Effects on the Brain
As THC enters the brain, it causes the user to feel euphoric—or
high—by acting on the brain's reward system, which is made up of regions
that govern the response to pleasurable things like sex and chocolate,
as well as to most drugs of abuse. THC activates the reward system in
the same way that nearly all drugs of abuse do: by stimulating brain
cells to release the chemical dopamine.
Along with euphoria, relaxation is another frequently reported effect
in human studies. Other effects, which vary dramatically among
different users, include heightened sensory perception (e.g., brighter
colors), laughter, altered perception of time, and increased appetite.
After a while, the euphoria subsides, and the user may feel sleepy or
depressed. Occasionally, marijuana use may produce anxiety, fear,
distrust, or panic.
Marijuana use impairs a person's ability to form new memories
(see below)
and to shift focus. THC also disrupts coordination and balance by
binding to receptors in the cerebellum and basal ganglia—parts of the
brain that regulate balance, posture, coordination, and reaction time.
Therefore, learning, doing complicated tasks, participating in
athletics, and driving are also affected.
Marijuana users who have taken large doses of the drug may experience
an acute psychosis, which includes hallucinations, delusions, and a
loss of the sense of personal identity. Short-term psychotic reactions
to high concentrations of THC are distinct from longer-lasting,
schizophrenia-like disorders that have been associated with the use of
cannabis in vulnerable individuals. (See "
Is There a Link Between Marijuana Use and Mental Illness?")
Our understanding of marijuana's long-term brain effects is limited.
Research findings on how chronic cannabis use affects brain
structure,
for example, have been inconsistent. It may be that the effects are too
subtle for reliable detection by current techniques. A similar
challenge arises in studies of the effects of chronic marijuana use on
brain
function. Although imaging studies (functional MRI; fMRI)
in chronic users do show some consistent alterations, the relation of
these changes to cognitive functioning is less clear. This uncertainty
may stem from confounding factors such as other drug use, residual drug
effects (which can occur for at least 24 hours in chronic users), or
withdrawal symptoms in long-term chronic users.
Marijuana, Memory, and the Hippocampus
Memory impairment from marijuana use occurs because THC alters how
information is processed in the hippocampus, a brain area responsible
for memory formation.
Distribution of cannabinoid receptors in the rat brain. Brain image
reveals high levels (shown in orange and yellow) of cannabinoid
receptors in many areas, including the cortex, hippocampus, cerebellum,
and nucleus accumbens (ventral striatum).
Most of the evidence supporting this assertion comes from animal
studies. For example, rats exposed to THC in utero, soon after birth, or
during adolescence, show notable problems with specific learning/memory
tasks later in life.
Moreover, cognitive impairment in adult rats is
associated with structural and functional changes in the hippocampus
from THC exposure during adolescence.
As people age, they lose neurons in the hippocampus, which decreases
their ability to learn new information. Chronic THC exposure may hasten
age-related loss of hippocampal neurons. In one study, rats exposed to
THC every day for 8 months (approximately 30 percent of their life-span)
showed a level of nerve cell loss (at 11 to 12 months of age) that
equaled that of unexposed animals twice their age.
An enduring question in the field is whether individuals who quit
marijuana, even after long-term, heavy use, can recover some of their
cognitive abilities. One study reports that the ability of long-term
heavy marijuana users to recall words from a list was still impaired 1
week after they quit using, but returned to normal by 4 weeks. However,
another study found that marijuana's effects on the brain can build up
and deteriorate critical life skills over time. Such effects may be
worse in those with other mental disorders, or simply by virtue of the
normal aging process.
Effects on General Physical Health
Within a few minutes after inhaling marijuana smoke, an individual's
heart rate speeds up, the bronchial passages relax and become enlarged,
and blood vessels in the eyes expand, making the eyes look red. The
heart rate—normally 70 to 80 beats per minute—may increase by 20 to 50
beats per minute, or may even double in some cases. Taking other drugs
with marijuana can amplify this effect.
Limited evidence suggests that a person's risk of heart attack during
the first hour after smoking marijuana is four times his or her usual
risk. This observation could be partly explained by marijuana raising
blood pressure (in some cases) and heart rate and reducing the blood's
capacity to carry oxygen. Such possibilities need to be examined more
closely, particularly since current marijuana users include adults from
the baby boomer generation, who may have other cardiovascular risks that
may increase their vulnerability.
Consequences of Marijuana Abuse
Acute (present during intoxication)
-
Impairs short-term memory
-
Impairs attention, judgment, and other cognitive functions
-
Impairs coordination and balance
-
Increases heart rate
-
Psychotic episodes
Persistent (lasting longer than intoxication, but may not be permanent)
-
Impairs memory and learning skills
-
Sleep impairment
Long-term (cumulative effects of chronic abuse)
-
Can lead to addiction
-
Increases risk of chronic cough, bronchitis
-
Increases risk of schizophrenia in vulnerable individuals
-
May increase risk of anxiety, depression, and amotivational syndrome*
* These are often reported co-occurring symptoms/disorders
with chronic marijuana use. However, research has not yet determined
whether marijuana is causal or just associated with these mental
problems.
The smoke of marijuana, like that of tobacco, consists of a toxic
mixture of gases and particulates, many of which are known to be harmful
to the lungs. Someone who smokes marijuana regularly may have many of
the same respiratory problems that tobacco smokers do, such as daily
cough and phlegm production, more frequent acute chest illnesses, and a
greater risk of lung infections. Even infrequent marijuana use can cause
burning and stinging of the mouth and throat, often accompanied by a
heavy cough. One study found that extra sick days used by frequent
marijuana smokers were often because of respiratory illnesses.
6
In addition, marijuana has the
potential to promote cancer
of the lungs and other parts of the respiratory tract because it
contains irritants and carcinogens—up to 70 percent more than tobacco
smoke. It also induces high levels of an enzyme that converts certain
hydrocarbons into their cancer-causing form, which could accelerate the
changes that ultimately produce malignant cells. And since marijuana
smokers generally inhale more deeply and hold their breath longer than
tobacco smokers, the lungs are exposed longer to carcinogenic smoke.
However, while several lines of evidence have suggested that marijuana
use may lead to lung cancer, the supporting evidence is inconclusive.
8
The presence of an unidentified active ingredient in cannabis smoke
having protective properties—if corroborated and properly
characterized—could help explain the inconsistencies and modest
findings.
Within a few minutes after inhaling marijuana smoke, an individual's
heart rate speeds up, the bronchial passages relax and become enlarged,
and blood vessels in the eyes expand, making the eyes look red.
A significant body of research demonstrates negative effects of THC
on the function of various immune cells, both in vitro in cells and in
vivo with test animals. However, no studies to date connect marijuana's
suspected immune system suppression with greater incidence of infections
or immune disorders in humans. One short (3-week) study found marijuana
smoking to be associated with a few statistically significant negative
effects on the immune function of AIDS patients; a second small study of
college students also suggested the possibility of marijuana having
adverse effects on immune system functioning. Thus, the combined
evidence from animal studies plus the limited human data available seem
to warrant additional research on the impact of marijuana on the immune
system. (See "
The Science of Medical Marijuana")
Adapted from Caspi et al., Biol Psychiatry, May 2005.
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