Also known as ganja, weed, reefer, and grass, cannabis-marijuana is a psychoactive herb that comes from the hemp plant. This mind-altering substance is an illegal drug in most states.


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

Before the 1960s, many Americans had never heard of marijuana, but today it is a widely used illegal drug. It is usually smoked as a cigarette (called a "joint" or a "nail") or in a pipe or bong. In recent years, marijuana has appeared in blunts (cigars that have been emptied of tobacco and refilled with marijuana). Marijuana cigarettes or blunts have come to sometimes include crack cocaine, a combination known by various street names, such as "primos" or "woolies." Joints and blunts are also sometimes dipped in PCP and are called "happy sticks," "wicky sticks," "love boat" or "tical." Some users mix marijuana into foods 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. Terms from years ago, such as pot, herb, grass, weed, Mary Jane and reefer are still used. You might also hear the names Aunt Mary, skunk, boom, gangster, kif, or ganja. There are also street names for different strains or "brands" of marijuana, such as Bubble Gum, Northern Lights, Fruity Juice, Afghani No.1, and a number of Skunk varieties. One book of American slang lists more than 200 terms for various kinds of marijuana.

Cannabis is a term that 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 strength or potency of the THC it contains. The strength of the drug is measured by the average amount of THC in test samples of marijuana confiscated by law enforcement agencies. They found:

  • Most ordinary marijuana contained, on average, 7 percent THC.
  • Sinsemilla, made from the buds and flowering tops of female plants, contained 12 percent THC on average, ranging from less than 1 percent to 27 percent.
  • Hashish (the sticky resin from female plant flowers) had an average of 10 percent THC, ranging from 1 percent to 26 percent.

The "Monitoring the Future Survey," conducted yearly, includes students from the 8th, 10th, and 12th grades. The recent survey found that 15.7 percent of that year's 8th-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 about 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; the way the drug is taken (by smoking or eating); the experience and expectations of the user; the setting where the drug is used; and whether alcohol or other drugs are also being 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 with 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."

Regarding children, parents should be aware of changes in a child's behavior, although this may be difficult with teenagers. Parents 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 and other deodorizers
  • Use of eye drops
  • Clothing, posters, jewelry promoting drug use

Unpleasant effects that may occur include depersonalization (inability to distinguish oneself from others), changed body image, disorientation, and acute panic reactions or severe paranoia.

Some cases of 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, sinussitis, bronchitis, and asthma in heavy users
  • Possible serious effects on the immune system

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 the THC in marijuana. Generally, traces (metabolites) of THC can be detected by standard urine testing methods several days after a smoking session. However, in heavy, chronic users, traces can sometimes be detected for weeks after they have stopped using marijuana.

Harmful Effects

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

Marijuana hinders the user's short-term memory (memory for recent events), and he or she may have trouble handling complex tasks. With the use of more potent varieties of marijuana, even simple tasks can be difficult.

Because of the drug's effects on perceptions and reaction time, users could be involved in auto crashes. Drug users 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; timing, movements and coordination are all affected by THC.

Effects on Driving

Marijuana affects many skills required for safe driving: alertness, good concentration and coordination, and quick reaction time. These effects can last up to 24 hours after smoking marijuana. Marijuana use can make it difficult to judge distances as well as react to signals and other sights and sounds on the road.

Studies show that approximately 6 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 shows that after smoking marijuana, people demonstrate the same lack of coordination on standard drunken-driver tests as do people 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, there are studies raising serious health concerns. For example, one study examined the health statuses of 450 daily non-tobacco 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.

Findings show that the regular use of marijuana may play a role in cancer and problems in the respiratory and immune systems.


It is hard to find out 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 every day.

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

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. Cancer of the respiratory tract and lungs may also be promoted by marijuana smoke, since it contains irritants and carcinogens. 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.

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 pre- and early school years, marijuana-exposed children have been reported to exhibit more behavioral problems and difficulties with sustained attention and memory than nonexposed children.

