Marijuana usage in the United States is increasing, with THC targeting cannabinoid receptors in the brain. While heavy usage has been shown to cause brain damage, the long-term effects of light-moderate usage remain unclear. More research is still needed to fully understand marijuana's effects on mental health and cognition, as well as potential differences between casual and chronic users.

1. The Effects of Tetrahydrocannibinol
on the Brain

2. Usage Statistics
Source:2002 National Survey on Drug Use and Health (NSDUH)
Source: 2010 NSDUH
Marijuana is more prevalent than any other type of illicit drug and it is
increasing in the overall number of users who have tried it.

3. Methods of usage and Reaching the Brain
• Marijuana is most commonly smoked, but it is also sometimes ingested.
• Smoking involves rapid absorption of THC (the main psychoactive agent in
marijuana) as it is absorbed in the alveoli in the lungs and travels through
the bloodstream to the brain in just a few seconds after inhalation.
• Consumption of cannabis takes longer, as it must first be absorbed in the
stomach, travel to the liver and then to the rest of the brain.
Source: Howstufforks.com

4. Methods of Action in the Brain
In the brain THC targets endocannabinoid receptors which
are normally used as receivers for naturally produced
endocannabinoids.
Specifically THC targets the CB1and CB2 receptors
These receptors are found in various parts of the brain (see
image below).

5. Methods of Action in the Brain (cont.)
• The main effects of marijuana are caused by its target of CB1
receptors, as these are found primarily in the brain while CB2 receptors
are found primarily in the immune system.
• Once bound to the CB1 receptors THC interferes with the binding of
natural cannabinoid receptors and causes a decrease in adenylyl cyclase
activity, inhibition of calcium channels and reduced inhibition of
potassium channels.
• As a result of the widespread locations of endocannabinoid receptors for
THC to target, marijuana can have a variety of effects ranging from
euphoria and introspection to paranoia and increased appetite.
Source: Cancer and Natural
Medicines, March 21, 2010

6. Long-Term Effects of Marijuana Use
(Psychosis and Depression)
• The overall effects of marijuana over time are still hotly contested.
• One of the common correlation with marijuana usage is between
usage and depression and psychosis.
• A large-scale study in Britain1 found that there was a significantly
higher risk (1.5 times greater) of psychosis among adolescents who
used marijuana than among those that haven’t.
• The Bettering the Evaluation of Care of Health (BEACH) study in
Australia found that 48% of people who admitted marijuana usage
suffered from a mental problem, with 19% suffering from
depression and 9% with psychosis.
1. Miettunen, Jouko.
“Assosiaction of cannabis
use with prodromal
symptoms of psychosis in
adoescence.” British
Journal of
Psychiatry, 2008.

7. Long-Term Effects of Marijuana
Use(Correlation vs Causation)
• However it is not certain whether marijuana actually causes
psychological disorders or whether people with disorders or if
people with mental health issues are simply more likely to use
marijuana.
• This idea of self medication is supported by a recent study2
suggesting the cannabidiol (a component of marijuana) is effective
as an antipsychotic drug.
• Another study3 found that depressed marijuana users reported less
symptoms of depression than non-users with depression.
+ =
2. Zuardi, A.W. “Cannabidiol, a Cannabis
sativa constituent, as an antipsychotic 3. Denson, Thomas. “Decreased
drug.” Brazilian Journal of Medicine, April Depression in marijuana users.” Addictive
2006. Behaviors, April 2006.

8. Long-Term Effects of Marijuana Use
(Gateway Drug)
• One common attack on marijuana has been its effect
as a gateway drug, either through social factors or
physiological changes made to brain chemistry by the
drug itself.
• However, a recent study4 found that adolescents who
used marijuana were no more likely to develop a
substance abuse issue than those who did not.
4. Tarter, Ralph. “Predictors of
Marijuana Use in Adolescents
Before and After Licit Drug Use:
Examination of the Gateway
Hypothesis.” American Journal of
Psychiatry, 2006.

9. Long Term Effects of Marijuana Use
(Brain Damage)
• There have been mixed results from studies performed
regarding marijuana and intelligence. A study5 found
no decrease in IQ in those who smoked lightly or
moderately, but there was a decrease in heavy users.
• Another study6 however found significant shrinkage in
the hippocampus (12%, controls memory and emotion)
and the amygdala (7.1%, controls fear and aggression).
• However a separate study7 found that any damage is
reversible except in heavy users.
5. Fried, Peter. “Current and former 6. “Heavy pot smoking linked to 7. Bolla KI. “Dose-related
marijuana use: preliminary findings of a smaller brains.” New Scientist, June neurocognitive effects of
longitudinal study of effects on IQ in 2008. marijuana use.”
young adults.” Canadian Medical Neurology, November 2002.
Association Journal, April 2002.

10. Long Term Effects of Marijuana Use
(Addiction)
• Cannabis withdrawal has been studied8, and withdrawal
usually lasts for 10-14 days and is associated with irritability
and anxiety along with decreased appetite, indicating there
that there is at least some physical dependence associated
with marijuana usage
• Overall the addictive and withdrawal effects of marijuana
are much less than those of opiates, stimulants or
alcohol, and are never life-threatening.
8. Kouri, Elena M. “Abstinence symptoms during
withdrawal from chronic marijuana use.”
Experimental and Clinical
Psychophramacology, Nov 2000.

11. Areas for Further Study
• While there are many areas in need of study
regarding marijuana and the
brain, understanding better how marijuana
affects light and moderate users, and the
relationship between smoking and mental
health are perhaps most important.

12. Conclusions
• The main conclusion here is that marijuana still
requires additionally study. Many of the results found
are conflicting or inconclusive, and even strong
correlations do not necessarily show causation.
• The one thing that is clear is that heavy marijuana
usage is bad for the brain. It causes shrinkage of
important parts of the brain and heavy enough usage
can make this damage permanent.
• Given that over 50% of youths will try marijuana at
some point during their lifetime, and this trend seems
to be increasing rather than decreasing, the need for
greater understanding in this area is definite.