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Discussion of Whether Marijuana Has Negative Effects on Mental Health

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Table of contents

  1. Introduction
  2. Evidence supporting that marijuana has a negative impact on mental health
    Evidence supporting that marijuana does not have a negative impact on mental health
  3. Conclusion
  4. References


With the recent country-wide legalization of marijuana in Canada, the recreational and medical use of marijuana is becoming more frequent. Marijuana is known to have many different effects ranging from pain relief to temporarily inducing anxiety. There are many uncertainties about its effect on people’s mental health and much debate on whether it helps or not. The public concern and interest over the potential dangers of marijuana has led to research on whether or not the use of marijuana has a negative impact on a person’s mental health. With extensive consideration, it was determined that marijuana use does have a negative impact on mental health.

Evidence supporting that marijuana has a negative impact on mental health

It has been shown that people who chronically use marijuana show symptoms of anxiety and depression, however, it is unclear whether these symptoms are caused by the use of marijuana or if they are pre-existing. In multiple studies it was found that long-term use of marijuana can increase the symptoms of depression and anxiety. One study was done with 1601 students aged 14-15 to determine if the use of marijuana can cause symptoms of depression and anxiety. Each student indicated their frequency of marijuana usage, symptoms of depression and anxiety, use of alcohol and illicit drugs and antisocial behaviour. They all had a follow-up report 7 years after these initial inquiries as well as a report every 6 months leading up to the final year of the study. The participants had not shown any signs of mental illness at their initial evaluation. Out of the male participants that used marijuana weekly, 49% of them reported symptoms of depression and anxiety. The results for the females showed that the girls who used marijuana almost daily had five times more people with depression and anxiety than the girls who did not use marijuana. Similarly, it was found that females who used marijuana daily as teenagers were four times more likely to form symptoms of depression and anxiety later on, and females who used it weekly as teenagers were twice as likely to form those symptoms later on. The study indicated that those who started frequently using marijuana had greatly increased depression and anxiety as opposed to those who did not use it or used it very little.

Frequent marijuana use may also increase the risk of developing schizophrenia. A study was done with 45570 Swedish conscripts aged 17-21 analyzing their drug use, alcohol use, social background, psychiatric history and psychiatric state. The study was done over a period of 15 years and each participants’ psychiatric state was tested by a psychiatrist and a diagnosis was formed if necessary. The results showed that those who used marijuana were 2. times more likely to develop schizophrenia than those who did not use at all. As the use of marijuana increased for participants, their likelihood of developing schizophrenia compared to non users increased by 6 times. All of this was concluded with the consideration of all social and medical external factors. Although the incidence of people who used marijuana in this study was not frequent, the results show a clear trend that marijuana use can lead to the onset of schizophrenia.

There has also been research that demonstrates that the use of marijuana may worsen already existing mental illnesses such as PTSD. A 19-year study was done to observe the effects of marijuana on 2276 patients with PTSD. Each person was admitted to a treatment program and classified as a never-user (did not use marijuana at the time of admission and discharge), stopper (did use marijuana at admission but stopped after discharge), continuing user (used at both admission and discharge), or starters (did not use at admission but did at discharge). The study analyzed their symptoms of PTSD, violent behaviour as well as drug and alcohol use. It was found that continuing users and starters had a large increase in the severity of their symptoms of PTSD as well as increased violence, drug and alcohol use. The never-users and stoppers had the smallest and least severe symptoms of PTSD. It was therefore concluded that frequent marijuana use could lead to worsening symptoms of PTSD and the stopping of marijuana use can help decrease the severity of PTSD.

Evidence supporting that marijuana does not have a negative impact on mental health

Other studies have shown that marijuana can be used to treat mental illnesses and therefore have a positive impact on those people’s mental state. There has been a study that shows people who had severe social anxiety and a phobia of public speaking had greatly lessened symptoms after using marijuana. It has already been shown that CBD in marijuana has anxiolytic effects (reduction of anxiety) on the limbic and paralimbic areas of the brain, which are mainly associated with emotions and thinking. This study randomly selected 24 subjects with generalized social anxiety disorder (SAD) and 12 without (healthy controls). In a double-blind procedure, 12 SAD subjects were given CBD, another 12 were given placebo and the remaining 12 healthy controls were given no medication. Each subject had not used marijuana or any other illicit drug within the past year and no more than 5 times in their lifetime. After the subjects were given their according doses, they had to perform a 4 minute speech in front of cameras. Measurements of factors indicating anxiety levels were taken before the subject’s dose of CBD or placebo, 80 minutes after ingestion, right before their speech, during their speech, 15 minutes after the speech and 35 minutes after the speech. The results from this experiment indicated that patients who were given CBD had significantly reduced anxiety during their speech. When they were compared with the healthy controls, it was revealed that their levels of discomfort, alertness and cognitive impairment were very similar, indicating that CBD helped SAD patients achieve a healthy mindset. It can be concluded from this experiment that marijuana does have a quick therapeutic effect on people experiencing anxiety.

