Marijuana – Understand the current research on weed


In this piece we aim to equip the reader with studies and research articles to give them an overview understanding and easy access to resources for their own purposes.

We will categorize each topic and provide links to relevant articles or studies. We will also provide bullet points to surmise all or part of what the article or study says so you can most efficiently find what you’re looking for.

What Is Marijuana?

Overview of cannabis

Difference between cannabis & marijuana

National Institute on Drug Abuse Report –

  • The term cannabis describes a specific plant containing ingredients that are psychoactive, i.e. alters the mind. The term is also used to describe cannabis products in general.
  • Marijuana and cannabis are often used interchangeably. However, more accurately marijuana specifically refers to cannabis products made from dried flowers, leaves, stems, and seeds of the cannabis plant.
  • Many people who use marijuana experience a sensation of euphoria and great relaxation. Other effects of the drug can be heightened sensory perception, laughter, changed perception of time and an increased appetite.
  • Not all users have a pleasant experience however. After consuming the drug some users experience anxiety, fear, distrust or panic. (This description excludes potential side effects which will be delved into later in this article)
  • Of the 120 currently known ingredients (cannabinoids) inside of cannabis, the major components are cannabidiol (CBD) and delta-9-tetrahydrocannabinol (THC).
  • The concentration of THC in cannabis has increased over time. In the early 90s, the average THC content in confiscated cannabis samples was roughly 3.7%. In 2013, the average was 9.6%. With the increase of THC levels there are concerns surrounding the increased likelihood of negative consequences for users.


THC vs CBD –

National Institute on Drug Abuse Report –


  • THC is the compound inside of marijuana which causes psychoactive effects. In other words, its what causes a “high”.
  • THC binds to CB1 receptors in the brain associated with learning, memory, movement, pain sensation & inflammation.
  • THC also binds to CB2 receptors associated with memory, emotions, immune cells, bone cells, and spleen and liver cells.
  • The consequence of THC binding itself to so many areas of the body and brain as listed above is what causes such powerful psychological & physiological effects.
  • Due to the listed effects above, using marijuana causes impaired thinking and interferes with a users ability to perform complicated tasks. It also impairs ones ability to regulate balance, posture, coordination and reaction time. Thus, when under the influence a person must ensure to not operate heavy machinery such as a vehicle.


  • According to a 2018 review article, CBD may reduce the ability for THC to bind to CB1 & CB2 receptors, thus may reduce the effect of THC.
  • The WHO stated in 2017 that CBD might provide therapeutic benefits for specific issues such as chronic pain, degenerative diseases, anxiety, depression & more.

Medical Cannabis

Cannabis & Pain

Medical Marijuana Overview –

  • The term ‘Medical Cannabis’ refers to physician recommended use of cannabis and its components (called cannabinoids) to treat or improve symptoms.
  • Chronic pain is the most commonly used reason for medical cannabis prescription. However, it may only modestly reduce the pain, like all currently available drugs for chronic pain treatment. Interestingly, it is not effective against acute pain.
  • Due to THC being very low or even non existent inside of medical cannabis, users report very low or no alteration in consciousness

Side Effects

  • Side effects and safety of short term medical cannabis use seem to be generally mild to moderate, well tolerated and transient. However, more research must be done.
  • There is little research done on the side effects and safety of long term medical cannabis use.

Negative Effects of Marijuana

  • Imaging studies in humans show regular users in adolescence display impaired neural connectivity in specific regions of the brain associated with a broad range of executive functions such as learning, memory, and impulse control compared with non-users.


National Institute on Drug Abuse Report –

43% of Canadian Marijuana Users Use Daily –

  • The age old question is whether marijuana is addictive. Studies show that with use over time marijuana can cause changes to the brain which result in addiction of the user. Of users who are likely to become addicted It is estimated that:
    • 9% of total users will become dependent on the substance.
    • 17% of users who start using marijuana in their teenage years.
    • 25 – 50% among daily users.
  • Of all Canadian marijuana smokers, 43% report to use daily, while 29% report to use at least once per week.
  • According to the 2013 NSDUH, marijuana accounted for 4.2 million of the total 6.9 million Americans dependent on or abusing illegal drugs.
  • When looking at addiction withdrawal symptoms we see that they’re linked with mild withdrawal syndrome. Frequent users often report the following symptoms which peak within the first week but can last two weeks after quitting:
    • Irritability
    • Mood and sleep difficulties
    • Decreased appetite
    • Cravings
    • Restlessness
    • and/or physical discomfort

Laziness & Academic Achievement

Self-Efficacy & Marijuana –

Marijuana Does Cause Laziness –

Marijuana & Academic Achievement

Marijuana Cravings & Academic Decline –

Marijuana Leading To Less Motivation –

Rats & Marijuana Laziness –

  • Hierarchical regression models used in one study showed that marijuana usage predicted lower initiative and persistence.
  • Frequent marijuana use is related to academic difficulties according to multiple studies recording college aged students (university).
  • Two studies conducted in laboratory environments showed that when a person has consumed the equivalent of one marijuana joint they are less willing to work for money. These studies also found that those who’ve consumed marijuana for extended periods, and those who’ve only done it for a short time, after 12 hours of not consuming the drug their motivation returned to normal levels compared to the control group.
  • However, both of these studies had small sample sizes of 40 people and only captured results in one aspect and time in their lives. Thus, longitudinal studies must be conducted.
  • In 2016 researchers at the University of British Columbia gave THC to laboratory rats to see what effects would take place regarding motivation levels. The study showed that after consumption of THC the rats were less willing to initiate complex tasks for a large reward, with most rats instead opting to complete an easier task despite the smaller prize.
  • It is important to note that the current studies linking marijuana to laziness are not conclusive regarding the relationship between the two.

