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Marijuana is the most commonly abused illicit drug in the United States. A dry, shredded green/brown mix of flowers, stems, seeds, and leaves of the hemp plant Cannabis sativa, it usually is smoked as a cigarette (joint, nail), or in a pipe (bong). It also is smoked in blunts, which are cigars that have been emptied of tobacco and refilled with marijuana, often in combination with another drug. It might also be mixed in food or brewed as a tea. As a more concentrated, resinous form it is called hashish and, as a sticky black liquid, hash oil. Marijuana smoke has a pungent and distinctive, usually sweet-and-sour odor. There are countless street terms for marijuana including pot, herb, weed, grass, widow, ganja, and hash, as well as terms derived from trademarked varieties of cannabis, such as Bubble Gum, Northern Lights, Fruity Juice, Afghani #1, and a number of Skunk varieties.

most abused illicit drug in the US

The main active chemical in marijuana is THC (delta-9-tetrahydrocannabinol). The membranes of certain nerve cells in the brain contain protein receptors that bind to THC. Once securely in place, THC kicks off a series of cellular reactions that ultimately lead to the high that users experience when they smoke marijuana.

Extent of Use

In 2004, 14.6 million Americans age 12 and older used marijuana at least once in the month prior to being surveyed. About 6,000 people a day in 2004 used marijuana for the first time—2.1 million Americans. Of these, 63.8 percent were under age 181. In the last half of 2003, marijuana was the third most commonly abused drug mentioned in drug-related hospital emergency department (ED) visits in the continental United States, at 12.6 percent, following cocaine (20 percent) and alcohol (48.7 percent)2.

6,000 people every day used marijuana for the first time in 2004

Prevalence of lifetime,* annual, and use within the last 30 days for marijuana remained stable among 10th- and 12th-graders surveyed between 2003 and 2004. However, 8th-graders reported a significant decline in 30-day use and a significant increase in perceived harmfulness of smoking marijuana once or twice and regularly3. Trends in disapproval of using marijuana once or twice and occasionally rose among 8th-graders as well, and 10th-graders reported an increase in disapproval of occasional and regular use for the same period3.

Percentage of 8th-Graders Who Have Used Marijuana:
Monitoring the Future Study, 2005

Percentage of 10th-Graders Who Have Used Marijuana:
Monitoring the Future Study, 2005

Percentage of 12th-Graders Who Have Used Marijuana
Monitoring the Future Study, 2005

* "Lifetime" refers to use at least once during a respondent’s lifetime. "Annual" refers to use at least once during the year preceding an individual's response to the survey. "30-day" refers to use at least once during the 30 days preceding an individual’s response to the survey.

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 movement4.

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 abuse indicate some changes in the brain similar to those seen after long-term abuse of other major drugs. 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 system5 and changes in the activity of nerve cells containing dopamine6. 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 an abuser's risk of heart attack more than quadruples in the first hour after smoking marijuana7. The researchers suggest that such an effect might occur from marijuana's 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 nonsmokers8. Many of the extra sick days among the marijuana smokers in the study were for respiratory illnesses.

risk of heart attack more than quadruples; potential to promote lung cancer

Even infrequent abuse 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 airways9. Smoking marijuana possibly increases the likelihood of developing cancer of the head or neck. A study comparing 173 cancer patients and 176 healthy individuals produced evidence that marijuana smoking doubled or tripled the risk of these cancers10.

Marijuana abuse also has the potential to promote cancer of the lungs and other parts of the respiratory tract because it contains irritants and carcinogens9,11. In fact, marijuana smoke contains 50 to 70 percent more carcinogenic hydrocarbons than does tobacco smoke12. It also induces high levels of an enzyme that converts certain hydrocarbons into their carcinogenic form—levels that may accelerate the changes that ultimately produce malignant cells13. 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 be more harmful to the lungs than smoking tobacco.

Other Health Effects

Some of marijuana's adverse health effects may occur because THC impairs the immune system's ability to fight disease. 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 inhibited14. In other studies, mice exposed to THC or related substances were more likely than unexposed mice to develop bacterial infections and tumors15,16.

Effects of Use on Learning and Social Behavior

Research clearly demonstrates that marijuana has the potential to cause problems in daily life or make a person's existing problems worse. Depression17, anxiety17, and personality disturbances18 have been associated with chronic marijuana use. 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 marijuana’s adverse impact on memory and learning can last for days or weeks after the acute effects of the drug wear off19,20,25.

some cognitive abilities may be restored in individuals who quit smoking marijuana, even after long-term heavy use

Students who smoke marijuana get lower grades and are less likely to graduate from high school, compared with their nonsmoking peers21,22,23,24. A study of 129 college students found that, among those who smoked the drug at least 27 of the 30 days prior to being surveyed, critical skills related to attention, memory, and learning were significantly impaired, even after the students had not taken the drug for at least 24 hours20. These "heavy" marijuana abusers had more trouble sustaining and shifting their attention and in registering, organizing, and using information than did the study participants who had abused 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 abusers to recall words from a list remained impaired for a week after quitting, but returned to normal within 4 weeks25. Thus, 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 among postal workers found that employees who tested positive for marijuana on a pre-employment urine drug test had 55 percent more industrial accidents, 85 percent more injuries, and a 75-percent increase in absenteeism compared with those who tested negative for marijuana use26. In another study, heavy marijuana abusers reported that the drug impaired several important measures of life achievement including cognitive abilities, career status, social life, and physical and mental health27.

