What are the acute
effects of marijuana use?

Marijuana's effects begin immediately after the drug enters the brain and last from 1 to 3 hours. If marijuana is consumed in food or drink, the short-term effects begin more slowly, usually in 1/2 to 1 hour, and last longer, for as long as 4 hours. Smoking marijuana deposits several times more THC into the blood than does eating or drinking the drug.³⁰

Within a few minutes after inhaling marijuana smoke, an individual's heart begins beating more rapidly, 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, in some cases, even double.¹⁶ This effect can be greater if other drugs are taken with marijuana.³¹

As THC enters the brain, it causes a user to feel euphoric - or "high" - by acting in the brain's reward system, areas of the brain that respond to stimuli such as food and drink as well as 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.^32,33,34

A marijuana user may experience pleasant sensations, colors and sounds may seem more intense, and time appears to pass very slowly. The user's mouth feels dry, and he or she may suddenly become very hungry and thirsty. His or her hands may tremble and grow cold. The euphoria passes after awhile, and then the user may feel sleepy or depressed. Occasionally, marijuana use produces anxiety, fear, distrust, or panic.

Marijuana use impairs a person's ability to form memories, recall events (see Marijuana, Memory, and the Hippocampus), and shift attention from one thing to another.^8,35 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 of movement, and reaction time.¹¹ Through its effects on the brain and body, marijuana intoxication can cause accidents. Studies show that approximately 6 to 11 percent of fatal accident victims test positive for THC. In many of these cases, alcohol is detected as well.^{36, 37, 38}

In a study conducted by the National Highway Traffic Safety Administration, a moderate dose of marijuana alone was shown to impair driving performance; however, the effects of even a low dose of marijuana combined with alcohol were markedly greater than for either drug alone³⁹. Driving indices measured included reaction time, visual search frequency (driver checking side streets), and the ability to perceive and/or respond to changes in the relative velocity of other vehicles.

Marijuana users who have taken high doses of the drug may experience acute toxic psychosis, which includes hallucinations, delusions, and depersonalization - a loss of the sense of personal identity, or self-recognition.^10,16 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.

Marijuana's Effects on the Brain When marijuana is smoked, its active ingredient THC travels throughout the body, including the brain, to produce its many effects. THC attaches to sites called cannabinoid receptors on nerve cells in the brain, affecting the way those cells work. Cannabinoid receptors are abundant in parts of the brain that regulate movement, coordination, learning and memory, higher cognitive functions such as judgment, and pleasure. Brain Region Functions Associated With Region Brain regions in which cannabinoid receptors are abundant Cerebellum Body movement coordination Hippocampus Learning and memory Cerebral cortex, especially cingulate, frontal, and parietal regions Higher cognitive functions Nucleus accumbens Reward Basal Ganglia Substantia nigra pars reticulata Entopeduncular nucleus Globus pallidus Putamen Movement control Brain regions in which cannabinoid receptors are moderately concentrated Hypothalamus Body housekeeping functions (body temperature regulation, salt and water balance, reproductive function) Amygdala Emotional response, fear Spinal cord Peripheral sensation, including pain Brain stem Sleep and arousal, temperature regulation, motor control Central gray Analgesia Nucleus of the solitary tract Visceral sensation, nausea and vomiting

How does marijuana use
affect physical health?

Marijuana use has been shown to increase users' difficulty in trying to quit smoking tobacco⁴⁰. This was recently reported in a study comparing smoking cessation in adults who smoked both marijuana and tobacco with those who smoked only tobacco. The relationship between marijuana use and continued smoking was particularly strong in those who smoked marijuana daily at the time of the initial interview, 13 years prior to the followup interview.

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 do.⁴¹ Many of the extra sick days used by the marijuana smokers in the study were for respiratory illnesses.

