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How Marijuana Really Affects Behavior

A large review compiles the evidence on how marijuana affects lives.

Key points

  • As marijuana becomes decriminalized, the evidence shows an increase in its use.
  • Marijuana alters cognitive function and reduces motivation.
  • There is also a clear relationship between marijuana use and psychosis.

Weed, pot, grass: Whatever you want to call it, marijuana is steadily becoming legal in more U.S. states. The recreational use of cannabis is legal in 14 states and has been decriminalized, meaning it carries a very small penalty, in 16 others. In addition, 36 states allow the use of marijuana for medical reasons with a doctor’s prescription.

As marijuana becomes decriminalized, it’s no surprise that the evidence shows an increase in its use. Although many believe it to be harmless, marijuana contains compounds with psychoactive properties that affect a wide range of brain functions including pain, motivation, memory, mood, and reward processing.

A new systematic review looks at the body of evidence to answer the question: Does marijuana use lead to negative behavioral consequences?

To answer that question, researchers from the University of Toronto collected 124 studies conducted from 1995 to 2020 that looked at how recreational marijuana use affected mental health and functioning. They identified four main areas where marijuana had negative consequences for mental health.

How Marijuana Affects Cognition

The researchers found chronic marijuana use impacts certain aspects of cognition including memory, decision-making, and attention. Specifically, they found that using more marijuana led to larger lapses in memory, mainly among participants who began using marijuana as adolescents.

They also found that cognitive impairments occurring while users were high were greater among participants who had not built up a tolerance to marijuana.

How Marijuana Affects Motivation

Compared with cognition, there is less evidence on how marijuana use affects motivation. The lack of standardized ways to define and measure motivation makes it more difficult for researchers to study. In addition, the review authors only found cross-sectional studies on how marijuana affects motivation, which provides a snapshot in time rather than randomized, controlled trials which could provide more definitive causal explanations.

Even considering the limitations, there is some evidence that chronic marijuana use leads to reduced motivation, “a loss of desire to work or compete” and less interest in setting and attaining goals.

How Marijuana Affects Psychosocial Functioning

A considerably large set of data shows when young people use marijuana consistently during their development, they are less likely to finish high school and less likely to complete a college degree. High schoolers who use marijuana regularly are less likely to attend class, complete their homework and get, or even value, good grades. Some of the evidence suggests that people who start using marijuana in early life and who use it frequently have less economic success compared to the general population.

The review authors found one study conducted in New Zealand that followed a group of children through middle adulthood while tracking their marijuana use. Participants were more likely to use marijuana chronically as adults if their parents used it, if they were diagnosed with a conduct disorder, if they were classified as novelty-seeking, or if they experienced trauma as a child. And those who used marijuana frequently as adults were more likely to experience mental health problems and abuse additional substances, even after controlling for other factors.

The Impact on Psychiatric Health

There is some evidence that frequent marijuana use leads to mental health problems, but the issue is complicated and not fully explained by the data. For example, many studies show that the more marijuana someone uses beginning in adolescence, the more likely they will experience symptoms of depression. But the studies are inconsistent, and some of them don’t account for other factors, such as nicotine or alcohol use.

The evidence on marijuana and anxiety is also mixed. Some studies show that marijuana use leads to increased feelings of anxiety. But there are other randomized-controlled studies of cannabidiol, or CBD, that find CBD reduces anxiety on social stress tests.

Researchers don’t understand the mechanisms that connect marijuana use, depression, and anxiety. They hypothesize that marijuana use causes a neurobiological chain of events that can lead to mental health problems. But mental health symptoms could also be related to the psychosocial aspects of marijuana use; for example, lower educational attainment is also associated with a risk of depression and anxiety.

Researchers did find a strong, clear relationship between marijuana use and psychosis, a condition where the mind loses some contact with reality. Heavy, more frequent marijuana users and those who begin using marijuana earlier in life are at a greater risk for psychosis.

Taken together, these data paint a cautionary picture on marijuana use with some clear take-home messages: There is evidence that marijuana, especially when started at a young age and used frequently, may lead to mental health problems and affect the user’s educational attainment and employment options. The strongest evidence suggests that marijuana use could lead to psychosis, and may also contribute to depression and anxiety, and affect cognitive function.

Visit Cornell University’s Bronfenbrenner Center for Translational Research’s website for more information on our work solving human problems.

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