Casual Marijuana Use and Psychological Issues

Research findings regarding casual marijuana use and its effect on psychological functions such as intelligence, motivation and mental health issues are issues worthy of discussion. While we assume that too much of anything is bad for you, how exactly does occasional marijuana usage affect a person’s psychological health?

Controversy: Marijuana and Cognitive Functions

There is a large body of research that has investigated the cognitive functioning in both heavy and casual marijuana users. Many of these studies are targeted at adolescent usage because the vast majority of studies looking at marijuana usage in adults over the age of 25 have indicated that there are no significant effects on intelligence (Hatfield, 2013). The research findings on marijuana usage and its effects on intelligence, or IQ scores, in adolescents have been mixed. There have been studies that have found that users have no significant changes in their IQ scores compared to non-users and other studies that report significant effects (Hatfield, 2013).

Most of the studies demonstrating an effect of marijuana usage on IQ scores note that these effects occur with heavier, chronic marijuana usage. For example, in a highly publicized and much cited recent study Meier et al. (2012) followed 1037 individuals from birth until they were 38 years old. Marijuana use was measured at ages 18 – 21 – 26 – 32 – and 38 years of age. Cognitive neuropsychological testing was performed well before any of the participants had used marijuana and then later after a persistent pattern of marijuana use. Statistically significant drops in IQ scores were noted from adolescence to adulthood in individuals who persistently used marijuana, with greater usage resulting in higher drops in IQ scores.

However, in a direct response to the Meier et al. Mokrysz et al. (2014) compared IQ scores between 2612 adolescents who reported using only marijuana once as compared to chronic users. The results indicated that there was a statistically significant difference in IQ scores with lower scores being associated with more chronic users; however, the statistical analysis indicated that this was more likely due to heavy alcohol usage and other lifestyle factors and not to marijuana usage. Thus, the researchers concluded that Meier et al.’s results were due to other lifestyle variables and any change in IQ scores observed in these studies is not due to marijuana usage. Similar mixed findings have been found with memory and other cognitive functions (e.g., see Ashtari et al., 2011).

What is the bottom line? One of the striking things is that the mean decline on an IQ score or a memory score performance observed in the majority of these studies is most consistent with something called measurement error. Whenever you give someone a standardized test there is always the potential for fluctuation in the score due to errors of administration, scoring, etc. by the person giving the test or in the motivation, physical state etc. of the person taking the test. This is especially relevant when individuals are given the tests more than once over a time period of years. In order to compensate for error neuropsychologists will often report “confidence intervals”, a range of scores/values that reflect where the true score of the person being tested probably lies. When looking at this research the so-called drops in scores, even though they are statistically significant, are often within this band of normal variation, or once the expected normal variation in the score is considered any differences beyond this range are not significant. Thus, what the majority of these studies are most likely measuring are the normal variations in test scores that could occur due to being given the test more than once over a period of time.

The research here is inconclusive. What is known is that when a person uses marijuana there are temporary changes in their attention span, memory, and other thinking abilities while they are under the influence of the drug. Thus, while the long-term effects of casual marijuana usage on cognition are not well defined, the short-term temporary effects that occur when one is actively using marijuana are known. Anyone using marijuana should not be driving an automobile, working with machinery, etc. until the temporary effects have dissipated.

Marijuana Use and Motivation

Certainly anyone who maintains a constant “buzz” by chronically using marijuana will most likely present as less industrious or lacking motivation. But, does casual marijuana use result in decreased motivation?

The National Institute of Health (NIH, 2014) reports that marijuana users are more likely to do more poorly in school or drop out of school; however, such findings may not be due to marijuana’s effect on their motivation but could be due to any number of other factors including pre-existing personality issues, mental illness, lifestyle issues, etc.

A number of research studies have linked marijuana usage to a decrease in motivation (often called the amotivational syndrome) via a decline in the neurotransmitters such as dopamine or serotonin or to other factors (e.g., see Bloomfield et al., 2014). On the other side of the coin, the so-called “amotivational syndrome” where individuals have some type of disorder that leads to lack of motivation, is not really a formal disorder in any diagnostic classification system (American Psychiatric Association [APA], 2013).

This is an easy issue to resolve. In the Bloomfield et al. study as in most of these studies there are a number of methodological issues that make the study inconclusive such as a small sample size, the amount of marijuana being used by the participants, etc. Moreover, most of the studies of this type use participants who are chronic users or very heavy marijuana users. Based on the status of the current research it is safe to say that casual marijuana use has not been definitively linked to laziness, apathy, or lack of motivation. Likewise reports that marijuana usage increases creativity are invalid and not supported by research (e.g., see Kowal et al., 2014).

Marijuana and Mental Illness

Certainly research has indicated that even casual marijuana usage and certainly heavy use can exacerbate the symptoms of people who have some form of mental illness including schizophrenia, clinical depression, bipolar disorder, anxiety, suicidality, etc. (NIH, 2014). Moreover, it is well-documented that individuals with mental illness have higher rates of substance abuse (APA, 2013). However, the connection between mental illness and higher rates of substance abuse has never been shown to be a causal one such that using a substance like marijuana causes someone to develop a mental illness. The use of any type of substance can increase temporary feelings of depression or anxiety in some people without mental illnesses; however, it is also certainly true that using a substance may decrease these feelings in certain people. Based on the research it is safe to say that anyone with a diagnosed psychiatric disorder should not be using marijuana, except when instructed to do so by their physician, and that casual marijuana use has not been definitively linked to causing any form of mental illness.


American Psychiatric Association. (2013). The DSM-5. Washington DC: Author.

Ashtari, M., Avants, B., Cyckowski, L., Cervellione, K. L., Roofeh, D., Cook, P., … & Kumra, S. (2011). Medial temporal structures and memory functions in adolescents with heavy cannabis use. Journal of psychiatric research, 45(8), 1055-1066.

Bloomfield, M. A., Morgan, C. J., Kapur, S., Curran, H. V., & Howes, O. D. (2014). The link between dopamine function and apathy in cannabis users: an [18F]-DOPA PET imaging study. Psychopharmacology, 231(11), 2251-2259.

Hatfeld, R. C. (2013). The everything guide to the human brain. Avon, MA: Adams.

Kowal, M. A., Hazekamp, A., Colzato, L. S., van Steenbergen, H., van der Wee, N. J., Durieux, J., … & Hommel, B. (2014). Cannabis and creativity: highly potent cannabis impairs divergent thinking in regular cannabis users. Psychopharmacology, 1-12.

Mokrysz, C., Gage, S., Landy, R., Munafò, M. R., Roiser, J. P., & Curran, H. V. (2014). Neuropsychological and educational outcomes related to adolescent cannabis use, a prospective cohort study. European Neuropsychopharmacoly, 24(2), S695.

National Institute of Health (2014). Drug Facts: Marijuana.

Dr. Hatfield is a clinical neuropsychologist with extensive experience assessing and treating neurological and psychiatric disorders. His areas of expertise include neurobiology, behavior, dementia, head injury, addiction, abnormal psychology, personality disorders, statistics, rehabilitation psychology and research methodology.

Copyright © 2022 MH Sub I, LLC. All rights reserved.
Terms of Use | Privacy Policy | Cookie Policy | Health Disclaimer | Do Not Sell My Personal Information |