cannabis and mental health

Does Cannabis Cause Schizophrenia?
Cannabis is now the most commonly used drug worldwide after alcohol. A 2019 Word Drug Report revealed that almost 200 million people consistently smoke marijuana globally . Marijuana, also know as weed, pot, bud, among other tags, comes from the dried flowers of Cannabis Sativa. The most common way of taking the drug is by smoking it as one would a handheld cigarette. However, users can also ingest marijuana as edibles or as vaporizers. Recently, more potent forms of the drug like Sinsemilla have been synthesized, which are now increasingly common for both recreational and medical use .
For the longest time, consistent use of marijuana has been thought to be a pivotal contributor to psychotic disorders like schizophrenia. Many studies conducted in the recent past have shown that cannabis users are overrepresented in studies involving schizophrenia. The findings and the increasing rate of marijuana use have prompted new debates in developed countries about whether the drug precipitates psychotic disorders like schizophrenia. This study will attempt to establish a causal link between cannabis and schizophrenia through an in-depth study of other scholars’ research.
Evidence of the “Yes” side
To properly determine whether there is a relationship between marijuana use and schizophrenia, one must approach this problem from two angles; first, the general and long-term effect of the drug, and second, the results from longitudinal studies conducted over the years .
Marijuana’s main effect is the feelings of euphoria and relaxation that it gives its users. Other common effects that may vary widely among users include increased appetite, heightened senses, uncontrollable laughter, or altered time perception. These effects are mostly due to how Tetrahydrocannabinol (THC), the primary psychoactive compound in cannabis, works on the part of the brain responding to pleasure, memory, concentration, sensory, and time perception.
THC mainly works by attaching itself to neural transmitters called cannabinoid receptors, thereafter activating these brain areas. This affects the neural communication network that plays a critical role in how the central nervous system functions. There is now evidence suggesting that prolonged use of cannabis may interfere with this system, resulting in profound effects .
A recent study with 2,000 teenagers as respondents showed that young people who smoked cannabis at least five times were more likely to develop recurrent psychotic symptoms over the course of 10 years. The study also revealed that the drug’s continued use increased one’s risk of persistent psychotic symptoms by almost 30% .
A 15-year-old study on 50,000 Swedish subjects also found that those who had, at some point in their lives, tried marijuana before 18 years were 2.4 times more likely to receive schizophrenic diagnoses later in life . This is, by far, one of the most substantial pieces of evidence suggesting cannabis use as a cause of schizophrenia. Similar findings have also been replicated in other cohort studies in Germany and new Zealand5. So far, the general consensus seems to be that there exists a strong relationship. However, results from some research attempt to dispute this evidence.
Evidence of the “No” side
Another crucial piece that comes up in numerous studies is cannabidiol (CBD), another major component of cannabis. CBD also works by inhibiting the degradation of anandamide uptake, increasing the levels of the latter in the central nervous system. Contrary to THC, CBD is more tolerated by the central nervous system and has fewer potent psychoactive effects on its own . In one study, CBD seemed to counteract most effects like anxiety, psychosis, and induced memory impairment caused by THC in healthy respondents. Similar evidence also shows that higher doses of CBD over THC is associated with a lower risk of psychotic symptoms and changes related to psychosis. This was clearly illustrated in a study where higher doses of CBD showed to reduce psychotic symptoms significantly in 6 patients with Parkinson’s disease . Through increasing the anandamide levels in the CNS, similar studies reflected an improvement in overall psychotic symptoms.
In general, findings from these researches suggest that rather than inducing psychosis, cannabis also has significant anti-psychotic properties. Further research would be required to build a strong case for this conclusion. At the moment, we can only compare the results from previous studies to draw a meaningful conclusion.
Comparing the “Yes” and “No” evidence
From the wealth of findings in most studies, most evidence shows a strong positive association between chronic marijuana use and schizophrenia. However, an association does not necessarily reflect causation . Researchers still have a difficult time trying to establish a strong link between the two. A Havard Medical School research led by Proal suggested that an increased familial risk for schizophrenia was more likely the underlying issue in post-symptomatic use and not the use of cannabis itself . This is a classic case of the “Chicken and Egg” phenomenon. Cannabis could also likely affect the onset of schizophrenia but is equally not likely the cause of the illness.
Cannabis has also been thought to be used by people with schizophrenia to ease their psychotic symptoms. However, the results from self-medication alone may be challenging to explain a relationship between chronic use and schizophrenia5.
Medical research has also found that individuals with specific genetic traits are more predisposed to schizophrenic diagnoses following long-term marijuana use. One study showed an increased risk of psychosis in adults with specific genetic variables in the COMT and AKT1 genotypes. AKT1 controls an enzyme that affects brain signaling while COMT controls a different enzyme that breaks down dopamine. It should be noted that some studies have suggested that abnormal dopamine levels in the prefrontal brain regions could be the cause of schizophrenia. A risk of psychosis was found to be higher chronic users with certain gene variables7.
Unfortunately, increased risk in people with genetic vulnerability is often only reported as a co-occurring symptom of chronic use. Researchers are yet to determine whether chronic marijuana use is a cause or if it is just associated with psychotic symptoms.
Regardless, one thing is common between marijuana and schizophrenia, psychosis. Psychosis as a symptom of schizophrenia disrupts thought processes making it hard to tell apart real events from hallucinations. In the same way, marijuana also affects coordination, movement, senses, and general perceptions, disrupting normal human behavior. That being said, psychosis is a symptom of both marijuana use and schizophrenia, which poses a challenge for audiences who would like a clear distinction between both.
So far, most research only seems to show an association between chronic marijuana use and schizophrenia. This, however, does mean that marijuana inherently causes schizophrenia. To properly understand causation and association, one can draw from tobacco use and cancer research. For many years, researchers noted an association between tobacco use and lung cancer. However, they could only define a causal relationship when they eventually determined exactly how cigarette smoking damaged the lungs and other parts of the body .
At the moment, the research on the effects of marijuana is still, and its earlier stages and most finding seems to be speculative. However, one cannot ignore the effects of cannabis components like THC on the brain. THC tends to provoke symptoms of schizophrenia in healthy subjects and aggravates symptoms in schizophrenic patients. At best, this shows that chronic marijuana use has a small causative effect on schizophrenia. Consequently, there needs to be more extensive research done on the effects of cannabis on mental health.

