The Enduring Gateway Myth

In my work with juvenile justice agencies and drug treatment professionals, I still hear people call cannabis (or marijuana) a "gateway" drug. This is one of those notions that just won't go away despite the availability of good information that disproves it. 
According to the gateway hypothesis, the use of cannabis leads to harder drugs. Drug users, according to the hypothesis, are enticed to move through a sequence of progressively more severe substance abuse, with each stage drawing them to the next stage and to the next drug. Most practitioners imagine that users must begin to crave a better high. Researchers have sometimes supported these ideas. 
Some researchers see support for the gateway hypothesis in survey data. Using data from the former NHSDA (now the NSDUH), Kandel and Yamaguchi reported a sequence of worsening drug use across individuals users. Virtually all cocaine users, for example, first used marijuana and alcohol. Other researchers see confirmation of the gateway hypothesis in studies of twins with different drug behaviors. Lynskey and associates, for example, examined several hundred pairs of same-sex twins and reported that the use of marijuana before age 17 in one sibling was associated with higher lifetime rates of drug use, drug dependence, and alcohol dependence in that sibling relative to the twin not using marijuana before age 17. 
Neither of these studies, however, actually supports the gateway hypothesis. The findings merely indicate that the individuals most likely to use drugs at some point tend to begin using them at an early age and also tend to begin their drug-taking careers with alcohol and marijuana. This phenomenon is not the same as the gateway hypothesis. 
A useful analogy is the difference between jogging and serious running. Marathon runners tend to start out jogging, but it would be incorrect to say that jogging leads to (or causes) marathons. The vast majority of joggers do not progress to marathon running. Nearly all marathoners would admit to being former joggers, but if we wanted to prevent marathon running for some reason it would be foolish and far too expensive to do so by intervening with all joggers. 
According to research by Morral and colleagues, marijuana use does not signal the beginning of an inevitable progression toward the use of other drugs. There is often a developmental pattern to drug initiation, with marijuana serving as the bridge between legal and illegal drugs for those who go on to other drugs, and it is true that individuals who begin to use drugs at a very young age are statistically more likely to develop problems with abuse and dependence later in life. However, the use of one drug does not necessarily lead to the use of other drugs. Very few drug users follow the entire drug progression sequence.
Research by Labouvie and White showed that drug use during adolescence does not always lead to continued use over time, nor does it necessarily lead to serious drug problems in adulthood. Most individuals stop voluntarily at some point during the drug progression sequence. A study by Elliot showed that escalation from one drug to another is not even statistically likely. Once drug use has started, users are actually more likely to regress to earlier stages in the sequence than to advance to subsequent stages.
So, why does the gateway hypothesis seem true? The best explanation is relatively simple—opportunity. A study by Wagner and Anthony showed how marijuana users are more likely to find themselves in social situations where cocaine is available. Obviously, cocaine users feel more compatible with marijuana users than with non-marijuana users. Thus, a marijuana smoker is more likely to be around when cocaine users make cocaine available. Cannabis smokers may be more likely than non-smokers to use cocaine eventually, but this has nothing to do with the addictive properties of marijuana. It is a result of the friendship networks that develop among drug users. It is especially true for illegal drugs because users have to be careful about their social contacts in order to avoid detection and arrest. Seeing someone actually using marijuana would make a cocaine user more comfortable offering that person cocaine.
The same thing is true with tobacco. Youth who are tobacco users are more likely to be in social situations where cannabis is offered. This does not mean that tobacco leads to marijuana use or that a pack of Marlboros is a gateway drug, but once a young person starts using tobacco he or she is probably more likely to be around when someone offers to share their cannabis.
An objective interpretation of research about teenagers and cannabis underscores the importance of screening and assessment. The gateway hypothesis should not be used to justify policies that treat all young marijuana users as if they are destined to become future addicts. A few teenage marijuana users will turn out to be future casualties of drug abuse, but most will not. After all, 40 to 50 percent of all youth in the U.S. try cannabis at some point before age 18. Distinguishing the future drug abuse victim from the typical teen pot smoker is critically important for families and communities. Effective and objective assessment requires the use of tools that are applied consistently and professionally, that are free of cultural and class bias, and that are designed to address the needs of youth and families and not the financial stability of treatment providers. 
 
References
Elliot, Delbert S. (1993). “Health-Enhancing and Health-Compromising Lifestyles.” In Promoting the Health of Adolescents: New Directions for the Twenty-First Century, edited by Susan G. Millstein, Anne C. Petersen, and Elena O. Nightingale (119–45). New York: Oxford University Press.
Kandel, Denise B., and Kazuo Yamaguchi (2002). “Stages of Drug Involvement in the U.S. Population.” In Stages and Pathways of Drug Involvement: Examining the Gateway Hypothesis, edited by Denise B. Kandel (65–89). Cambridge, U.K.: Cambridge University Press.
Labouvie, Erich, and Helene R. White (2002). “Drug Sequences, Age of Onset, and Use Trajectories as Predictors of Drug Abuse/Dependence in Young Adulthood.” In Stages and Pathways of Drug Involvement: Examining the Gateway Hypothesis, edited by Denise B. Kandel (19–41). Cambridge, U.K.: Cambridge University Press.
Lynskey, Michael T., Andrew C. Heath, Kathleen K. Bucholz, Wendy S. Slutske, Pamela A. F. Madden, Elliot C. Nelson, Dixie J. Statham, and Nicholas G. Martin (2003). “Escalation of Drug Use in Early-Onset Cannabis Users vs. Co-Twin Controls.” Journal of the American Medical Association 289(4): 427–33. 
Morral, Andrew R., Daniel F. McCaffrey, and Susan M. Paddock (2002). “Reassessing the Marijuana Gateway Effect.” Addiction 97:1493–1504. 
Wagner, Fernando, and James C. Anthony (2001). “Into the World of Illegal Drug Use: Exposure Opportunity and Other Mechanisms Linking the Use of Alcohol, Tobacco, Marijuana, and Cocaine.” American Journal of Epidemiology 155(10): 918–25.