Gateway drug theory
The gateway drug theory (also called gateway theory, gateway hypothesis and gateway effect) states that use of less deleterious drugs precedes, and can lead to, future use of more dangerous hard drugs or crime. It is often attributed to the earlier use of one of several licit substances, including tobacco or alcohol, as well as cannabis.
The reverse gateway theory posits that earlier regular cannabis use predicts later tobacco initiation and/or nicotine dependence in those who did not use tobacco before.
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The hypothesis is that the use of soft drugs like cannabis leads to the use of harder drugs via a sequence of stages. This is based on the observation that many consumers who use cocaine or heroin have previously used cannabis, and most have used alcohol or tobacco; the hypothesis is that progression of drug use initiation continues from there to other drugs like cocaine or heroin. Some research supports that cannabis use predicts a significantly higher risk for subsequent use of "harder" illicit drugs, while other research does not. Some research finds that even alcohol represents a "gateway" drug, leading to the use of tobacco, marijuana, and other illicit substances.
While some research shows that many hard drug users used cannabis or alcohol before moving on to the harder substances, other research shows that some serious drug abusers have not used alcohol or cannabis first. The latter is evident in Japan, where the overwhelming majority of users of illicit drugs do not use cannabis first. One study finds no evidence that medical marijuana laws lead to an increase in cocaine or heroin usage.
The risk factor for using drugs in cannabis users may be higher because few people try hard drugs prior to trying cannabis, not because cannabis users increasingly try hard drugs such as certain substituted amphetamines (e.g., methamphetamine). For example, cannabis is typically available at a significantly earlier age than other illicit drugs.
Jacob Sullum analyzed the "gateway" theory in a 2003 Reason magazine article, noting that the theory's "...durability is largely due to its ambiguity: Because it's rarely clear what people mean when they say that pot smoking leads to the use of "harder" drugs, the claim is difficult to disprove.":
- "Notice that none of these interpretations involves a specific pharmacological effect of the sort drug warriors seem to have in mind when they suggest that pot smoking primes the brain for cocaine or heroin. As a National Academy of Sciences panel observed in a 1999 report, 'There is no evidence that marijuana serves as a stepping stone on the basis of its particular drug effect.' Last year the Canadian Senate's Special Committee on Illegal Drugs likewise concluded that 'cannabis itself is not a cause of other drug use. In this sense, we reject the gateway theory.'"
The National Institute on Drug Abuse have noted that while most cannabis users do not go on to use "harder" substances, reported data is consistent with the theory that cannabis is a gateway drug. However, they also suggest an alternative explanation. It may be that some individuals are more prone to using drugs and that these people are more likely to start with readily available substances such as cannabis, tobacco, and alcohol.
Alcohol and tobacco
Alcohol tends to precede cannabis use, and it is rare for those who use hard drugs to not have used alcohol or tobacco first; the 2005 National Survey of Drug Use and Health (NSDUH) in the United States found that, compared with lifetime nondrinkers, adults who have consumed alcohol were statistically much more likely to currently use illicit drugs and/or abuse prescription drugs in the past year. Effects were strongest for cocaine (26 times more likely), cannabis (14 times more likely), and psychedelics (13 times more likely). In addition, lifetime drinkers were also six times more likely to use or be dependent on illicit drugs than lifetime nondrinkers.
According to the NIDA, "People who abuse drugs are also likely to be cigarette smokers. More than two-thirds of drug abusers are regular tobacco smokers, a rate more than triple that of the rest of the population."
Electronic cigarettes as gateway to smoking
Much of the literature on electronic cigarettes has expressed fears that they would, especially if marketed by big tobacco, be a gateway to smoking tobacco for young people. In 2015 a report commissioned by Public Health England, a government agency, examined the evidence for this, and concluded that "We strongly suggest that use of the gateway terminology be abandoned until it is clear how the theory can be tested in this field", They quoted a study which concluded that: "Although the concept of the gateway theory is often treated as a straightforward scientific theory, its emergence is rather more complicated. In effect, it is a hybrid of popular, academic and media accounts – a construct retroactively assembled rather than one initially articulated as a coherent theory”.
Another theory is that prohibition itself is a gateway to "hard" drug use. Many support this theory referencing the introduction to the black market. Once utilizing the black market, sellers have incentive to sell higher margin products, unrestricted by greater regulation on more harmful substances. The research has found an increase of drug-related incidents in counties of the United States that ban alcohol as well. 
Alternative explanations for the correlation between the use of soft drugs (e.g., marijuana) and the use of hard drugs (e.g., cocaine, heroin) include, but are not limited to:
- The Common Liability to Addiction theory (CLA) argues that a person's involvement with drugs and its various degrees, including potential development and severity of addiction, are based on biobehavioral mechanisms that are largely not drug-specific. Within the CLA framework, the sequence of drug use initiation - the essence of the "gateway theory" - is opportunistic and trivial: the "gateway" drugs, that is, the substances used first, are merely those that are (usually) available at an earlier age (thus usually licit) than those used later (usually, hard drugs). In an extensive review addressing the CLA and the "gateway" theory, it was pointed out (Vanyukov et al., 2012) that the "gateway" sequence applies only to the initiation of use of different drugs rather than different levels or extent of drug involvement (from use to dependence), questioning its relevance to addiction as a medical problem. Despite that, the "gateway theory" has significantly and, arguably, adversely influenced policy formation, intervention, and research.