Researchers are not certain whether any effects of marijuana 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 (such as what happened a few minutes ago). It is hard to learn while high—a working short-term memory is required for learning and performing tasks that call for more than one or two steps.

Among a group of longtime heavy marijuana users in Costa Rica, researchers found that the people had great trouble when asked to recall a short list of words (a standard test of memory). People in that study group also found it very hard to focus 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, can be used for treating the nausea and vomiting that go along with certain cancer treatments and is available by prescription. Another chemical related to THC, nabilone, has also been approved by the Food and Drug Administration for treating cancer patients who suffer from nausea. The oral THC is also used to help AIDS patients eat more to keep up their weight.

Scientists are studying whether marijuana, THC and related chemicals in marijuana (called cannabinoids) may 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 is addictive because it causes compulsive, uncontrollable drug craving, seeking and use, even in the face of negative health and social consequences. In 2004, more than 298,317 people entering drug treatment programs reported marijuana as their primary drug of abuse. 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 ones. Certainly, youngsters who have already begun to smoke cigarettes and, or, use alcohol are at high risk for marijuana use.

Also, research suggests 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 brothers and sisters in the home are models for children to follow.

Some young people who take drugs do not get along with their parents. Some have a network of friends who use drugs and urge them to do the same (peer pressure). All aspects of a child's environment—home, school and neighborhood—help to determine whether the child will try drugs.

Children who become more heavily involved with marijuana can become dependent, and that is their prime reason for using the drug. Others mention psychological coping as a reason for use—to deal with anxiety, anger, depression, boredom and so forth. But marijuana use is not an effective method for coping with life's problems, and staying high can be a way of simply not dealing with the problems and challenges of growing up.

Researchers have found that children and teens (both male and female) who are physically and 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.

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 has been estimated at more than 104 times greater for those who have tried marijuana than for those who have never tried it. 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.

Marijuana may affect the brain in some of the same ways that other drugs do. Researchers are examining the possibility that long-term marijuana use may create changes in the brain that make a person more at risk of becoming addicted to other drugs, such as alcohol or cocaine. While not all young people who use marijuana go on to use other drugs, further research is needed to determine who will be at greatest risk.


Through treatment that is tailored to individual needs, patients can learn to control their condition and live normal, productive lives. People in drug-addiction treatment learn behavioral changes and often take medications as part of their treatment regimens.

The ultimate goal of all drug-abuse treatment is to enable the patient to achieve lasting abstinence, but the immediate goals are to reduce drug use, improve the patient'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 more healthful lifestyles.

Up until a few years ago, it was hard to find treatment programs specifically for marijuana users. Treatments for marijuana dependence were much the same as therapies for other drug-abuse problems. These included detoxification, behavioral therapies and regular attendance at meetings of support groups, such as Narcotics Anonymous.

Recently, researchers have been testing 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. From the studies, drug treatment professionals are learning which characteristics of users are predictors of treatment success and which approaches to treatment can be most helpful.

Further progress in treatment to help marijuana users includes a number of programs set up to help adolescents. Some of these programs are in university research centers, where most of the young clients report marijuana as their drug of choice. Others are in independent adolescent treatment facilities. Family physicians are also a good source for information and help in dealing with adolescents' marijuana problems.

Find a Treatment Program here.


  • New England Journal of Medicine
  • Pediatrics
  • Archives of General Psychiatry
  • International Journal of Clinical Pharmacology and Therapeutics
  • National Institute on Drug Abuse, Child Abuse and Neglect (NIDA)
  • Journal of Clinical Pharmacology
  • Science
  • Behavioral Pharmacology
  • National Institute on Drug Abuse
  • Western Journal of Medicine
  • Journal of the American Medical Association
  • University of Michigan, News and Information Services
  • National Institutes of Health -U.S. National Library of Medicine
Last reviewed 11/24/2014