Cannabis & Driving

National Institute on Drug Abuse Report –

  • A meta-analysis of multiple studies showed the risk of being involved in a car accident roughly doubles when under the influence of Marijuana. On top of this, drivers impaired by marijuana are 3 – 7 times more likely to be the cause of the accident than those who had not used drugs or alcohol.

Long Term Effects

National Institute on Drug Abuse Report –

  • Substantial evidence from animal research and a growing level of human studies indicate that frequent exposure to marijuana in the development period of ones life can cause long term or even permanent negative changes in the brain.
  • Rats exposed to marijuana before birth, soon after birth or during adolescence show notable problems with specific learning and memory tasks later in life.

IQ & Cannabis

IQ & Cannabis Use Controversy –

IQ & Cannabis Use Ole Rogeberg Controversy

  • The IQ & cannabis debate started in 2012 when New Zealand researchers conducted a longitudinal study of 1037 New Zealanders from Dunedin. They tested subjects at the age of 13 and again at 38. The researchers then concluded that cannabis users who took up use in adolescence and continued it for years suffered an average of an 8 point IQ decrease when tested again at 38.
  • Controversy around the IQ & Cannabis relationship began when Economist Ole Rogeberg disputed the conclusion with his own analysis of the New Zealand researchers results which he argued showed that those who suffered an IQ decrease came from low socio-economic individuals who instead of dropping by IQ points from marijuana use, their score at 13 was temporarily heightened by school attendance, and their result at age 38 was instead a return to their natural baseline.
  • In 2016 a review of two longitudinal twin studies was conducted. The researchers in these studies measured the IQ of their subjects at 9 & 12, and again at 17 & 20. The 2016 review study showed that marijuana users did have lower IQ score than non-marijuana users. However, there was no evidence of a dose-response relationship between frequency of use and IQ drop. On top of this, in twin studies IQ differences between the non user twin and the user twin did not show.
  • The 2016 review study concluded that declines in IQ may not be a direct result of marijuana use, but instead there are underlying factors that cause marijuana initiation and low academic achievement.
  • The evidence of the relationship between IQ and cannabis use is not yet conclusive. A currently undergoing study following 11,000 10 year old’s in the United States is seeking to finalize the controversial debate. Unfortunately, it will be a few years before we start to see results.


Schizophrenia & Marijuana

Marijuana & Schizophrenia risk –

13% of Schizophrenia Might Be Able To Be Stopped –

  • You may be at risk of developing schizophrenia if you regularly use marijuana. According to a 2017 study, regular users had a 1.37 times increase of risk for developing the condition than those who did not use marijuana.
  • A study from 2018 showed an increase of 2% in cases of serious mental health issues in states that have made medical marijuana legal.
  • Marijuana use in youth (under the age of 18) may lead to the development of schizophrenia later in life. A longitudinal Swedish study within the military of 50,465 participants discovered that those who used the drug before 18 were 2.4 times more likely to develop schizophrenia than those who did not. The chances of risk increased if it was regular use.
  • Those who have a genetically higher chance to develop schizophrenia after regular use of marijuana are also genetically more likely to initiate and regularly use the substance.
  • Marijuana use is observably associated with an increased risk of schizophrenia, whether the relationship is causal is not yet known. However, there is substantial evidence of this relationship.
  • Assuming that the relationship is causal, one longitudinal study has estimated that 13% of all schizophrenic cases can be avoided if all marijuana use was eliminated.


Marijuana’s Impact on Depression –

Depression & Marijuana Use –

  • Marijuana is commonly used to deal with stress, anxiety & depression. Studies show that even though marijuana helps with all of these symptoms in the short term during and after use, regular use can exacerbate depression in the long term.
  • Thankfully, the exacerbated depression can go away after ∼2 days of no marijuana use, with no significant differences after 28 days of no use.
  • Those who do have depression have 130% higher odds of any marijuana use and 216% higher odds for daily marijuana use according to 2015 – 2016 data. This is a vast increase from 46% higher likelihood of any use and 37% higher likelihood of daily use using 2005 – 2006 data.

Gateway Drug?

  • Studies show that rats who are exposed to THC during adolescence is associated with an altered reward system in the brain, resulting in an increased likelihood that the animal will self administer harder drugs (such as heroin) when given the opportunity.

Article Last Updated 08/12/2020

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