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 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. Although the specific causes of these symptoms remain unknown, they appear to occur more frequently when a high dose of cannabis is consumed in food or drink rather than smoked. Such 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 syndrome1
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 (Polen et al. 1993).
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 (Hashibe et al. 2006). 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")
1 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.