1. United Nations. World Drug Report. Accessed November 17, 2020
2. Mack A, Joy J. Marijuana as Medicine? The Science Beyond the Controversy: HOW HARMFUL IS MARIJUANA? National Academies Press (US); 2000.
3. Kuepper R, Os JV, Lieb R, Wittchen H, Höfler M, Henquet C. Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 10-year follow-up cohort study. BMJ. 2011 Mar 1;342:d738. doi: 10.1136/bmj.d738.
4. NIDA. What is marijuana? . National Institute on Drug Abuse. April 13, 2020. Accessed November 18, 2020.
5. Hall W and Degenhardt L. Cannabis use and the risk of developing a psychotic disorder. World Psychiatry. 2008 Jun; 7(2): 68–71. doi: 10.1002/j.2051-5545.2008.tb00158.x.
6. Andréasson S, Allebeck P, Engström A, Rydberg U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet. 1987 Dec 26;2(8574):1483-6. doi: 10.1016/s0140-6736(87)92620-1.
7. Manseau MW, Goff DC. Cannabinoids and schizophrenia: Risks and therapeutic potential. Neurotherapeutics. 2015 Oct;12(4):816-24. doi: 10.1007/s13311-015-0382-6.
8. Zuardi AW, Crippa JAS, Hallak JEC, et al. Cannabidiol for the treatment of psychosis in Parkinson’s disease. J Psychopharmacol. 2009;23:979–983. doi: 10.1177/0269881108096519.
9. Rothman K, Greenland S. Causation and causal inference in epidemiology. Am J Public Health. 2005, 95(S1), S114-150.
10. Proal AC, Fleming J, Galvez-Buccollini JA, Delisi LE. A controlled family study of cannabis users with and without psychosis. Schizophr Res. 2014 Jan;152(1):283-8. doi: 10.1016/j.schres.2013.11.014.
11. Proctor RN. The history of the discovery of the cigarette–lung cancer link: evidentiary traditions, corporate denial, global toll. BMJ. 2012;21:87-91.
Justification for the topic
As a student surrounded by cannabis users, I have often wondered whether the short-term effects of using cannabis could have long-standing consequences on mental health. I opted to pursue this topic because I wanted to find out what studies showed about the chronic use of marijuana. On a personal level, I hoped that results from this study would increase my knowledge beyond what I already on the effects marijuana has on one’s current mental state. The government only just legalized recreational marijuana, but we won’t comprehend the results for years to come. We are currently aware that long-term use may pose a risk to mental health. However, the legalization may present more opportunities to investigate ways to reduce the possibility of harmful effects. Like other prior studies on the subject, this research could prove useful to the general public and the government when forming policies on marijuana use.

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