Even infrequent marijuana 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 illnesses, a heightened risk of lung infections, and a greater tendency toward obstructed airways.⁴

Cancer of the respiratory tract and lungs may also be promoted by marijuana smoke.⁴ A study comparing 173 cancer patients and 176 healthy individuals produced strong evidence that smoking marijuana increases the likelihood of developing cancer of the head or neck, and that the more marijuana smoked, the greater the increase.¹⁸ A statistical analysis of the data suggested that marijuana smoking doubled or tripled the risk of these cancers.

Marijuana has the potential to promote cancer of the lungs and other parts of the respiratory tract because it contains irritants and carcinogens.⁴² In fact, marijuana smoke contains 50 percent to 70 percent more carcinogenic hydrocarbons than does tobacco smoke.⁴³ 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.⁴⁴ 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 does.

Some adverse health effects caused by marijuana 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.¹⁷ In other studies, mice exposed to THC or related substances were more likely than unexposed mice to develop bacterial infections and tumors.^15,45

One study has indicated that a person's risk of heart attack during the first hour after smoking marijuana is four times his or her usual risk.⁴⁶ The researchers suggest that a heart attack might occur, in part, because marijuana raises blood pressure and heart rate and reduces the oxygen-carrying capacity of blood.

Marijuana, Memory, and the Hippocampus Marijuana's damage to short-term memory seems to occur because THC alters the way in which information is processed by the hippocampus, a brain area responsible for memory formation. Laboratory rats treated with THC displayed the same reduced ability to perform tasks requiring short-term memory as other rats showed after nerve cells in their hippocampus were destroyed.65 In addition, the THC-treated rats had the greatest difficulty with the tasks precisely during the time when the drug was interfering most with the normal functioning of cells in the hippocampus. As people age, they normally lose neurons in the hippocampus, which decreases their ability to remember events. Chronic THC exposure may hasten the age-related loss of hippocampal neurons. In one series of studies, rats exposed to THC every day for 8 months (approximately 30 percent of their lifespan), when examined at 11 to 12 months of age, showed nerve cell loss equivalent to that of unexposed animals twice their age.66

The Body's Natural THC-Like Chemicals THC owes many of its effects to its similarity to a family of chemicals called the endogenous cannabinoids, which are natural Cannabis-like chemicals. Because a THC molecule is shaped like these endogenous cannabinoids, it interacts with the same receptors on nerve cells, the cannabinoid receptors, that endogenous cannabinoids do, and it influences many of the same processes. Research has shown that the endogenous cannabinoids help control a wide array of mental and physical processes in the brain and throughout the body, including memory and perception, fine motor coordination, pain sensations,67 immunity to disease, and reproduction.68 When someone smokes marijuana, THC overstimulates the cannabinoid receptors, leading to a disruption of the endogenous cannabinoids' normal control. This overstimulation produces the intoxication experienced by marijuana smokers. Over time, it may degrade some cannabinoid receptors, possibly producing permanent adverse effects and contributing to addiction and risk for a withdrawal syndrome.69,70

The Science of Medical Marijuana THC, the main active ingredient in marijuana, produces effects that potentially can be useful for treating a variety of medical conditions. It is the main ingredient in a medication that is currently used to treat nausea and increase appetite in patients with wasting due to cancer chemotherapy or AIDS, to relieve pain, and to treat glaucoma. Scientists are investigating other potential medical uses for cannabinoids, including treating brain damage due to stroke and neurological disorders such as Parkinson's disease and Tourette's syndrome.71 Some clinical trials of smoked marijuana for therapy are underway, but the inconsistency of THC dosage in different marijuana samples poses a major hindrance to valid trials and to the safe and effective use of the drug. Moreover, the adverse effects of marijuana smoke on the respiratory system will offset the helpfulness of smoked marijuana for some patients. Finally, little is known about the many chemicals besides THC that are in marijuana, or their possible deleterious impact on patients with medical conditions.

The National Institute on Drug Abuse (NIDA) is part of the National Institutes of Health (NIH), a component of the U.S. Department of Health and Human Services. Questions? See our Contact Information. Last updated on Saturday, January 18, 2003.