Cannabis (marijuana) is a psychoactive herb that comes from the cannabis plant. This mind-altering substance, also known as ganja, weed, reefer, and grass, is an illegal drug in most states, though laws are changing rapidly. 


Marijuana is a green or gray mixture of dried, shredded flowers and leaves of the plant Cannabis sativa.

Marijuana is usually smoked as a cigarette (typically called a "joint") or in a pipe or bong. Marijuana also appears in "blunts" (cigars that have been emptied of tobacco and refilled with marijuana) and "vapes," or electronic cigarettes (E-cigs). Marijuana cigarettes or blunts sometimes include crack cocaine, a combination known by various street names like "primos" or "woolies." Joints and blunts are also sometimes dipped in PCP and called "happy sticks," "wicky sticks," "love boat," or "tical." Some users mix marijuana in food or use it to brew tea.

Slang terms for drugs change quickly, and they vary from one part of the country to another. They may even differ across sections of a large city.

Cannabis refers to marijuana and other drugs made from the same plant. Strong forms of cannabis include sinsemilla (sin-seh-me-yah), hashish (hash, for short), and hash oil.

All forms of cannabis are mind-altering (psychoactive) drugs; they all contain delta-9-tetrahydrocannabinol (THC), the main active chemical in marijuana. They also contain more than 400 other chemicals.

Marijuana's effect on the user depends on the amount of THC to which the person is exposed. The potency of the drug is measured by the average amount of THC in test samples confiscated by law enforcement agencies. These samples showed the following: 

  • Most ordinary marijuana contained, on average, seven percent THC.
  • Sinsemilla, made from the buds and flowering tops of female plants, contained 12 percent THC on average, but ranged from less than one percent to 27 percent.
  • Hashish (the sticky resin from female plant flowers) had an average of 10 percent THC, but ranged from one percent to 26 percent.

The Monitoring the Future Survey, conducted yearly, includes students from the eighth, 10th, and 12th grades. A recent survey found that 15.7 percent of eighth-graders had tried marijuana at least once, and that among 10th-graders, 14.2 percent were current users (that is, had used within the past month). Among 12th-graders, 42.3 percent had tried marijuana at least once, and approximately 18 percent were current users.


If someone is high on marijuana, he or she might:

  • Seem dizzy and have trouble walking
  • Seem silly and giggly for no reason
  • Have very red, bloodshot eyes
  • Have a hard time remembering things that just happened
  • Become very sleepy after a few hours, as the early effects fade

The effects of marijuana on each person depend on the type of cannabis and how much THC it contains, how the drug is taken (by smoking or eating), the experience and expectations of the user, the setting, and whether alcohol or other drugs are also used. Some people feel nothing at all when they first try marijuana; others may feel high (intoxicated and/or euphoric).

It's common for marijuana users to become engrossed in ordinary sights, sounds, or tastes, and trivial events may seem extremely interesting or funny. Time seems to pass very slowly, so minutes feel like hours. Sometimes the drug causes users to feel thirsty and very hungry—an effect called "the munchies."

Unpleasant effects that may occur include depersonalization (being a detached observer of oneself), changed body image, disorientation, acute panic reactions, or severe paranoia.

Rarely, severe delirium, hallucinations, and violence have also been reported, though such cases should raise suspicion that the marijuana may have been laced with another agent, such as PCP.

Marijuana has specific effects that may decrease a user's ability to perform tasks requiring a great deal of coordination, such as driving a car. Visual tracking is impaired, and the sense of time is typically prolonged.

Learning may be greatly affected because the drug reduces a user's ability to concentrate. Studies have shown that learning may become "state-dependent," meaning that information acquired or learned while under the influence of marijuana is best recalled in the same state of drug influence.

Other marijuana effects may include:

  • Blood-shot eyes
  • Increased heart rate and blood pressure
  • Bronchodilatation (widening of the airways)
  • In some users, bronchial (airway) irritation leading to bronchoconstriction (narrowing of the airways) or bronchospasm (airway spasms, leading to narrowing of the airways)
  • Pharyngitis, sinusitis, bronchitis, and asthma in heavy users
  • Possible serious effects on the immune system

Parents who are concerned about a teen's marijuana use should look for withdrawal, depression, fatigue, carelessness with grooming, hostility, and deteriorating relationships with family members and friends. In addition, changes in academic performance, increased absenteeism or truancy, lost interest in sports or other favorite activities, and changes in eating or sleeping habits could be related to drug use. However, these signs may also indicate problems other than use of drugs.

