmacological effects. The primary psychoactive constituent of cannabis is delta-9-tetrahydrocannabinol (THC). Other plant cannabinoids include delta-8-tetrahydrocannabinol; cannabinol; and cannabidiol (CBD); CBD is the second major psychoactive constituent of cannabis.3 The ratios of these and other cannabinoids vary enormously in preparations of cannabis, and little information exists about the concentration of each of the particular cannabinoids in commonly used cannabis products. Concerns have been expressed regarding the large increase in the potency of cannabis and the surrounding health implications. In the 1960s, the THC content was thought to be in the range of 1% to 3%; today it can reach up to 20%.4
The endogenous cannabinoid system consists of 2 types of G-protein-coupled receptors: cannabinoid 1 (CB1) and cannabinoid 2 (CB2) receptors. CB1 receptors are the most abundant in the brain, while CB2 receptors predominate on immune cells. CB1 receptors are highly concentrated in brain regions implicated in the putative neural circuitry of psychosis and cognitive function. These include the hippocampus, prefrontal cortex, anterior cingulate, basal ganglia, cerebellum, and cortex, with lower levels present in the thalamus, hypothalamus, and amygdala. Activation of CB1 receptors mediates the behavioral and physiological effects of both endogenous and exogenous cannabinoids in the brain.4
An important role of the CB1 receptor is to modulate neurotransmitter release in a manner that maintains homeostasis by preventing excessive neuronal activity in the CNS.5 CB1 receptors are localized on presynaptic neuron terminals on both inhibitory and excitatory neurons, yet they predominate on γ-aminobutyric acid interneurons.6 It is the inhibitory neurons that are thought to mediate most of the effects of cannabinoids. In addition, the action of cannabinoids includes interactions, albeit indirectly, with the dopaminergic system.
THC is a partial agonist at the CB1 receptors, where it has modest affinity and low intrinsic activity. In contrast, CBD shows very little affinity for CB1 receptors. Moreover, the precise molecular mechanism of action of CBD remains unclear. The main endocannabinoids are anandamide and 2-arachidonylglycerol. In contrast to classic neurotransmitters, endocannabinoids can function as retrograde synaptic messengers—they are released from postsynaptic neurons and travel backward across synapses, activating CB1 on presynaptic axons and suppressing neurotransmitter release.
1. United Nations Office on Drugs and Crime (UNODC). World Drug Report 2010. Vienna: United Nations; 2010.
2. Koskinen J, Löhönen J, Koponen H, et al. Rate of cannabis use disorders in clinical samples of patients with schizophrenia: a meta-analysis. Schizophr Bull. 2010;36:1115-1130.
3. Iversen LL. The Science of Marijuana. 2nd ed. New York: Oxford University Press; 2008.
4. Ameri A. The effects of cannabinoids on the brain. Prog Neurobiol. 1999;58:315-348.
5. Pertwee RG. The diverse CB1 and CB2 receptor pharmacology of three plant cannabinoids: delta9-tetrahydrocannabinol, cannabidiol and delta9-tetrahydrocannabivarin. Br J Pharmacol. 2008;153:199-215.
6. Eggan SM, Lewis DA. Immunocytochemical distribution of the cannabinoid CB1 receptor in the primate neocortex: a regional and laminar analysis. Cereb Cortex. 2007;17:175-191.
7. Szabo B, Siemes S, Wallmichrath I. Inhibition of GABAergic neurotransmission in the ventral tegmental area by cannabinoids. Eur J Neurosci. 2002;15:2057-2061.
8. Murray RM, Morrison PD, Henquet C, Di Forti M. Cannabis, the mind and society: the hash realities. Nat Rev Neurosci. 2007;8:885-895.
9. Morrison PD, Murray RM. From real-world events to psychosis: the emerging neuropharmacology of delusions. Schizophr Bull. 2009;35:668-674.
10. Moore TH, Zammit S, Lingford-Hughes A, et al. Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review. Lancet. 2007;370:319-328.
11. McLaren JA, Silins E, Hutchinson D, et al. Assessing evidence for a causal link between cannabis and psychosis: a review of cohort studies. Int J Drug Policy. 2009;21:10-19.
12. Andréasson S, Allebeck P, Engström A, Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet. 1987;2:1483-1486.
13. Arseneault L, Cannon M, Poulton R, et al. Cannabis use in adolescence and risk for adult psychosis: longitudinal prospective study. BMJ. 2002;325:1212-1213.
14. Caspi A, Moffitt TE, Cannon M, et al. Moderation of the effect of adolescent-onset cannabis use on adult psychosis by a functional polymorphism in the catechol-O-methyltransferase gene: longitudinal evidence of a gene X environment interaction. Biol Psychiatry. 2005;57:1117-1127.
15. Lachman HM, Papolos DF, Saito T, et al. Human catechol-O-methyltransferase pharmacogenetics: description of a functional polymorphism and its potential application to neuropsychiatric disorders. Pharmacogenetics. 1996;6:243-250.
16. Bilder RM, Volavka J, Lachman HM, Grace AA. The catechol-O-methyltransferase polymorphism: relations to the tonic-phasic dopamine hypothesis and neuropsychiatric phenotypes. Neuropsychopharmacology. 2004;29:1943-1961.
17. Fergusson DM, Horwood LJ, Swain-Campbell NR. Cannabis dependence and psychotic symptoms in young people. Psychol Med. 2003;33:15-21.
18. Bowers MB Jr, Mazure CM, Nelson JC, Jatlow PI. Psychotogenic drug use and neuroleptic response. Schizophr Bull. 1990;16:81-85.
19. Linszen DH, Dingemans PM, Lenior ME. Cannabis abuse and the course of recent-onset schizophrenic disorders. Arch Gen Psychiatry. 1994;51:273-279.
20. Rabin RA, Zakzanis KK, George TP. The effects of cannabis use on neurocognition in schizophrenia: a meta-analysis. Schizophr Res. 2011;128:111-116.
21. Zuardi AW, Crippa JA, Hallak JE, et al. Cannabidiol, a Cannabis sativa constituent, as an antipsychotic drug. Braz J Med Biol Res. 2006;39:421-429.
22. Fadda P, Robinson L, Fratta W, et al. Differential effects of THC- or CBD-rich cannabis extracts on working memory in rats. Neuropharmacology. 2004;47:1170-1179.
23. Henquet C, Krabbendam L, Spauwen J, et al. Prospective cohort study of cannabis use, predisposition for psychosis, and psychotic symptoms in young people. BMJ. 2005;330:11.
24. Tien AY, Anthony JC. Epidemiological analysis of alcohol and drug use as risk factors for psychotic experiences. J Nerv Ment Dis. 1990;178:473-480.
25. van Os J, Bak M, Hanssen M, et al. Cannabis use and psychosis: a longitudinal population-based study. Am J Epidemiol. 2002;156:319-327.