- Teenagers' trust of adults erodes when authority-figures exaggerate or make up the dangers of the "gateway" drugs, leading teenagers to regard all anti-drug messages as nonsense.
- The peer environments in which "gateway" drugs are used can sometimes overlap with the ones in which harder drugs are used, especially in societies that prohibit the substances or impose very high age-limits.
- Vanyukov MM, Tarter RE, Kirillova GP, et al. (June 2012). "Common liability to addiction and "gateway hypothesis": theoretical, empirical and evolutionary perspective". Drug Alcohol Depend (Review). 123 Suppl 1: S3–17. doi:10.1016/j.drugalcdep.2011.12.018. PMC . PMID 22261179.
- Pudney, Stephen (December 2002). "The road to ruin? Sequences of initiation into drug use and offending by young people in Britain" (PDF). Home Office Research Study 253. London: Home Office Research, Development and Statistics Directorate. ISBN 1-84082-928-1. ISSN 0072-6435. Retrieved 2009-04-04.
- Peters EN, Budney AJ, Carroll KM (August 2012). "Clinical correlates of co-occurring cannabis and tobacco use: a systematic review". Addiction (Review). 107 (8): 1404–17. doi:10.1111/j.1360-0443.2012.03843.x. PMC . PMID 22340422.
- Peters EN, Budney AJ, Carroll KM (August 2012). "Clinical correlates of co-occurring cannabis and tobacco use: a systematic review". Addiction. 107 (8): 1404–17. doi:10.1111/j.1360-0443.2012.03843.x. PMC . PMID 22340422.
- Chu, Yu-Wei Luke (2015-05-01). "Do Medical Marijuana Laws Increase Hard-Drug Use?". Journal of Law and Economics. 58 (2): 481–517. doi:10.1086/684043.
- Sullum, Jacob (24 Jan 2003), Marijuana as a "gateway" drug, Reason, retrieved 2014-04-01
- "Is marijuana a gateway drug?". drugabuse.gov. National Institute of Drug Abuse.
- Illicit Drug Use among Lifetime Nondrinkers and Lifetime Alcohol Users, NSDUH, 2005
- The National Institute on Drug Abuse (NIDA), part of the NIH, a component of the U.S. Department of Health and Human Services. – Nicotine Craving and Heavy Smoking May Contribute to Increased Use of Cocaine and Heroin – Patrick Zickler, NIDA NOTES Staff Writer. Retrieved October, 2006.
- McNeill, A, SC (2015). "E - cigarettes: an evidence update A report commissioned by Public Health England" (PDF). www.gov.uk. UK: Public Health England. p. 38. Retrieved 24 August 2015.
- Bell, K. and H. Keane, "All gates lead to smoking: The ‘gateway theory’, e-cigarettes and the remaking of nicotine"., Social Science & Medicine, 2014. 119, quoted McNeill, pp. 37-38
- "Prohibition Is the Real "Gateway Drug"". The Huffington Post. 29 September 2015.
- Fernandez, Jose M. and Gohmann, Stephan and Pinkston, Joshua C., Breaking Bad: Are Meth Labs Justified in Dry Counties? (August 25, 2015). Available at SSRN: http://ssrn.com/abstract=2650484 or http://dx.doi.org/10.2139/ssrn.2650484
- Conlin, Michael; Dickert-Conlin, Stacy; Pepper, John. "Effect of Alcohol Prohibition on Illicit-Drug-Related Crimes, The." Journal of Law and Economics 48.1 (2005): 215-234.
- Brecher, Edward M. (1972). "Heroin on the youth drug scene - and in Vietnam". Licit and illicit drugs; the Consumers Union report on narcotics, stimulants, depressants, inhalants, hallucinogens, and marijuana - including caffeine, nicotine, and alcohol. Boston: Little, Brown. ISBN 0-316-10717-4.
- Ginzler JA, Cochran BN, Domenech-Rodríguez M, Cauce AM, Whitbeck LB (2003). "Sequential progression of substance use among homeless youth: an empirical investigation of the gateway theory". Subst Use Misuse (Review). 38 (3-6): 725–58. doi:10.1081/ja-120017391. PMID 12747403.
- Kenkel D, Mathios AD, Pacula RL (January 2001). "Economics of youth drug use, addiction and gateway effects". Addiction (Review). 96 (1): 151–64. doi:10.1080/09652140020017021. PMID 11177526.
- The classification of cannabis under the Misuse of Drugs Act 1971 (Section 4.6 "Does cannabis use lead on to other drug use?")
- How did the marijuana gateway myth get started? Schaffer Library of Drug Policy.