Effects of Exposure During Pregnancy

Research has shown that some babies born to women who abused marijuana during their pregnancies display altered responses to visual stimuli28, increased tremulousness, and a high-pitched cry, which may indicate neurological problems in development29. During the preschool years, marijuana-exposed children have been observed to perform tasks involving sustained attention and memory more poorly than nonexposed children do30,31. In the school years, these children are more likely to exhibit deficits in problem-solving skills, memory, and the ability to remain attentive30.

Addictive Potential

Long-term marijuana abuse can lead to addiction for some people; that is, they abuse 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 abusing the drug. People trying to quit report irritability, sleeplessness, and anxiety32. They also display increased aggression on psychological tests, peaking approximately one week after the last use of the drug33.

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 nonidentical male twins to report similar responses to marijuana abuse, indicating a genetic basis for their response to the drug34. (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.34

Treating Marijuana Problems

The latest treatment data indicate that, in 2002, marijuana was the primary drug of abuse in about 15 percent (289,532) of all admissions to treatment facilities in the United States. Marijuana admissions were primarily male (75 percent), White (55 percent), and young (40 percent were in the 15-–19 age range). Those in treatment for primary marijuana abuse had begun use at an early age; 56 percent had abused it by age 14 and 92 percent had abused it by 1835.

92 percent of those treated had abused marijuana by age 18

One study of adult marijuana abusers found comparable benefits from a 14-session cognitive-behavioral group treatment and a 2-session individual treatment that included motivational interviewing and advice on ways to reduce marijuana use. Participants were mostly men in their early thirties who had smoked marijuana daily for more than 10 years. By increasing patients' awareness of what triggers their marijuana abuse, both treatments sought to help patients devise avoidance strategies. Abuse, dependence symptoms, and psychosocial problems decreased for at least 1 year following both treatments; about 30 percent of the patients were abstinent during the last 3-month followup period36.

Another study suggests that giving patients vouchers that they can redeem for goods—such as movie passes, sporting equipment, or vocational training—may further improve outcomes37.

Although no medications are currently available for treating marijuana abuse, recent discoveries about the workings of the THC receptors have raised the possibility of eventually developing a medication that will block the intoxicating effects of THC. Such a medication might be used to prevent relapse to marijuana abuse by lessening or eliminating its appeal.

1 Results from the 2004 National Survey on Drug Use and Health: National Findings (Office of Applied Studies, NSDUH Series H–27, DHHS Publication No. SMA 05–4061). Rockville, MD, 2004. NSDUH is an annual survey conducted by the Substance Abuse and Mental Health Services Administration. Copies of the latest survey are available from the National Clearinghouse for Alcohol and Drug Information at 800-729-6686.

2 These data are from the annual Drug Abuse Warning Network, funded by the Substance Abuse and Mental Health Services Administration, DHHS. The survey provides information about emergency department visits that are induced by or related to the use of an illicit drug or the nonmedical use of a legal drug. The latest data are available at 800-729-6686 or online at www.samhsa.gov.

3 These data are from the 2005 Monitoring the Future Survey, funded by the National Institute on Drug Abuse, National Institutes of Health, DHHS, and conducted annually by the University of Michigan’s Institute for Social Research. The survey has tracked 12th-graders’ illicit drug use and related attitudes since 1975; in 1991, 8th- and 10th-graders were added to the study. The latest data are online at www.drugabuse.gov.

4 Herkenham M, Lynn A, Little MD, Johnson MR, et al. Cannabinoid receptor localization in the brain. Proc Natl Acad Sci, USA 87(5):1932–1936, 1990.

5 Rodriguez de Fonseca F, et al. Activation of cortocotropin-releasing factor in the limbic system during cannabinoid withdrawal. Science 276(5321):2050–2054, 1997.

6 Diana M, Melis M, Muntoni AL, et al. Mesolimbic dopaminergic decline after cannabinoid withdrawal. Proc Natl Acad Sci 95(17):10269–10273, 1998.

7 Mittleman MA, Lewis RA, Maclure M, et al. Triggering myocardial infarction by marijuana. Circulation 103(23):2805–2809, 2001.

8 Polen MR, Sidney S, Tekawa IS, et al. Health care use by frequent marijuana smokers who do not smoke tobacco. West J Med 158(6):596–601, 1993.

9 Tashkin DP. Pulmonary complications of smoked substance abuse. West J Med 152(5):525–530, 1990.

10 Zhang ZF, Morgenstern H, Spitz MR, et al. Marijuana use and increased risk of squamous cell carcinoma of the head and neck. Cancer Epidemiology, Biomarkers & Prevention 8(12):1071–1078, 1999.