Another study has shown that marijuana is beneficial to people with schizophrenia. In this study, patients with schizophrenia were randomly selected and used in a double-blind test. Similar to the public speaking experiment, 45 patients were given CBD and 45 patients were given placebo. They were all assessed before and after their treatments and the results showed that after 6 weeks, patients who used CBD marijuana had lower levels of schizophrenia symptoms than the placebo group. According to the treating clinician, the CBD treated groups’ symptoms had improved and were considered less unwell compared to the placebo group. This can conclude that CBD marijuana helps in reducing symptoms of schizophrenia.

There has also been research that shows marijuana can help with bipolar disorder. Over a period of 4 weeks, 12 subjects with bipolar who smoke marijuana (MJBP), 18 with bipolar that do not smoke marijuana (BP), 23 without bipolar that do smoke marijuana (MJ) and 21 healthy controls (HC) rated their mood 3 time daily including right after each use of marijuana. The results demonstrated that the MJBP group had significantly lowered their bipolar symptoms such as lower levels of anger, tension and depression in comparison to the BP group. It showed that there were some short-term benefits to smoking marijuana for those with bipolar, however there was no evidence to show it had long-term benefits. Overall these studies concluded that marijuana has a positive effect on existing mental health disorders.


In the study which concluded that marijuana had a negative impact on patients with PTSD, there seemed to be some confounding effects. In each patient that frequently used marijuana and had worsened symptoms of PTSD, they also had an increased use of other drugs and alcohol. It is therefore unclear in this study if the drugs and alcohol are actually a factor that worsened their symptoms rather than the marijuana. In the study which concluded that marijuana can cause schizophrenia, the results were determined with strict consideration of all external factors. The other conditions that could possibly influence the development of schizophrenia such as family background, socioeconomic class, living conditions, alcohol use, other drug use, etc. were all considered and adjusted so as not to affect results. In the case of marijuana causing depression and anxiety, it was not stated in the experiment whether a low or high dose of THC marijuana was used. If the patients were given high doses, this would explain why they had increased anxiety because high doses of THC produce anxiogenic reactions. In addition, none of these studies had any actual medical reasoning or explanation to their conclusions, instead all of it was based on observations and surveys. However, each of these studies were conducted over long periods of time (many years) and therefore do provide a more accurate conclusion on the true effect on a person’s mental health.

In comparison, the study that concluded CBD helped reduce symptoms of anxiety during public speaking was a double-blind design with randomization which decreases the bias of the experiment as well as increases its validity. However, the results of this experiment do not necessarily prove that marijuana has a positive impact on mental health because the results were very short term and situational. It was proven to help temporary relieve anxiety of public speaking but it cannot determine any long-term effects or its effects in other occurrences of anxiety. For the study of marijuana reducing bipolar symptoms, the results showed that MJBP subjects all had overall worse symptoms of bipolar than the BP subjects. This may indicate that even though their symptoms improved right after marijuana use, these improvements are very temporary and do not actually help with long-term bipolar disorder.


Overall, the research claiming that marijuana has a positive impact on mental health contained a lack of evidence and was too specific. Not many external factors that could affect results were taken into consideration in each experiment. The validity and significance of the experiment claiming that marijuana helped with bipolar symptoms had little significance because it showed that the improvements only lasted for a short period of time after marijuana use. Overall the evidence showing that marijuana helps mental health only proved true for short-term relief and specific situations. Short-term relief of symptoms does not indicate actual improvement in mental health because mental disorders are long-lasting issues that need long-term solutions. The research claiming that marijuana has a negative impact on mental health provides good long-term evidence. It provided information on the development of anxiety, depression and schizophrenia due to marijuana use as well as the worsening of PTSD symptoms due to marijuana use. Although one experiment contained some uncertainty, the studies were all done with the use of good procedures that helped make the overall experiment a very accurate and reliable source of evidence. Therefore, it can be concluded that the use of marijuana does have a negative impact on a person’s mental health by causing the development of symptoms or worsening existing symptoms.


  • Patton, GC. et al. Cannabis use and mental health in young people: cohort study. BMJ. 325, 1195-1198 (2002)
  • Andreasson, S. & Allebeck, P. & Engstrom, A. & Rydberg, U. Cannabis and schizophrenia. A longitudinal study of Swedish conscripts. Lancet. 330, 1483-1486 (1987)
  • Wilkinson, ST. & Stefanovics, E. & Rosenheck, RA. Marijuana use is associated with worse outcomes in symptom severity and violent behavior in patients with posttraumatic stress disorder. J Clin Psychology. 76, 1174-1180 (2015)
  • Bergamaschi, MM. et al. Cannabidiol Reduces the Anxiety Induced by Simulated Public Speaking in Treatment-Naive Social Phobia Patients. Neuropsychopharmacology. 36, 1219-1226 (2011)
  • McGuire, P. et al. Cannabidiol (CBD) as an Adjunctive Therapy in Schizophrenia: A Multicenter Randomized Controlled Trial. Am J Psychiatry. 175, 225-231 (2018)
  • Sagar, KA. et al. Joint Effects: A Pilot Investigation of the Impact of Bipolar Disorder and Marijuana Use on Cognitive Function and Mood. PLoS ONE. 11, (2016)

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