In addition, parents should be aware of:

  • Signs of drugs and drug paraphernalia, including pipes and rolling papers
  • Odor on clothes and in the child's bedroom
  • Use of incense or other deodorizers
  • Use of eye drops
  • Clothing, posters, or jewelry promoting drug use

Length of Time Present in Body

Regular marijuana users, upon stopping use of the drug, may experience withdrawal effects. These may include agitation, insomnia, irritability, and anxiety.

Additionally, fatty tissues in various organs readily absorb THC, and traces (metabolites) of the chemical can generally be detected by standard urine testing methods up to several days after marijuana use. In heavy, chronic users, however, traces can sometimes be detected for weeks after marijuana use has stopped.

Harmful Effects

Marijuana can be harmful in a number of ways, through both immediate effects and damage to health over time.

Marijuana hinders users' short-term memory and the ability to handle complex tasks. More potent varieties of marijuana can make even simple tasks difficult.

 They also may become involved in risky sexual behavior. There is a strong link between drug use and unsafe sex and the spread of HIV.

Under the influence of marijuana, students may find it hard to study and learn. Young athletes could find their performances are off because timing, movements, and coordination are all affected by THC.

Effects on Driving

Because of its effect on perceptions, marijuana disrupts many skills required for safe driving: alertness, concentration, coordination, and quick reaction time. The drug can make it difficult to judge distances or react quickly to signals and other sights and sounds on the road. These effects can last up to 24 hours after marijuana use.

Studies show that approximately six percent to 11 percent of fatal-accident victims tested positive for THC. In many of these cases, alcohol was detected as well. When users combine marijuana with alcohol, as they often do, the hazards of driving can be more severe than with either drug alone. 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 alone.

In one study, researchers found that out of 150 reckless drivers who were tested for drugs at an arrest scene, 33 percent tested positive for marijuana and 12 percent tested positive for both marijuana and cocaine. Data also show that after smoking marijuana, people demonstrate the same lack of coordination on standard drunken-driving tests as do those who have had too much to drink.

Long-Term Effects of Marijuana

While all of the long-term effects of marijuana use are not yet known, some research raises serious health concerns. For example, one study examined the health status of 450 daily non-tobacco-smoking marijuana smokers. They found that the marijuana smokers had more sick days and more doctor visits for respiratory and other problems than did a similar group who did not smoke either substance. Regular use of marijuana may play a role in cancer and problems in the respiratory and immune systems. Additionally, research has shown that marijuana use can trigger psychotic symptoms in people who have a genetic vulnerability to schizophrenia.  

Lungs and Airways

People who smoke marijuana regularly may develop many of the same breathing problems that tobacco smokers have, such as daily cough and phlegm production, more frequent chest colds, a heightened risk of lung infections, and a greater tendency toward obstructed airways.


It is hard to determine whether marijuana alone can cause cancer because many people who smoke marijuana also smoke cigarettes and use other drugs. Marijuana smoke contains some of the same cancer-causing compounds as tobacco, sometimes in higher concentrations. Studies show that someone who smokes five joints per day may be taking in as many cancer-causing chemicals as someone who smokes a full pack of cigarettes. Moreover, marijuana smokers usually inhale more deeply and hold their breath longer than tobacco smokers, which increases the lungs' exposure to carcinogenic smoke. Thus, puff for puff, smoking marijuana may increase the risk of cancer more than smoking tobacco does.

Tobacco smoke and marijuana smoke may work together to change the tissue lining of the respiratory tract and lungs. Marijuana smoking could contribute to early development of head and neck cancer in some people.

Some individuals receiving cancer treatment have found, however, that THC can help them with appetite and nausea issues.

Immune System

The immune system protects the body from many agents that cause disease. It is not certain whether marijuana damages the immune systems of people, but both animal and human studies have shown that marijuana impairs the ability of T-cells in the lungs' immune defense system to fight off some infections.

Effects on Pregnancy

Doctors advise pregnant women not to use any drugs because they might harm the growing fetus. Although one animal study has linked marijuana use to loss of the fetus very early in pregnancy, two studies in humans found no association between marijuana use and early pregnancy loss. More research is necessary to fully understand the effects of marijuana use on pregnancy.

Some scientific studies have found that babies born to women who use marijuana during their pregnancies display altered responses to visual stimulation, increased tremors and a high-pitched cry, which may indicate problems with nervous system development. During preschool and early school years, marijuana-exposed children have been reported to exhibit more behavioral problems and difficulties with sustained attention and memory than unexposed children.

Researchers are not certain whether any effects of marijuana use during pregnancy persist as a child grows; however, because some parts of the brain continue developing through adolescence, it's possible that certain kinds of problems will become more evident as a child matures.

