Everyone knows that the brain is complicated. Yet, even with all of the scientific and medical advances that have been made, we still don’t know everything there is to know about the brain. What we do know, is that it is responsible for nearly everything we do.

The brain is always on, processing a constant stream of information. Then, based on this information, it tells the body how to react. However, something so complex doesn’t just develop and start operating perfectly overnight. Like the rest of the body, the brain has to undergo puberty to develop and reach its “adult” state. This starts in adolescence, but doesn’t stop until the mid-20s. (Yes, this means that you can vote, buy a car, get a loan, and are even considered an adult in the eyes of society before your brain has fully matured.)

So, what is puberty for the human brain?

Scientifically, puberty for the brain is a time when it undergoes growth and refinement, also known as neuromaturation. In other words, the brain is remodelling itself to ensure more efficient communication takes place between its different regions. This is done through an increase in white matter and a decrease in grey matter.

However, as any teenager can attest to, it’s also a time of increased vulnerability. Especially if something unexpected or foreign is added to the mix, and you have no idea how to handle it.

The active ingredient in marijuana is tetrahydrocannabinol (THC), and it is a foreign substance in both the body and brain.
In animals, studies have shown that the adolescent brain is particularly vulnerable to the effects of THC, and that exposure can result in permanent structural changes with persistent cognitive and behavioural changes.

In humans, current research has shown similar trends.

Structurally, imaging studies of the brains of marijuana users have shown disordered white matter. And, a greater degree of disorder was found with a greater use of high potency – high THC level – marijuana. However, because of how the studies were designed, it is not possible to prove whether the white matter changes in the brain occurred because of marijuana use, or if the white matter changes in the brain are what lead people to use marijuana.

Now, while the extent to which marijuana effects the structure of the brain has not been 100% proven, marijuana use does effect behaviour and cognition.

Among adolescents, issues with memory, learning, problem-solving, attention and impulsivity were observed. Further, these issues didn’t just disappear after the adolescents stopped using marijuana – some actually remained after weeks of abstinence. And, it’s not just adolescents experiencing behaviour and cognition deficits. In adults, numerous studies have reported impairment in inhibition, decision making, and executive control.

One thing to note is that these issues are similar to the symptoms experienced by people with ADHD. So, does this mean that there is a relation between ADHD and marijuana use?

Multiple studies have shown that, compared to the general population, ADHD youth begin using substances earlier and are at greater risk of developing a substance use disorder (SUD). And, it’s not just youths. In general, people with ADHD are approximately two-times as likely to develop a SUD. Plus, if they do develop a SUD, they become addicted at a much younger age, use more substances, and are hospitalized more often than people without ADHD.

But why?

One hypothesis is that risky substance use may be due to the difficulties people with ADHD have around regulating their consumption – aka impulsivity issues. However, research has suggested that this may also be due to how the brain is structured. Dopamine imbalance in the brain has been shown to play a role in both ADHD and SUD. Additionally, this might be why there is also an over-representation of ADHD among people with SUD, with approximately one in four people with SUD meeting criteria for ADHD.

So if this is the case, and people with ADHD are more likely to develop SUD, why are they using marijuana?

An analysis of a forum about marijuana use and ADHD found that, despite a lack of clinical recommendations or research suggesting beneficial effects of marijuana for ADHD, most people were using marijuana because they believed that it would help with their ADHD symptoms, or help them better manage side effects. In other words, subjectively, they felt less anxious, more in control and, overall, better when they used marijuana.

Additionally, it was found that the course, rather than the severity, of ADHD symptoms impacted future substance use. In other words, adolescents with worsening ADHD symptoms predicted higher levels of early adult substance use. In contrast, adolescents with improving, or even stable, ADHD symptoms predicted lower levels of early adult substance use. And, in other research, it has been found that treating ADHD actually decreases substance use among people with ADHD.

But what happens when people with ADHD use marijuana?

Initially, the hypothesis was that people who use marijuana and have ADHD would experience a worsening of their ADHD symptoms, because marijuana has been shown to mimic symptoms of ADHD in non-ADHD people.

One study examined the impact of ADHD and marijuana use on executive functioning in young adults. This study found that the ADHD group performed worse than the control subjects in areas of verbal memory, processing speed, cognitive interference, decision-making, working memory, and response inhibition. However, this is typical of people with ADHD when compared to the general population. The key finding of the study was that, when marijuana was added, there was no significant impact on executive functioning among marijuana users with ADHD. In other words, people with ADHD who used marijuana did NOT improve, but they also did not get worse.

In general, when people with ADHD have reported an improvement when using marijuana, this is because of the anti-anxiety effects of the drug. Objectively, marijuana does not improve ADHD symptoms. However, subjectively, people report an improvement because they experience less stress about having ADHD as their overall anxiety decreases.

So, this raises the alternative hypothesis: if people with ADHD (subjectively) feel better, they might be using marijuana as a form of self-medication. This hypothesis is supported by some ADHD teens who say that marijuana helps them cope, as well as one small study discussed below.

Now, the study did have some faults, and the results were not statistically significant. So, the results need to be interpreted with caution, and further research into this topic is still needed. However, the study does provide some early evidence supporting the self-medication theory of marijuana use in ADHD. As the study noted, there were minor improvements in ADHD symptoms among the people using the cannabinoid spray as opposed to the placebo.

Additionally, while the study did not report any negative cognitive effects, the authors proposed two explanations for their findings that are worth mentioning. The first, is that the duration of the study was too short and the study group small. This means, marijuana use could still have been harmful and worsened ADHD symptoms, but it was just too early/the duration of marijuana use too short to be able to tell. The second is that the spray used in the study had a 1:1 of cannabinoidiol (CBD)-to-THC. This 1:1 ratio is NOT typical of what is being bought on the streets. Commonly purchased marijuana has been found to have much higher levels of THC, and these levels are not balanced with CBD. This is important, because previous studies have suggested that THC is responsible for the cognitive issues caused by marijuana use, while CBD is likely to protect against these impairments.

Now, the decision to use marijuana is a personal decision. Hopefully, this blog will help you make an informed decision about whether marijuana use is right for you. So, if moving forward you choose to use marijuana, here are 10 recommendations, based on medical research, for lower-risk marijuana use:

  1. The most effective way to avoid marijuana-use related health issues is to abstain from use.
  2. Avoid using marijuana at a young age (<16yrs).
  3. Choose low potency THC or a balanced THC-to-CBD marijuana products.
  4. Abstain from using synthetic cannabinoids (which have been found to have 80-90% more THC).
  5. Avoid smoking marijuana and give preference to non-smoking methods.
  6. Avoid deep or other risky inhalation practices.
  7. Avoid high frequency (e.g., daily or near daily) marijuana use.
  8. Abstain from marijuana-impaired driving.
  9. Populations at higher risk for marijuana-related health problems (pregnant women, adolescents and children, etc.) should avoid use altogether.
  10. Avoid combining previously mentioned risky behaviours (e.g., early inhalation and high-frequency use).
  11. Speak to your doctor about any health concerns you may have related to marijuana use.


Amanda Marchak, BSc Materials Engineering, MD Candidate (Class of 2019)
Amanda Marchak completed a Bachelors of Science in Materials Engineering at the University of Alberta, with a special interest in biomedical applications. After realizing that her interests were more clinical-based as opposed to research-based, she began pursuing a medical degree at the University of Calgary. Amanda is set to graduate in 2019, and hopes to pursue a career in pediatrics. As a volunteer at the the CanLearn Society in Calgary, Alberta, she works in collaboration with a team of psychologists and physicians to help provide information to individuals, and their families, about ADHD and Learning Disabilities.