Effects on the Brain
Scientists have learned a great deal about how THC acts in the brain to
produce its many effects. When someone smokes marijuana, THC rapidly
passes from the lungs into the bloodstream, which carries the chemical to
organs throughout the body, including the brain.
In the brain, THC connects to specific sites called cannabinoid receptors on
nerve cells and influences the activity of those cells. Some brain areas have
many cannabinoid receptors; others have few or none. Many cannabinoid
receptors are found in the parts of the brain that influence pleasure,
memory, thought, concentration, sensory and time perception, and
coordinated movement.
The short-term effects of marijuana can include problems with memory
and learning; distorted perception; difficulty in thinking and problem
solving; loss of coordination; and increased heart rate. Research findings
for long-term marijuana use indicate some changes in the brain similar to
those seen after long-term use of other major drugs of abuse. For example,
cannabinoid (THC or synthetic forms of THC) withdrawal in chronically
exposed animals leads to an increase in the activation of the stress-response
system and changes in the activity of nerve cells containing dopamine.
Dopamine neurons are involved in the regulation of motivation and reward,
and are directly or indirectly affected by all drugs of abuse.
Effects on the Heart
One study has indicated that a user’s risk of heart attack more than
quadruples in the first hour after smoking marijuana. The researchers
suggest that such an effect might occur from marijuana’s effects on blood
pressure and heart rate and reduced oxygen-carrying capacity of blood.
Effects on the Lungs
A study of 450 individuals found that people who smoke marijuana
frequently but do not smoke tobacco have more health problems and miss
more days of work than nonsmokers. Many of the extra sick days among
the marijuana smokers in the study were for respiratory illnesses.
Even infrequent use can cause burning and stinging of the mouth and
throat, often accompanied by a heavy cough. 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 illness, a heightened risk of lung infections, and a
greater tendency to obstructed airways. Smoking marijuana increases the
likelihood of developing cancer of the head or neck, and the more
marijuana smoked the greater the increase. A study comparing 173 cancer
patients and 176 healthy individuals produced strong evidence that
marijuana smoking doubled or tripled the risk of these cancers.
Marijuana use also has the potential to promote cancer of the lungs and
other parts of the respiratory tract because it contains irritants and
carcinogens. In fact, marijuana smoke contains 50 to 70 percent more
carcinogenic hydrocarbons than does tobacco smoke. It also produces high
levels of an enzyme that converts certain hydrocarbons into their
carcinogenic form levels that may accelerate the changes that ultimately
produce malignant cells. Marijuana users usually inhale more deeply and
hold their breath longer than tobacco smokers do, which increases the
lungs’ exposure to carcinogenic smoke. These facts suggest that, puff for
puff, smoking marijuana may increase the risk of cancer more than
smoking tobacco.
Other Health Effects
Some of marijuana’s adverse health effects may occur because THC impairs
the immune system’s ability to fight off infectious diseases and cancer. In
laboratory experiments that exposed animal and human cells to THC or
other marijuana ingredients, the normal disease-preventing reactions of
many of the key types of immune cells were inhibited. In other studies,
mice exposed to THC or related substances were more likely than
unexposed mice to develop bacterial infections and tumors.
Effects of Heavy Marijuana Use on Learning and Social Behavior
Depression, anxiety, and personality disturbances have been associated
with marijuana use. Research clearly demonstrates that marijuana has
potential to cause problems in daily life or make a person’s existing
problems worse. Because marijuana compromises the ability to learn and
remember information, the more a person uses marijuana the more he or
she is likely to fall behind in accumulating intellectual, job, or social skills.
Moreover, research has shown that marijuana’s adverse impact on memory
and learning can last for days or weeks after the acute effects of the drug
wear off.
Students who smoke marijuana get lower grades and are less likely to
graduate from high school, compared with their non-smoking peers. A
study of 129 college students found that, for heavy users of marijuana
(those who smoked the drug at least 27 of the preceding 30 days), critical
skills related to attention, memory, and learning were significantly
impaired even after they had not used the drug for at least 24 hours. The
heavy marijuana users in the study had more trouble sustaining and
shifting their attention and in registering, organizing, and using
information than did the study participants who had used marijuana no
more than 3 of the previous 30 days. As a result, someone who smokes
marijuana every day may be functioning at a reduced intellectual level all of
the time.
More recently, the same researchers showed that the ability of a group of
long-term heavy marijuana users to recall words from a list remained
impaired for a week after quitting, but returned to normal within 4 weeks.
Thus, it is possible that some cognitive abilities may be restored in
individuals who quit smoking marijuana, even after long-term heavy use.
Workers who smoke marijuana are more likely than their coworkers to
have problems on the job. Several studies associate workers’ marijuana
smoking with increased absences, tardiness, accidents, workers’
compensation claims, and job turnover. A study of municipal workers
found that those who used marijuana on or off the job reported more
“withdrawal behaviors” such as leaving work without permission,
daydreaming, spending work time on personal matters, and shirking tasks
that adversely affect productivity and morale. In another study, marijuana
users reported that use of the drug impaired several important measures of
life achievement including cognitive abilities, career status, social life, and
physical and mental health.
Addictive Potential
Long-term marijuana use can lead to addiction for some people; that is,
they use the drug compulsively even though it interferes with family,
school, work, and recreational activities. Drug craving and withdrawal
symptoms can make it hard for long-term marijuana smokers to stop using
the drug. People trying to quit marijuana report irritability, sleeplessness,
and anxiety. They also display increased aggression on psychological tests,
peaking approximately one week after the last use of the drug