Effects on the Brain

THC affects the nerve cells in the part of the brain where memories are formed. This makes it hard for the user to recall recent events or to learn anything. Short-term memory is required for learning and for performing tasks that call for more than one or two steps.

Researchers studying a group of longtime heavy marijuana users in Costa Rica found that they had great trouble when asked to recall a short list of words (a standard test of memory). They also had difficulty focusing their attention on the tests given to them.

As people age, they normally lose nerve cells in a region of the brain that is important for remembering events. Chronic exposure to THC may hasten the age-related loss of these nerve cells. In one study, researchers found that rats exposed to THC every day for eight months (about one-third of their life span) showed a loss of brain cells comparable to rats that were twice their age. It is not known whether a similar effect occurs in humans. Researchers are still learning about the many ways that marijuana could affect the brain.

Medicinal Uses

There has been much debate about the possible medical use of marijuana. Under U.S. law since 1970, marijuana has been a Schedule I controlled substance. This means that the drug, at least in its smoked form, has no commonly accepted medical use.

In considering possible medical uses of marijuana, it is important to distinguish between whole marijuana and pure THC or other specific chemicals derived from cannabis. Whole marijuana contains hundreds of chemicals, some of which are clearly harmful to health.

THC, manufactured into a pill that is taken by mouth, not smoked, is available by prescription and can be used to treat the nausea and vomiting that go along with certain cancer treatments and to help AIDS patients eat more to maintain their weight. Another chemical related to THC, nabilone, has also been approved by the Food and Drug Administration for treating cancer patients who suffer from nausea.

Scientists are studying whether marijuana, THC, and related chemicals in marijuana (called cannabinoids) have other medical uses.


The main active chemical in marijuana is THC. In 1988, it was discovered that the membranes of certain nerve cells contain protein receptors that bind THC. Once securely in place, THC kicks off a series of cellular reactions that lead to the high that users experience when they smoke marijuana.


Marijuana can be addictive because it can cause compulsive, uncontrollable drug craving, seeking, and use, even in the face of negative health and social consequences. Some heavy marijuana users show signs of withdrawal when they do not use the drug. They develop symptoms such as restlessness, loss of appetite, trouble sleeping, weight loss, and shaky hands.

Children and young teens start using marijuana for many reasons. Curiosity and the desire to fit into a social group are common motivating factors. Young people who have already begun to smoke cigarettes and/or use alcohol are at high risk for marijuana use. They may have family problems or a network of friends who use drugs and urge them to do the same (peer pressure).

Studies suggest that the use of alcohol and drugs by other family members plays a strong role in whether children start using drugs. Parents, grandparents, and older siblings in the home are models for children to follow.

Researchers have also found that children and teens (both male and female) who are physically or sexually abused are at greater risk of using marijuana and other drugs and of beginning drug use at an earlier age than other young people.

All aspects of a child's environment—home, school, and neighborhood—help to determine whether the child will try drugs.

Does Using Marijuana Lead to Other Drugs?

Long-term studies of high school students and their patterns of drug use show that very few young people use other drugs without first trying marijuana. The risk of using cocaine is estimated to be more than 100 times greater for those who have tried marijuana than for those who have not. Although there are no definitive studies on the factors associated with the movement from marijuana use to use of other drugs, growing evidence shows that a combination of biological, social, and psychological factors are involved. For this reason, marijuana is often referred to as a "gateway" drug.


Treatment is typically tailored to individual needs, and people who would like to stop using drugs can learn to control their cravings and live productive lives. People in drug-addiction treatment learn behavioral strategies to change their habits and often take medications as part of their treatment regimens.

The ultimate goal of all drug-abuse treatment is to enable people to achieve lasting abstinence, but the immediate goals are to reduce drug use, improve the person's ability to function, and minimize medical and social complications. Like people with diabetes or heart disease, people in treatment for drug addiction will need to change behaviors to adopt healthier lifestyles.

Historically, treatments for marijuana dependence were much the same as therapies for other drug-abuse problems. These included detoxification, behavioral therapies, and regular attendance at support group meetings.

More recently, researchers have introduced different ways to attract marijuana users to treatment and help them abstain from drug use. There are currently no medications for treating marijuana dependence. Treatment programs focus on counseling and group support systems. Drug treatment researchers are identifying which characteristics of users are predictors of treatment success and which approaches to treatment can be most helpful.

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  • National Institute on Drug Abuse, Child Abuse and Neglect (NIDA) National Institute on Drug Abuse National Institutes of Health - U.S. National Library of Medicine

Last reviewed 02/22/2019