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(5).
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(6) and changes in the activity of nerve cells containing dopamine(7). 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 users risk of heart attack more than quadruples in the first hour after smoking marijuana(8). The researchers suggest that such an effect might occur from marijuanas 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(9). 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(10). Smoking marijuana increases the likelihood of developing cancer of the head or neck, and the more marijuana smoked the greater the increase(11). 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(12, 13). In fact, marijuana smoke contains 50 to 70 percent more carcinogenic hydrocarbons than does tobacco smoke(14). It also produces high levels of an enzyme that converts certain hydrocarbons into their carcinogenic formlevels that may accelerate the changes that ultimately produce malignant cells(15). 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 marijuanas adverse health effects may occur because THC impairs the immune systems 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(16). In other studies, mice exposed to THC or related substances were more likely than unexposed mice to develop bacterial infections and tumors(17, 18).
Effects of Heavy Marijuana Use on Learning and Social Behavior
Depression(19), anxiety(20), and personality disturbances(21) have been associated with marijuana use. Research clearly demonstrates that marijuana has potential to cause problems in daily life or make a persons 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 marijuanas adverse impact on memory and learning can last for days or weeks after the acute effects of the drug wear off(22, 23).
Students who smoke marijuana get lower grades and are less likely to graduate from high school, compared with their non-smoking peers(24, 25, 26, 27). 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(28). 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(29). 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 behaviorssuch as leaving work without permission, daydreaming, spending work time on personal matters, and shirking tasksthat adversely affect productivity and morale(30). 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(31).
Effects on Pregnancy
Research has shown that babies born to women who used marijuana during their pregnancies display altered responses to visual stimuli, increased tremulousness, and a high-pitched cry, which may indicate neurological problems in development(32). During infancy and preschool years, marijuana-exposed children have been observed to have more behavioral problems than unexposed children and poorer performance on tasks of visual perception, language comprehension, sustained attention, and memory(33, 34). In school, these children are more likely to exhibit deficits in decision-making skills, memory, and the ability to remain attentive(35, 36, 37)</FONT>.
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 report irritability, sleeplessness, and anxiety(38). They also display increased aggression on psychological tests, peaking approximately one week after the last use of the drug(39).
Genetic Vulnerability
Scientists have found that whether an individual has positive or negative sensations after smoking marijuana can be influenced by heredity. A 1997 study demonstrated that identical male twins were more likely than non-identical male twins to report similar responses to marijuana use, indicating a genetic basis for their response to the drug(40). (Identical twins share all of their genes.)
It also was discovered that the twins shared or family environment before age 18 had no detectable influence on their response to marijuana. Certain environmental factors, however, such as the availability of marijuana, expectations about how the drug would affect them, the influence of friends and social contacts, and other factors that differentiate experiences of identical twins were found to have an important effect.
