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Long-Term Effects of Marijuana on the Brain – What to Know

effects of weed on the brain

Do you know that marijuana is the most commonly used psychotropic drug in the United States, right after alcohol? Numbers show that in 2018, 11.8 million young adults in the U.S. reported marijuana use in the past year. The use of marijuana is more common in men than in women. Even though cannabis consumption keeps increasing, most people are not aware of all the effects of marijuana on the brain. In this post, we discuss what effects marijuana could have on brain health with long-term use. Are there any risks? Scroll down to find out.

Short-term effects of marijuana 

We’re going to start with a brief rundown of short-term effects on brain and body associated with marijuana use. They include:

  • Impaired memory
  • Lower anxiety
  • Disrupted motor control
  • Bigger appetite
  • Faster heart rate
  • Sleep pattern changes
  • Decreased pain

How marijuana affects cognitive functions long-term ?

Despite the ever-growing use of marijuana, the effects of long-term use are still not thoroughly elucidated. The reason is simple; a vast majority of studies on cannabis focused on short-term effects and results that people can experience. 

However, some long-term effects of marijuana are short-term carry-over effects. 

Why does that happen?

The body fat attracts THC, the psychoactive cannabinoid responsible for the “high” effect. Since the cellular walls in the body are comprised of fat, it’s easy for cells to absorb THC. What happens next is that THC is re-released gradually over a certain period of time. 

If you’ve ever wondered how a person can test positive for marijuana even when not using it for several days now, you know the reason. Traces of THC persist in the body for up to a month after you stop smoking. This also explains why some short-term effects of marijuana use may last for a while.

Before you start thinking this is just a theory, it’s worth noting the scientific evidence confirms it. For example, a study from the Addictive Behaviors showed that people do experience some effects of marijuana for a while even after they stop using it. The main objective of this cross-sectional study was to examine relationships between recent and past cannabis use on neurocognitive functioning. 

For this purpose, the scientists enrolled 158 subjects into the study. Of these, 68 participants were recent users, 41 were past users, and 49 subjects were non-users. Past users were those who haven’t used marijuana for more than 28 days or four weeks.

Results showed that recent marijuana users demonstrated significantly worse attention and working memory performance than non-users. Their information processing speed and executive functioning were also worse than in people who have never used marijuana. Interestingly, there was no statistically significant difference between recent and past cannabis users in terms of neurocognitive performance. 

Although cognitive functions were the worst in recent marijuana users, past users performed more poorly on measures of executive function than non-users. This implies that the effects of cannabis on the brain don’t go away once a person stops using it; they remain for a while. 

These findings dictate that as long as you have THC in the body, marijuana will continue to affect the brain. 

While it’s evident that marijuana can impair cognitive functions, the effects on the brain itself and its structure and functions need more attention and further investigation. This subject is not totally clear, but several studies have inspected it, and we’re going to discuss their findings below.

Long-term effects of weed on the brain

As the rates of marijuana use increase, so do the questions about its influence on the brain. The research mostly focuses on short-term effects, as mentioned above, but studies on the long-term impact of cannabis are emerging, and we can expect even more of them in the future.

A study from the Proceedings of the National Academy of Sciences (PNAS) found that marijuana use can change brain structures, but further longitudinal study approaches are necessary to uncover more on this subject. The research included 110 participants, of which 62 were non-users, and 48 people were cannabis users. Marijuana users were enrolled in the study if they currently use marijuana regularly at least four times a week over the last six months.

The findings showed that heavy, chronic marijuana users have lower orbitofrontal cortex (OFC) gray matter volume compared to their counterparts who don’t use cannabis. The orbitofrontal cortex is the area of a prefrontal cortex that sits above the eye sockets. It is located at the front of your brain. This part of the brain has extensive connections with sensory areas and limbic system structures that are involved in memory and emotion. The OFC also receives information about the sight of the objects.

The effects of marijuana use on OFC aren’t as shocking if we bear in mind that OFC is the main area in the reward network. Moreover, OFC is abundant in cannabinoid receptor CB1, and it’s significantly implicated in addictive behaviors. Since CB1 receptors, which are a part of the endocannabinoid system, are positioned on excitatory terminals of cortical projection neurons, then downregulation of these receptors negatively affects the plasticity of OFC. While scientists couldn’t identify all the mechanisms through which marijuana use decreases OFC gray matter volumes, they theorize it could be down to changes in cell size, neuronal loss, and reduction in CB1 density. 

In other words, the PNAS study confirms the unfavorable effect of marijuana on OFC connectivity but also emphasizes the complexity of this problem. You see, greater functional connectivity of OFC was linked to an earlier age of onset of regular use, while chronic marijuana use led to slower connectivity. This relationship only paints a picture of how complicated the effects of marijuana on the brain really are. Although some evidence shows that CB1 receptor down-regulation may return to normal values with marijuana abstinence, scientists concluded that more research is needed to confirm that.