11 Sridhar KS, Raub WA, Weatherby, NL Jr., et al. Possible role of marijuana smoking as a carcinogen in the development of lung cancer at a young age. Journal of Psychoactive Drugs 26(3):285–288, 1994.

12 Hoffman D, Brunnemann KD, Gori GB, et al. On the carcinogenicity of marijuana smoke. In: VC Runeckles, ed, Recent Advances in Phytochemistry. New York. Plenum, 1975.

13 Cohen S. Adverse effects of marijuana: Selected issues. Annals of the New York Academy of Sciences 362:119–124, 1981.

14 Adams IB, Martin BR: Cannabis: pharmacology and toxicology in animals and humans. Addiction 91(11):1585–1614, 1996.

15 Friedman H, Newton C, Klein TW. Microbial infections, immunomodulation, and drugs of abuse. Clin Microbiol Rev 16(2):209–219, 2003.

16 Zhu LX, Sharma M, Stolina S, et al. Delta-9-tetrahydrocannabinol inhibits antitumor immunity by a CB2 receptor-mediated, cytokine-dependent pathway. J Immunology 165(1):373–380, 2000.

17 Brook JS, Rosen Z, Brook DW. The effect of early marijuana use on later anxiety and depressive symptoms. NYS Psychologist 35–39, January 2001.

18 Brook JS, Cohen P, Brook DW. Longitudinal study of co-occurring psychiatric disorders and substance use. J Acad Child and Adolescent Psych 37(3):322–330, 1998.

19 Pope HG, Yurgelun-Todd D. The residual cognitive effects of heavy marijuana use in college students. JAMA 275(7):521–527, 1996.

20 Block RI, Ghoneim MM. Effects of chronic marijuana use on human cognition. Psychopharmacology 100(1–2):219–228, 1993.

21 Lynskey M, Hall W. The effects of adolescent cannabis use on educational attainment: A review. Addiction 95(11):1621–1630, 2000.

22 Kandel DB, Davies M. High school students who use crack and other drugs. Arch Gen Psychiatry 53(1):71–80, 1996.

23 Rob M, Reynolds I, Finlayson PF. Adolescent marijuana use: Risk factors and implications. Aust NZ J Psychiatry 24(1):45–56, 1990.

24 Brook JS, Balka EB, Whiteman M. The risks for late adolescence of early adolescent marijuana use. Am J Public Health 89(10):1549–1554, 1999.

25 Pope HG, Gruber AJ, Hudson JI, et al. Neuropsychological performance in long-term cannabis users. Arch Gen Psychiatry 58(10):909–915, 2001.

26 Zwerling C, Ryan J, Orav EJ. The efficacy of pre-employment drug screening for marijuana and cocaine in predicting employment outcome. JAMA 264(20):2639–2643, 1990.

27 Gruber AJ, Pope HG, Hudson JI, et al. Attributes of long-term heavy cannabis users: A case control study. Psychological Medicine 33(8):1415–1422, 2003.

28 Fried PA, Makin JE. Neonatal behavioural correlates of prenatal exposure to marihuana, cigarettes and alcohol in a low risk population. Neurotoxicology and Teratology 9(1):1–7, 1987.

29 Lester BM, Dreher M. Effects of marijuana use during pregnancy on newborn crying. Child Development 60(23/24):764–771, 1989.

30 Fried PA. The Ottawa prenatal prospective study (OPPS): Methodological issues and findings. It’s easy to throw the baby out with the bath water. Life Sciences 56(23–24):2159–2168, 1995.

31 Fried PA, Smith AM. A literature review of the consequences of prenatal marihuana exposure: An emerging theme of a deficiency in aspects of executive function. Neurotoxicology and Teratology 23(1):1–11, 2001.

32 Kouri EM, Pope HG, Lukas SE. Changes in aggressive behavior during withdrawal from long-term marijuana use. Psychopharmacology 143(3):302–308, 1999.

33 Haney M, Ward AS, Comer SD, et al. Abstinence symptoms following smoked marijuana in humans. Psychopharmacology 141(4):395–404, 1999.

34 Lyons MJ, Toomey R, Meyer JM, et al. How do genes influence marijuana use? The role of subjective effects. Addiction 92(4):409–417, 1997.

35 These data from the Treatment Episode Data Set (TEDS) 2003: Substance Abuse Treatment Admissions by Primary Substance of Abuse, According to Sex, Age Group, Race, and Ethnicity, funded by the Substance Abuse and Mental Health Services Administration, DHHS. The latest data are available at 800-729-6686 or online at www.samhsa.gov.

36 Stephens RS, Roffman RA, Curtin L. Comparison of extended versus brief treatments for marijuana use. J Consult Clin Psychol 68(5):898–908, 2000.

37 Budney AJ, Higgins ST, Radonovich KJ, et al. Adding voucher-based incentives to coping skills and motivational enhancement improves outcomes during treatment for marijuana dependence. J Consult Clin Psychol 68(6):1051–1061, 2000.