Evidence also shows that marijuana use can affect white matter, and the younger a person is when he starts smoking cannabis, the greater the damage. This study showed that marijuana use had no impact on cortical volumes.

The journal PLoS One published a study which found that morphological brain alterations are observed in adults and adolescents who use marijuana. These alterations are especially pronounced in the medial temporal and frontal cortices and cerebellum. The effects are associated with the amount of exposure to cannabis.

When we’re talking about brain volume and cannabis use, it’s important to mention that evidence shows people with psychosis are more prone to these changes. However, the impact of marijuana on the brains of people with psychological problems and risks of psychosis should be further elucidated. 

However, not all studies on this subject agree that marijuana is responsible for brain changes. For instance, the JAMA Psychiatry published a study which showed that exposure to marijuana was related to smaller changes in the left amygdala and right ventral striatum volumes. That being said, these changes weren’t that significant. They were within normal variations, scientists explain. This led the scientists to conclude that differences in amygdala volume in marijuana users could be attributed to common factors ranging from genetic to environmental origin. The causal influence (of cannabis use on changes in amygdala volume) has little support. 

Just a reminder, the amygdala is an important part of the brain, and it plays a key role in emotion and behavior.

Brain development changes with marijuana

Human brain changes and develops throughout life, but it is the most malleable in a period between birth and the 21st birthday. Since many teenagers start smoking marijuana and continue to do so well into adulthood, it’s impossible not to wonder how cannabis affects developing brains. A study from the Brain journal attempted to answer that question. 

A team of researchers compared MRI scans of subjects who have started using marijuana in adolescence with the scans of participants who have never used cannabis. In marijuana users, the neural connections between the right and left brain hemispheres were decreased. This finding means that internal communication in the brains of marijuana users is lower than in non-users.

Despite the importance of the study and its results, you still need to keep in mind a few things. Like a vast majority of studies on the effects of cannabis, this one also doesn’t show what happens when a person stops using marijuana for a certain period of time. In other words, this research doesn’t show whether internal communication in the brain improves once a long-time marijuana user decides to stop. Some impairments could have been in the brain before marijuana use in the first place.

On that note, another study found that people who started using cannabis in their adolescent years had lower IQs in their 30s compared to childhood. The frequent cannabis use is associated with a loss of six to eight IQ points measured in mid-adulthood. 

The greatest drop in cognitive performance was among people who started using marijuana the youngest and who smoked most heavily. The weak cognitive performance was independent of their socioeconomic status and other parameters. Interestingly, this study showed that abstaining from marijuana doesn’t fully restore neuropsychological functioning. In other words, those subjects didn’t restore the lost IQ points after abstaining from marijuana. 

Another piece of evidence shows that young marijuana users experience a significant decline in verbal ability and general knowledge between the preteen years and late adolescence or early adulthood. That being said, those who started using cannabis at an older age already had low scores at the beginning of the study. More precisely, they had low scores on the brain functions even before they started using cannabis.

Can long-term marijuana use harm my brain?

The influence of marijuana use on the brain is a subject of many studies, but many of them have conflicting results. Several studies have shown that marijuana can, indeed, affect cognitive performance and induce some structural changes in the brain. 

But, this doesn’t mean marijuana can destroy the brain entirely. 

Long-term and heavy cannabis usage (e.g., from adolescence to adulthood) can slow down the brain, but won’t scramble it. 

The negative effects of marijuana on the brain are more pronounced in adolescent cannabis smokers who continue using it into adulthood. 

A lot more studies are necessary to determine just how much marijuana use can impair the brain’s functions. 

One of the greatest obstacles that prevent scientists from learning all the short- and long-term effects of marijuana on the brain is that study participants often use multiple substances. Data about the participants’ health and mental functioning before the study is limited. This also doesn’t allow scientists to take a look at the broader picture. Changes in brain and cognitive functions can occur due to a wide range of reasons, including another illicit drug, but without enough data, it’s difficult for scientists to rule out other potential factors.

How marijuana affects memory?

As seen throughout this post, marijuana can have a significant impact on brain function and cognitive abilities, including memory. 

But have you ever wondered why smoking marijuana could impair your memory and make you forgetful?

Memory impairment from cannabis happens because THC changes the way the hippocampus processes information. Don’t forget that the hippocampus is an area of the brain responsible for the formation of memory.

As you age, the number of neurons in the hippocampus decreases. As a result, it becomes more difficult to learn and store new information. Chronic exposure to THC could accelerate the age-related loss of neurons in the hippocampus. In turn, this area of the brain can’t form memory properly. 


In this post, we discussed the long-term use effects of weed on the brain. Evidence on this subject is still conflicting, but scientists agree that early onset of marijuana use can induce structural changes in the brain and lower IQ scores. A lot more research is necessary to learn more about this subject.